Outsourcing Economics Explained ---

Guide to the Many Advantages of Outsourcing to Abrasive-Form
Many companies fail to consider the complete picture when trying to determine if outsourcing provides a more preferable route to pursue rather than keeping their creep feed grinding operation in-house.   In this white paper, we will examine the range of factors that determine an accurate cost-benefit analysis, including cost, delivery, quality, flexibility, and risk reduction.

Cost Advantages

Abrasive-Form will optimize your creep feed grinding operation to wring costs out of the process.  Abrasive-Form brings many advantages to bear on this process.

• Abrasive-Form has 26 years of experience and many capable experts in creep feed grinding, which is the central focus of the company.
  
• Abrasive-Form brings expertise in several disciplines including engineering, the best in supplies procurement, and efficient operational management.
  
• Your company benefits from Abrasive-Form's cross-training over several industries rather than managing with the limited problem solving techniques of one industry's isolated environment.
  
• Abrasive-Form can easily design fixtures to process multiple parts simultaneously, rather than be constrained to sequential operation processing or batch processing. 
  
• Abrasive-Form can coordinate with upstream suppliers such as foundries to design details into rough parts that make manufacturing easier down the road without affecting the design or quality of finished parts.
  
• Abrasive-Form operates with a much lower overhead structure than most large OEMs, thus having a far lower 'loading factor' to its direct labor costs
  
• Outsourcing to Abrasive-Form means eliminating the $500,000 cost of buying a creep feed grinder.  Add to that figure the cost to locate and electrify each machine on your shop floor and your company's savings could easily be in the range of $600,000 per unit.  In addition:
  
• Your company will save on the annual cost of machine maintenance and repairs.
  
• Abrasive-Form's electrical and mechanical maintenance staff has over 20 years of experience that is put to work to avoid any downtime.
  
• Your company will not need to maintain an inventory of spare parts for machine repair.
  
• Abrasive-Form always has backup machines to process your job such that there is never a job queue for machine repair or other reasons.  This eliminates all the costs of investing in backup machines. 
  
• By relying on Abrasive-Form's machines, your company also eliminates the costs of maintaining stocks of consumable items such as wheels, dressing tools, coolant, lubricating oils, hydraulic fluids, filters, etc.  This also eliminates time-consuming and irritating purchasing tasks for minor cost supplies.
  
• Abrasive-Form can bundle the purchase of consumable supplies with other customers' needs thus achieving far greater purchasing power and lower unit costs for supplies.
  
• Abrasive-Form maintains an extensive consumables inventory minimizing the chance that a stock-out will cause an interruption in service.
  
• Your company also eliminates the costs of hiring full time employees, both operators and supervisors, to run the machines and the considerable costs of training personnel to become grinding specialists. (Note:  Abrasive-Form feels that it takes at least three years experience for an operator to reach full efficiency production capabilities.)  By shouldering direct labor costs, Abrasive-Form also minimizes your company's labor-related overhead costs.
  
• There will be no need for your company to pay for an expensive learning curve to reach optimum output levels.  Abrasive-Form's experienced personnel can climb this curve much faster.
  
• Your company will no longer need to devote expensive manufacturing floor space to creep feed grinding tasks.
  
• Your company eliminates costs for work-in-progress when it outsources to Abrasive-Form, postponing expenses until finished parts are received.  This eliminates the need for cash investment until far later in a manufacturing cycle.
  
• Leaving the grinding to Abrasive-Form enables your company to focus energies on other manufacturing problems and/or to concentrate on core competencies.
  
• Abrasive-Form can guarantee cost reductions in multi-year contracts by employing continuous improvement techniques on your application. 

Delivery Advantages

• Abrasive-Form's application engineering group has many years of experience in minimizing the lead time to get a project up and running.  This lead time compression will enable your company to get a product to market quickly allowing your company to take full advantage of a greater portion of the component's "life cycle".  Your company will not lose volumes because fixturing and tooling was not prepared in time.
  
• Abrasive-Form's in-house staff of toolmakers can perform all tooling repairs promptly.  Abrasive-Form also stocks many wearable fixture components in-house.
  
• Abrasive-Form has over 50 qualified machine operators on staff.  Your company's project will not suffer due to vacations, illnesses or turnover because staff backups are in place.

Quality Advantages

• Usually, Abrasive-Form requires more stringent tolerances on part production than customers require.
  
• Abrasive-Form provides the quality documentation that is required by your company and can use the computer system of your company's choice. 
  
• Your company can lock in maximum scrap costs by downloading this responsibility to Abrasive-Form.

Flexibility Advantages

• Abrasive-Form will communicate production status and deliveries to your company in any combination of phone, fax, e-mail, or Internet updates
  
• Your company can save on ancillary operations such as deburring, heat treating, non-destructive testing, etc., by having Abrasive-Form manage these functions seamlessly into the total cycle time.
  
• Abrasive-Form will do whatever it takes to meet your company's delivery requirements and routinely schedules operators to work through holidays, vacations, and weekends to meet special delivery requests. 
  
• Abrasive-Form will almost always be able to use your company's tooling since its 38 creep feed grinding machines have a wide range of working envelopes.
  
• Abrasive-Form has the floor capacity to actually take delivery of your company's complete grinding machine and fixtures for processing parts in the shortest possible time period, if needed.  

Reduction of Risk Advantages

• Your company can eliminate the risks of uncertain parts volumes by outsourcing an application to Abrasive-Form.  This eliminates the nightmare of purchasing a complete machining system(s) only to find out that the life of an application is much shorter than the time used to payback an initial investment.
  
• Your company can mitigate the impact of not achieving an originally projected production rate that was used to justify capital expenditures.
  
• Your company can eliminate the risk that a project volume will double or triple and necessitate additional equipment purchases.  Abrasive-Form has plenty of available capacity to keep pace with a fast-growing project.

Paul K. Gibree
Product Engineer
Norton Company
1 New Bond Street
Worcester, MA 01606

It is widely accepted that grinding wheels with a high percentage of porosity are an absolute necessity for Creep Feed grinding. It is the purpose of this paper to understand why and to, highlight possible areas for thought when selecting the appropriate wheel.

There's no "Buzz Word" in the grinding industry today that sparks the interest and excitement of "CREEP FEED".

It is impossible to attend a seminar on Creep Feed without hearing the phrase "very soft very open" when referring to the grinding wheel without any real explanation of why or compared to what? Compared to conventional grinding, and what is actually going on. In contrast to conventional reciprocating grinding, Creep Feed is characterized by increased depths of cut and much slower table speeds.

Increasing the depth of cut or infeed is an easily observable change to any grinding system. Increasing the depth of cut also increases the length of arc of contact. In this example "length of arc" will refer to the distance an abrasive particle must follow while in contact with the work piece. For our example we will be using a 16" diameter grinding wheel. Setting the machine for a depth of cut of .001", will generate an arc of contact between the wheel and the work piece of .126". Increasing the depth of cut to .110" will increase this arc of contact from .126 to 1.266". (Figure #1).

Using the same 16" diameter grinding wheel of any given width and depth of cut .001" we have a given area of contact. By "area of contact" we mean that portion of the wheel actually in contact with the work piece during the grind. (Length of arc x width of wheel) (Figure #2). Within this area, based on abrasive size and structure there will be a given amount of abrasive particles. Increasing the depth of cut (Figure #3) will proportionately increase this area of contact. Again, keeping abrasive size and structure constant, we can expect an increase in the amount of active abrasive particles in contact with the material.

Applying a given force of the same amount to both areas results in the amount of force per individual abrasive particle within the larger area to decrease (Figure #4). This decrease in unit pressure per abrasive particle is the same principle we see in conventional grinding. One should be able to visualize at this time that changing the depth of cut directly effects the area of contact between the wheel and the work piece. Changing the area of contact in turn effects the unit pressure per abrasive particle causing the grinding wheel to act harder or softer.

As in conventional reciprocating grinding it is an accepted f act that as you increase the area of contact, you must adjust the grade to maintain the same performance. Changing the depth of cut although very obvious is not the only element to influence the number of active abrasive particles in relation to area. Form must also be taken into consideration.

Unlike conventional surface grinding where form has a less noticeable effect on the number of active abrasive grinding particles, Creep Feed with full depth grinding has a much more pronounced affect. For example, a straight faced grinding wheel has a fixed number of active abrasive particles for a given horsepower (Figure #9A), unless Figure B has a depth of cut greater than that of the formed area, the effect is the same as removing 1/4 the amount of material with the same horsepower. A wheel that might work well with the given horsepower in 9A could possibly grind unsatisfactory in condition 9B. Another thing to remember here is that the sides of the form sections are not actively grinding, but rather have a polishing effect. Keeping horsepower constant from form 9A to 9B would result in more force per active abrasive particle, or we would expect the wheel to act softer than in 9A.

The extreme example of a forms affect on the active abrasives grinding particles is represented in Figure 9C. In this case only 1/2 the material is now being removed by twice the active abrasive particles. Again maintaining the same horsepower as in 9A would result in a much lower force per active abrasive particle and in this instance we would expect the wheel to act harder. From these examples it becomes easier to visualize that when we say very soft it is relative to performing the same operation by conventional reciprocation with it's greatly reduce area of contact. And that a wheel that works very well on a particular material with set machine parameters would possibly require readjusting these parameters, as the depth of cut or form on the grinding wheel is changed to optimize results.

When referring to the cavities or porosity within a grinding wheel what we are actually referring to is the wheels structure. Structure is the relationship of abrasive grain and bonding material to the voids or spaces within the wheel Figure #5. For each grit size and grade combination there is an optimum structure. In creep feed it is confirmed that increased porosity is absolutely necessary. Testing has also indicated that it is beneficial if the cavities have a size relatively close to the size of the abrasive particles. Extremely large pore size is undesirable because the number of active abrasive particles at the interface of the grinding wheel and workpiece becomes extremely erratic which can have a direct result on performance and wheel wear. As pore size increases, uniform distribution throughout the grinding wheel also becomes extremely difficult to control (Figure #6).

As pore size decrease control of distribution increases and improved bond to pore relationships are possible. Unduly small pore sizes are also not highly desirable. If the pore size becomes too fine relative to the abrasive particle-size the wheels will not transport sufficient coolant or allow for ample chip clearance and grindability will decrease.

Pore size also has a direct bearing on the grinding wheels form holding capabilities. As pore size increases without regard for abrasive size it becomes increasingly difficult to maintain form. Although bond posts between individual abrasive particles strengthen, the large honeycomb matrix of the wheel oftentimes will leave the truing device trying to impart a form into air (Figure #7). This is especially observable on the edges of the wheels where large chunks of abrasive material may separate from the grinding wheel. Smaller cavities reduce the size of the honeycomb matrix and bond posts between individual abrasive particles becomes less random. This finer more uniformly distributed matrix means that the truing device will have an increased probability of imparting the form into the grinding wheel. The wheel will also accept more intricate forms and tend to maintain the form longer.

To summarize, although grinding wheels with very large pores and obviously open honeycomb structures are very impressive visually, they may not be the best choice for all grinding operations. Quality in a Creep Feed grinding wheel is:

• Proper size relationship between abrasive grains and wheel cavities - for chip clearance and coolant transportation.
• Controlled abrasive and pore distribution - for high dynamic balance.
• Greater bond strength - to accept the higher total forces with higher concentrations of porosity.
• Controlled bond degradation - at the interface of wheel and piece part.
• Reproducibility

References

1. Liv Z.C., "Characteristic Parameters of Pores in a Grinding Wheel", Journal of Huazhong University of Science & Technology, 1962.
2. Liv Z.C., "Research on the Pores of Grinding Wheels", Thesis for Master's Degree, Huazhong University of Science & Technology, 1981.
3. ZHU X.H.,, "On the Structure of Grinding Wheels in Creep Feed Grinding" University of Wisconsin, 1981.
4. Lewis,, Kenneth B., "The Grinding Wheel", The Grinding Wheel Institute, Cleveland, Ohio 1960.

Its speed over conventional reciprocating grinding increases directly with the depth of the form. More importantly, the process runs rings around such conventional straight-line machining operations as milling and broaching.

Published by Abrasive-Form, Inc., Roselle, Illinois

If your production involves milling, broaching and similar "linear" machining operations and/or grinding to demanding dimensional tolerances or finish requirements, odds are you can produce many of those parts faster, better and for less cost using a process called creep feed grinding (CFG). The process involves slowly feeding---Creep feeding---the workpiece past a form grinding wheel set to the full depth of cut. The desired detail (slot, shoulder, form, etc.) is produced in the workpiece in a single pass under the wheel.

The advantages of CFG are best understood by comparing it to conventional reciprocating grinding. In conventional grinding, the table reciprocates back and forth beneath a grinding wheel that infeeds at a rate of two or three "tenths" per pass. To grind a 0.100-inch-deep slot in mild steel at an in-feed of, say, 0.0002 inch would take 500 passes. A typical reciprocating grinder with a 12-inch stroke operating at 50 spm would require 10 minutes for the job, not counting interruptions for wheel dressing that might be necessary during the operation.

By contrast, with CFG the same 0.100-inch-deep slot can be accomplished in a single pass in a few seconds to a few minutes depending on the length of the cut. Machining speed is probably the greatest advantage of CFG. Additionally, parts are produced to very high accuracy, without burrs or distortion. And complex profiles can be ground into the workpiece as easily as simple ones. The advantages of CFG add up to a cost saving opportunity for most parts which require profiling operations.

As a rule of thumb, CFG can readily remove about 1 cc of metal (mild steel using an aluminum oxide wheel) per kilowatt per minute. One horsepower is roughly equivalent to 0.7 kilowatt, which means that a 10-horsepower CFG machine can remove about 7 cc of stock per minute.

Those of you who are accustomed to working in metrics have already figured out that 7 cc translates to a metal removal rate of about 1/2 cubic inch per minute for a 10-horsepower machine. How, then, can CFG compete favorably against conventional machining methods with their higher metal removal rates?

The answer is found in the sequence of operations typically required to machine a part from the solid. Consider the steps involved in machining a part on a horizontal milling machine. The operation begins with a "soft" (unheat treated) blank. Appropriate cutting tools must be assembled for the job; where an unusual profile is to be milled, custom cutters may be needed. The tools must be sharpened as they become worn, adding further to the cost of the operation.

Milling by its nature leaves burrs which must be removed in a secondary operation. Depending on the part and the nature of the cut, the deburring operation may be simple-or so difficult that its cost exceeds that of the initial machining. Either way, deburring involves another step and another handling.

After the "soft" workpiece has been machined and deburred, it must then be heat treated to the required strength and hardness. Frequently, the parts must be sent to a commercial heat-treater for processing, which not only adds to the cost of the part, but creates a built-in interruption of the production cycle.

The heat-treating process frequently creates distortions in the workpiece which must be corrected. Finally, while milled surfaces are acceptable for commercial tolerances, the workpiece may require a finish grind if close tolerances or fine finishes are specified. Profile surfaces may dictate the use of a form wheel, further increasing the cost of the part. The grinding operation will also give rise to alignment and holding problems.

Now, let's look at production of the same part by CFG. The process starts with a through-hardened blank. The grinding operation is typically performed in one setup on one machine. The part is free of burrs and distortion as it comes off the CF grinder and is ready for downstream operations or for use as is. The direct processing by CFG eliminates the costs of the secondary operations associated with milling, precludes the possibility of rejects due to errors in secondary operations, reduces in-process and handling costs, reduces labor and cutting toot costs, and makes possible faster delivery times on jobs.

There are two more very significant advantages of CFG. In the first place, the CFG part is metallurgically superior to its milled counterpart since it's free of machining stresses and heat-treat-related problems such as decarburization. Secondly, while some materials can only be machined with great difficulty if at all by milling, any material can be machined efficiently by CFG given the correct wheel type, coolant, etc.

CFG is not a black art. It is a straightforward, thoroughly predictable machining process. However, successful results cannot be achieved with inadequate or insufficient equipment or a less-than-complete commitment to making the process work. Buying a machine is not enough; CFG must be implemented from a systems standpoint. The plant or shop that tries to acquire CFG capability by upgrading a 10- or 15-year-old machine for the purpose is almost certain to fail. So, too, will the firm which expects simply to buy a machine, stick it in a corner, hook it up and do creep feed grinding.

Proper care must be given to the selection of the machine, type of wheel for the operation, type of coolant, coolant flow and capacity, coolant filtration, wheel dressing, and every other factor which affects the process. Like the proverbial chain, the CFG system will be only as effective as its weakest link.

A machine designed from the ground up for CFG should be specified since high levels of rigidity, accuracy and power are crucial. Unlike reciprocating grinding where stock removal and power consumption per pass are low, CFG cuts to full depth with a high arc of contact. CFG machines routinely utilize 100 percent of available horsepower, and the machine must be rigid enough to handle such loading. Not just the iron, but bearings, spindle, motor, table drive ... everything must be rigid.

Grinding wheel selections --- a study in its own right --- is also critical to success. Improper selection --- for example, using a wheel that is too dense for the work material and type of cut --- can result in burnt and/or out-of-tolerance parts.

Selection of the type of wheel can also make or break the job. Aluminum oxide wheels are suitable for most materials, but CBN wheels are usually more efficient for tool steels and such difficult-to machine metals as Inconel and Rene, and diamond wheels are usually best for carbides and ceramics.

Proper wheel dressing is critical to maintaining the stock removal capabilities of the grinding wheel. Crush roll dressing produces the sharpest, most free-cutting wheel. When brought into contact with the wheel, the crush roll fractures the bond on the face of the wheel, leaving grains that are completely intact, full and sharp.

Proper crush roll dressing requires rigid conditions. The wider or harder the wheel, the greater the rigidity required. Ideally, the crush roll should be mounted on the grinding head above the wheel.

Diamond roll dressing produces a wheel which is almost as sharp as a crush dressed wheel if performed correctly. The diamond roll dresses faster, lasts longer, and can maintain incredibly close tolerances. It is the ultimate production tool where circumstances permit its use.

In diamond roll dressing, the wheel must be rotated at an appropriate speed relative to the roll. Accordingly, the CFG machine must have a variable speed (d-c drive) spindle capable of rotating the wheel at the required speed. This is important, since most grinding machines have a-c (constant speed) spindles and are thus unsuitable for diamond roll dressing.

Coolant is another important consideration. It must be delivered to the grinding zone in sufficient volume and at sufficient pressure to cleanse the grinding wheel and prevent it from becoming loaded, to provide the lubricity needed to promote chip formation, and to keep the wheel and workpiece cool by drawing heat away. Effective coolant filtration is also needed to remove suspended particles. This is particularly important where part finish is critical.

These comments are not intended to discourage potential users of CFG. But the would-be user must be made aware that no component of the system can be ignored if the operation is to succeed. In CFG, as in good pizza making, there's no skimping on the family recipe.

Applications for CFG

In grinding operations involving modest metal removal (to depths of .010 inch), grinding times for creep feed grinding versus reciprocating grinding are roughly equivalent. As the form gets deeper however (0.020 to 0.030 inch), CFG begins to demonstrate a clear advantage over reciprocating grinding in terms of speed. The advantage increases directly with the depth of the form; CFG can be as much as 100 times faster for grinding a 1/4-inch deep slot.

Perhaps the greatest potential for CFG is as an alternative to horizontal milling operations. While the process is "limited" to straight-line machining, the requirements for this type of operation in industry are staggering. Imaginative fixturing and multipart setups further increase the efficiency of the process.

• Increased productivity and quality of parts requiring slots and/or profiles.
  
• Eliminates costly first operations such as milling, broaching or turning. The form or slot can be creep-feed ground to full depth from the solid.
  
• Fully hardened parts can be creep-feed ground from the solid--thus, in many cases eliminating the need for straightening operations required prior to conventional reciprocation grinding after milling and heat treating.
  
• Reduced handling time and cost. For example, a conventional machining process of a part may require:
• Milling
• Deburring
• Heat Treat
• Straightening
• Reciprocating Grinding.

Whereas with creep-feed, the milling, deburring, straightening, and extra handling operations can be eliminated.

• Reduced set-up time and costs due to combined operations.
  
• Reduced tooling costs (i.e., formed milling cutters, broaching tools, etc. as compared to grinding wheels).
  
• More parts processed per wheel life due to less wheel breakdown when compared to reciprocation grinding.
  
• Reduced wear on dressing tools due to the use of softer wheels.
  
• Wheel form maintained longer--thus, requiring less redressing and resulting in increased productivity.
  
• Reduced grinding wheel cost due to less wear and fewer number of dressing operations required.
  
• Improved tolerances and surface integrity as opposed to milling, broaching or turning.

Abrasive-Form believes many companies fail to consider the complete picture

by Dave Bode

Outsource creep-feed grinding process used in high-volume turbine part production, among other things -or keep it in-house? When contemplating outsourced manufacturing versus in-house capability, many companies fail to consider the complete picture as they try to determine whether outsourcing their creep-feed grinding requirements is preferable to keeping the operation in-house In order to make a more informed analysis and decision, a number of factors must be examined, including cost, delivery, quality, flexibility and risk reduction.

Explained John Harig, president of Abrasive-Form, Inc, one of North America's largest creep-feed grinding outsources, "In the current economic environment, we see a growing interest among top level manufacturing executives and managers to revisit existing processes and look for new ways to trim production costs. But often, however, we find that many of the real costs of in-house creepfeed grinding are overlooked, such as the overhead associated with direct labor costs, costs of purchasing and maintaining inventories of consumables, training costs, and management of ancillary operations like deburring or heat treating, among others.

"Many of the most profitable manufacturing operations outsource creep feed grinding so they can better concentrate on their core competencies. In fact, what we do best is take a project from the drawing to full implementation. Abrasive-Form has the most extensive track record with creep-feed grinding in North America and has been a partner to such companies for more than a quarter of a century."

Headquartered in Bloomingdale, Illinois, U.S.A., Abrasive-Form, Inc. (AF) was founded in 1976 by CEO Ken Kummer to specialize in close tolerance linear form grinding. Back then, linear form grinding, as well as creep-feed (CF) grinding, were relatively new processes in the U.S., Kummer noted little was known about their true economic value to manufacturing and the ability to eliminate processes by grinding forms from the heat-treated solid, as well as CF grinding's use in difficult-to-machine aerospace materials.

In the 1980s, the company converted itself into a production shop and purchased its first CNC creep-feed grinder, which enabled it to pursue a new tier of repetitive-run production parts. Also in the 1980s, AF began machining turbine components, which ultimately led to its recognition today as a leader in high volume turbine part production.

In the 1990s, Abrasive-Form dramatically increased its capacity through the addition of machines, capable machine operators and support staff. In 2000, the company merged with Roselle Tool, Inc., giving it an expanded capability to engineer and rapidly fabricate tooling for new projects Also in 2000, AF moved into a modem 5760 m' facility to accommodate its growth and currently has 38 machines and some 80 employees serving customers worldwide in industries as diverse as gas turbines, diesel engines, hydraulic pumps, hand tools, medical equipment and clipper blades.

Creep-feed grinding is a highly accurate, efficient method of machining intricate forms and slots into a wide variety of materials. In contrast to conventional grinding, where the grinding wheel is gradually lowered as it reciprocates over the workpiece, the CF grinding machine plunges a formed grinding wheel deep into a workpiece and then economically moves the piece under the wheel, yielding a finished part in only one pass. The grinding wheel's form is
automatically maintained by frequent dressing.

Further, unlike milling where multiple operations might require multiple setups with different tooling, AF can design innovative fixtures to process multiple operations or even multiple parts simultaneously, rather than be constrained to sequential operations or batch processing. This, of course, makes the operation more efficient and shortens cycle times.

Surface finishes achieved with CF grinding are normally much better than those obtained with conventional grinding, and the process typically does not leave troublesome burrs, Harig noted. Indeed, the process is described as a manufacturing technique combining rapid stock removal rates, complex part geometry and superior surface finish.
Other CF grinding benefits include: improved part quality due to the CF process' ability to hold extremely tight tolerances; elimination of part distortion during heat treating because parts are ground after hardening; improved fatigue resistance because the process interjects such mild residual compressive stresses; shorter lead times and lower inventories due to fewer manufacturing steps and reduced need for work in process and finished goods inventory; and applicability to almost any material including all steels, alloys carbide and ceramics.

Tremendously high forces are generated during CF grinding, requiring machines especially designed for rigidity and power. "The negative point about creepfeed grinding .if it can be considered a negative point .is that the equipment to do it economically is very expensive," explained AF Product Manager Neil Fehr. "The average machine with adequate support features costs around US$500,000, and machines can cost as much as US $l million. The machines are also costly to maintain and are best run by experienced specialists. There are few, if any, user-friendly books on the process, which has dozens of variable parameters happening all at once."

Most of Abrasive-Form's CF grinders, which range from 4 kW to 78 kW, are Magerle and Blohm CNC units with Siemens, GE Fanuc or Allen Bradley controls. Also included are Edgetek, Jones & Shipman and Bridgeport Harig machines.

Currently, according to Fehr, AF has had its greatest successes in industries like turbine component manufacturing and aerospace where parts contain high nickel alloys .because tools to mill or broach them are very expensive and cannot consistently hold CF grinding's tight tolerances of as small as two microns.

As a leading subcontractor to the gas turbine industry, AF which is ISO 9002/AS9000 certified said that since 1988 it has machined over 500,000 turbine blades, buckets, vanes, nozzles and shrouds. Customers include GE Power Systems, Rolls-Royce Energy Systems, Siemens Westinghouse and GE Aircraft Engines.

AF offers a wide range of grinding related services to suit its customers' requirements.  It can, for example, simply grind a customer's blanks or supply him with completed parts. The company, which is well-known for its in-house tooling design and production, can also design and build tooling to meet the customer's specific manufacturing objectives.

"In contrast to our customers, Creepfeed grinding is Abrasive-Form's core competency," said Kummer. "From the cost standpoint, we can often bring our years of experience, our highly capable people and our up-to-date machines to bear on our customers' precision linear form grinding requirements more cost effectively than they could do it themselves. '"

By John Webster,
Grinding Research Center,
University of Connecticut

While most people agree that grinding fluids are important for precision grinding, few give serious thought to the nozzle, hoses, and connectors that deliver the fluid to the grinding zone. However, the importance of nozzles and other parts of this delivery system on grinding wheel performance was the subject of a recent program at the Grinding Center.

There is not much research specifically on nozzles for grinding applications so background information came from the research in fire hose technology. Research by others showed that shape of a nozzle has great effect on the stream of fluid as it leaves the nozzle, and small eddy currents and secondary flows within a hose can slow the flow. Transitions at connectors create destructive flow patterns that persist through even long portions of hoses or pipes. These effects of secondary flows can be diminished with small honey-combed flow conditioner inserted in the hose.

But are these findings for fire hoses applicable to grinding applications and grinding fluids? Past work at the Center for Grinding Research show that fluid delivery is important, and the volume of fluid needs to exceed a certain amount before it benefits grinding. So the Research Center conducted a number of studies on the effects of nozzle design that included measurements of the influences of fluid flow rates, wheel speed, and wheel structure, as well as the effect of fluid delivery on grinding efficiency.

Research results

Test results show that secondary flows can be a serious problem. The lengths of pipe within a typical system are far too short to attenuate the effect of elbows and transitions along the fluid path. The cooling effects of the grinding fluids can be changed significantly by good nozzle design and optimum wheel characteristics. Nozzles designed with a convex internal surface and smooth surface finish increase the available flow rate to the process. The shape and size of the aperture (the opening in the nozzle for fluids) affects the shape of the jet stream. The greater the discharge distance for a nozzle, the more difficult it is to aim the jet.

Research conclusions are:

1. The proper delivery of coolant is affected by the nozzle design. In particular nozzle geometry, surface finish, nozzle fillets, break-edges and size of opening are very important.
   
2. Matching the speed of the fluid to wheel peripheral speed is best.
   
3. Burn can be reduced when fluids exceed a critical speed in the grinding zone and sufficient flow is available.
   
4. The coolant delivery systems should be designed with attention to changes in the shape and size of hoses, pipes, and connectors. Important are nozzles, nozzle positions, flow rate settings, pump capacity, flow conditioners, large diameter straight pipes, and flexible hoses. Increasing nozzle distance tends to reduce the cooling performance of the grinding fluid especially with nozzle designs that produce low coherence jet streams. A single or a cluster of round nozzles can replace expensive profiled nozzles.
   
5. In creepfeed grinding applications, a second nozzle is necessary to prevent burning on the back edge of the workpiece. The second nozzle should not reduce the flow of the primary nozzle when there is limited pump capacity.
   
6. With induced or high porosity wheels, water-based fluids give higher stock removal rates than straight oils. The opposite is true for wheels with dense structure.
   
7. Refrigeration of fluids will improve stock removal rates.

Abrasive-Form Inc, Roselle, IL, bills itself as the "partnershop" for a national customer base of more than 125 companies based on its ability to become "an extension of our major customers' plants," according to president, Ken Kummer.

The creep-feed (CF) grinding job shop has built its reputation on the value-added services it provides such as process engineering, fixture design, handling systems, grinding wheel development, and the high-precision capability of its creep-feed grinders. Included in the company's equipment inventory are about two dozen Maegerle and Blohm high-precision machines from United Grinding Technologies Inc, Miamisburg, OH.

Abrasive-Form employs all the disciplines of the complete manufacturing process to make it more than a "make me this part" job shop. Every project is started from scratch and relies on its own extensive library of information compiled from previous creep-feed grinding, fixturing, and parts handling solutions, as well as machine and operator capability.

"We take the part design and develop an optimal creep-feed process including dedicated fixturing and part handling, establish a correlated gaging operation, and set up CF equipment to run the job," Mr. Kummer explains. The company has even set up, totally debugged, and perfected CF grinding operations in customer plants while continuing to serve as a subcontractor.

Examples of Abrasive-Form's creep-feed grinding expertise abound, such as the tiny terminal clip for a radar computer board shown on the cover. The finished dimensions of these parts, which begin as a 12 ft long bar of aerospace alloy and are run in batches of 120,000 each, are 0.098" high and 0.062" wide.
For another "partnershop" customer, Abrasive-Form developed a single fixture that held a six-surface part and established six critical datum lines and points. Clamped on a Maegerle with a continuous wheel dresser for the surface forms, consecutive CF passes on each of the six surfaces yielded a completed part without any refixturing. Batch production was cut from a few months, using multiple setups for a single 100-part order, to a few days.

For another customer, Abrasive-Form developed a mini (12" long) grinding transfer fixture with eight stations. Beginning with a tool steel casting, parts are oriented and moved ahead one station after each pass of the grinding wheel across the eight stations. A finished part is delivered every two minutes. The grinding wheel has a special creep-feed porosity and graining size developed by Abrasive-Form with the wheel manufacturer for this particular job. Maegerle's rigidity and high precision are credited for grinding all eight surfaces in a single pass while holding a 32 RMS maximum surface finish with 0.005" maximum radii in the part's corners.

Based on its experience with solving customer problems over the last 20 years, Abrasive-Form has accumulated an extensive reference library for recognizing possible solutions and benchmarking performance. Established parameters are used as references in quoting jobs. They include material to be ground, wheel selection, handling and fixturing requirements, and critical specifications such as close tolerances and tight corners that must be held.

There's more to creep-feed grinding than the fantastically high metal removal rates and exceptional accuracies that can be achieved in a single pass, Mr. Kummer believes.

"They forget to focus on how to shorten cycle times and lower costs by presenting parts to the wheel more efficiently," he says. Unlike milling, where multiple operations could require multiple setups with different tooling, Abrasive-Form can often complete all operations simultaneously in one setup with creep-feed grinding.

"Presenting more parts to the wheel in less time makes the whole operation more efficient and is how the total cycle time can be considered optimized," Mr. Kummer explains.

In one instance, more than a half dozen machine tools were replaced by one Maegerle CF grinder with multiple fixtures and a fast station-to-station handling system. Queue time at machines was zeroed out. Scrap was reduced from 20% to less than 1%, and the customer eliminated a $20,000 a month expenditure on encapsulation.

Nearby, a CNC Blohm Profimat continuous-dress CF grinder is producing aerospace alloy turbine vanes in a shuttle-fixture operation that delivers a completed vane every 15 minutes. In the first fixture, the wheel grinds a nib, top step, wall, and bottom step on root ends. Two steps are ground in the second fixture. In both fixtures, "angel wings" are ground using the side of the wheel. Production is non-stop, with overall tolerances held as close as ±0.0002."

As "partnershop" relationships mature, customers work very closely with Abrasive-Form to plan machine time for their projects.

"Most partnershop customers schedule their work well in advance. They know when they can have their work done on their machine," says Mr. Kummer.

In a real sense, Abrasive-Form's machines, technicians, and engineers are "integrated" into the customers' production capabilities, says Mr. Kummer. "Dedicating our CF grinders, operators, and technicians to our customers' jobs is about as integrated as we can be without actually being on their floor," Mr. Kummer says. "This is why we call it partnershopping. We build a partnership relation with a customer to the point where they feel our shop is an extension of their own facility. As a result, they are very loyal. They stay-with us."

Creep-Feed grinding is no longer an alien concept to most people in the metalworking field, yet this advocate of the process believes that it is too often overlooked for applications where both overall productivity and quality would benefit substantially.

An Interview With John Besse
Creep-Feed Product Supervisor
Norton Company
Worcester, Massachusetts
Conducted By Mark Albert
Executive Editor

Let's start with the definition of creep-feed grinding.

Creep-feed grinding is actually a milling process, using the grinding wheel for the milling cutter. Traditionally in milling operations, you take during very slow "creep" feeds. In conventional surface grinding, the table reciprocates back and forth many times, like a pendulum, taking very, very small depths of cut per pass. In creep-feed, the table doesn't reciprocate. It slowly feeds across the workpiece to remove a very large amount of stock. Properly applied, this technique promises increased productivity and improved part quality.

Why is creep-feed grinding described as being "tailor made" for precision machining applications?

To answer this question, let's use a good example of a precision machining application: Turbine blades.

Creep-feed is a viable process that allows us to produce precision components in a highly productive manner without damage to the workpiece. These parts are made from superalloys that are very sensitive to heat and prone to thermal damage during grinding. Traditionally, we had to grind these parts very slowly and very carefully, which is very time consuming.

The parts are also very expensive---worth $500 to $1,500 apiece. Using a creep-feed process, we can produce finished parts with high dimensional accuracy and good part quality, in a much larger volume. An example would be a creep feed installation that does turbine blades. It grinds every part, every surface on the blade, without anyone touching it. It can produce a part every three to four minutes. That's an extreme improvement in productivity over the old ways.

How much of an improvement?

This cell cuts machining time by at least 15 to 20 minutes per part---or more. Using traditional methods, you would need several operations to finish several different surfaces; so the average production cycle time would probably be four to five times longer, not counting many more setups.

What are some advantages of creep-feed?

With difficult-to-grind alloys, burrs have historically been a problem and have required a separate operation for their removal. Therefore, the workpiece has to be handled again.

Creep-feed grinding creates very small chips which leave little or no burr; so the deburring operation can often be eliminated.

Creep-feed grinding allows you to consolidate operations---every time a step is added to a process, the exposure to possible errors increases---and to cut down on workpiece handling. These factors have the potential for a huge payback in terms of profitability and delivery.

In terms of quality, is the product comparable with those produced using traditional grinding methods?

It's arguably a better product. Because of the nature of the chip that is produced in creep-feed grinding, the process actually puts the workpiece in a residual compressive stress state, which improves surface integrity. The residual compressive stresses allow the part to run longer or be used longer without fatigue failure.

In general, creep-feed grinding also offers better control over part geometry.

What scale of investment is required to install a creep-feed operation? You can't add creep-feed wheels to your existing machines.

Creep-feed takes a major commitment to the process---a huge capital investment. A properly equipped creep-feed grinder may cost ten times more then a standard surface-grinding machine, but the technology allows you to utilize a cell approach that consolidates many operations into one machine. While creep-feed grinding may require a major capital investment up front, the point to remember is that it produces very expensive parts in a highly productive fashion. The payback is also very fast.

Creep-feed grinding calls for a change in outlook. The value of staying competitive in the long run has to become a priority. If managers are very short sighted, they can find a thousand reasons not to make this capital investment, but will soon be lagging behind the competition and struggling to survive.

Creep-feed is a sensitive operation. Everything has to be done right, starting with a willingness to make an adequate investment. It's not wise or cost effective to try to retrofit existing equipment to run creep-feed operations.

What are the pitfalls to be aware of?

Creep-feed grinding is an unforgiving operation. That's why it's not a good idea to retrofit---in the long haul, you're always going to be fighting it. In creep-feed, we're dealing with very powerful machines, machines rated at a much higher horsepower then conventional machines, because the process by nature removes a great deal of material quickly. A typical conventional grinder may be rated at 3 to 20 hp, but a grinder designed for creep-feed may be rated 25-200 hp.

You have to pay attention to coolant application, fixturing, dressing parameters, and grinding wheel specifications. These must all be right for the creep-feed process to work properly. If any one element is not properly taken care of, the system fails.

Naturally, big machines featuring high horsepower, high accuracy, aggressive coolant application system, and so on---require much more maintenance then standard, simple machines. Of course they are also much more expensive.

How would you characterize the growth of creep-feed grinding in the United States?

It's still relatively new, but because it is a new technology, its growth has been extremely fast---faster than a two-to-one ratio over the last five years. It's going to grow at two-to-one over the next five years as acceptance starts to level off, but that's still a remarkable growth rate. Five years ago there were maybe two or three major installations. Today, there are probably 25 to 30 major installations---either cells or high-volume creep-feed production shops---although there are hundreds of smaller installations in the field. About 75 percent of all these are in the aerospace market.

But, there's still some reluctance to accept creep-feed grinding.

I attribute this reluctance to the natural tendency to resist change. Successful creep-feed grinding calls for a radical change, and rakes a major commitment.

Plus, creep-feed grinding has been held back by a wait-and-see attitude. This attitude may offer security for the moment, but it can cost a shop its technical edge over time. There has been some reluctance on the part of some engineers who said creep-feed would never work. Now, it has definitely cost them in terms of market share, and some of them will have to revisit creep-feed when they see what their competitors are doing.

Should job shops take a look at creep-feed for their operations?

Although these machines are generally high-production units designed, built and set up to run thousands of parts, some job shops are using creep-feed grinding for 500- or 250-piece runs. In these situations, they have found creep-feed grinding to be very cost-effective for them, if not in terms of productivity, then in terms of improved part quality, yield and fewer rejections.

Or, in some cases, the nature of the operation may be so aggressive or so severe that the job can only be done profitably with creep-feed grinding. The creep-feed process could make sense for grinding just one part, if it allows that part to be produced in an hour instead of a week.

What advice can you share with shops and plants considering an investment in creep-feed grinding?

The most important thing that any prospective user must understand is how important every element of the process is and how different those elements may be from what they're used to. They're looking at buying an extremely rigid machine with high horsepower---much higher horsepower then in traditional grinding machines. They're looking at machines with very positive drives, usually ballscrews. And they must realize that rigidity is so important, tight down to designing the fixtures that are very rigid. This is important because the machine is only as rigid as its weakest link---in a lot of cases, that's the fixture.

Remember we're grinding with much higher forces to make that deep cut, and the actual horsepower needed to drive that wheel is extremely high. In total, the normal and tangential forces on the part in the fixture are very high and change depending on the application. They can be anywhere from 50 to 500 lbs per inch of wheel contact thickness. The wheel contact areas can produce up to 2,000 lbs of total force. So it's very important to make sure that fixtures are extremely rigid---at least as rigid as the rest of the machine. If they are not, the user will not be taking advantage of this machine and the benefits that make this process worthwhile.

Another example of how important every element in the process is, is the kind of grinding wheel selected for each application. The key here is a quality wheel with adequate, uniform porosity. One way you can test this is by blowing smoke through the wheel in order to see if the porosity is proper---if is like a filter, then you know that its porosity is what you need.

What do you look for in a creep-feed grinding wheel?

Consistent, uniform porosity is the chief requirement for a creep-feed grinding wheel. More then half of any creep-feed wheel is air. Creep-feed wheels are induced-pore products, manufactured to create many interconnected air holes throughout the wheel. The air holes conduct coolant into the cut and facilitate the flow of swarf out. Remember this wheel is taking deep cuts, so the wheel is essentially buried in the part. There is no place for the coolant or the swarf to go but to be carried into the wheel before it exits the cut. Therefore, I consider high-quality, induced-pore grinding wheel products an essential.

The quality factor is important because the pore distribution, uniform density and balance of the wheel are all extremely critical. All of these attributes are related. If the pores aren't distributed properly, for example, it affects density and balance.

To give you an idea of how important these wheel quality considerations are, I'd like to point out that balance and grade limits are cut in half for Norton Company's creep-feed products compared to standard wheels. There's simply a lot of demand on the wheel in the creep-feed process.

We also recognize that offering the technical expertise of experienced engineers is important since any shop or plant applying creep-feed will likely have questions and concerns that are vital to the success of their particular application.

What is next in creep-feed technology?

Superabrasives and ceramic abrasives are emerging. They're out there now, but there's a limited use for them. Here applications will grow slowly as the technology we use to make these products improves---and as the alloys get more and more difficult to grind.

While alloys are always being changed to make them lighter or improve their performance under higher heat, they're also becoming much harder to grind and machine. Naturally, the tools to grind these products have to keep improving, and we may find 20 years from now that a good portion of creep-feed grinding involves superabrasives or ceramic abrasives.

Superabrasives---such as cubic boron nitride (CBN)---excel where it's critical to hold very tight tolerances on intricate forms made of difficult-to-grind or hardened alloy materials. An example would be the grinding of small corner radii. Ceramic abrasives are a new class of products that promise to yield higher productivity, longer wheel life, and improved part quality compared to conventional aluminum oxide creep-feed wheels. The Norton SG seed-gel ceramic aluminum oxide products are in this class, for example.

Any final advice to those currently involved in---or thinking of getting involved in---creep-feed grinding?

There's no mystery to creep-feed grinding. It's not black magic. The science of creep-feed grinding is accessible and practical. Do your homework Attend technical seminars. Go out of you way to obtain information about this process. There are many technical articles and papers that have been published on creep-feed grinding. There is also a fair number of competent and reputable technical experts who offer consulting services, including ours at Norton Company.

Try first to have an understanding of what the process is, then you can look at applying it to some of your current processes---bottle neck operations, quality problems and labor intense operations.

But get smart first. The more you pay attention to the details of creep-feed grinding, the grater the edge you'll have over your competitor.

Creep-feed grinding (CFG) is undoubtedly the fastest growing abrasive-technology process in the US, along with the use of superabrasives. To machine materials of the future---ceramics, cermets, monocrystal ceramics, whisker-reinforced metals, nonmetals, etc---grinding will be the only process available. CFG will be the only economical solution for ceramics. Conventional milling, broaching, planing, and turning---even in their most up-to-date forms---will not be able to cut tomorrow's materials. Grinding will be the only way for cutting tool technology to catch up to material science.

Unfortunately, the US machine tool industry has not kept abreast of this evolving technology and has lost market share to off-shore competition. US industry, until recently, has depended on foreign machine tool builders to produce creep-feed grinding machines and modern grinding systems. US builders have had to play catch-up, with few companies fully embracing the new technology, because of the significant investment commitment required---both financially and technically in the machine-tool designs suitable for creep-feed and superabrasive grinding.

CFG is a high-precision, high stock-removal abrasive process. Even the most difficult-to-machine materials can be machined relatively burr-free with excellent surface integrity. Metallics are being machined at rates much faster than milling and in the hardened state. The savings are not just the result of fast stock removal. Add to this the elimination of costly deburring operations, straightening after heat treatment, inventorying of raw material and consumable tooling, risk of thermal or metallurgical damage to part surfaces, or the need for expensive near-net-shape technologies. This is why CFG is the choice of the aerospace industry.

New machine requirements

CFG has been in use for almost 30 years. The 1990's will be an opportune time to reassess the CFG process and develop a completely new concept in machine-tool design, a new generation addressing these present and future industry needs:

Higher speeds: Industry recognizes the need for higher peripheral wheel speeds, particularly with superabrasives. This expertise lies predominately in Europe, and US safety standards for use of high wheel speeds lag those in Europe. Major high-speed advantages are being lost here due to inadequate machine designs and lack of initiative to improve wheel-safety standards.

Higher precision: Precision is directly affected by machine-tool design. Although CNCs and pseudo-adaptive controls allow poor machine designs to perform somewhat satisfactorily, the only path to an advanced, more precise machine is through its basic design.
A significant contributor is the epoxy/concrete machine base, such as the Granitan patent held by Studer in Switzerland. US builders are relying heavily on the Swiss for machine-base technology, as well as its fabrication and manufacture.

Wheel technology: The latest abrasive technologies require superior machine tools from the standpoint of thermal and vibrational stability, as well as truing and dressing methods. Whether the raw grains are manufactured by GE or DeBeers, the majority of superabrasive wheels are made by foreign sources. The latest grinding wheel technologies are vitrified superabrasive wheels and high induced-porosity conventional wheels. Japan leads the way in superabrasive wheel technology, followed closely by the Europeans. Domestic wheels have improved dramatically in recent years with some wheel specifications equal to European. Although the dollar decline has made foreign products less attractive, European wheel vendors remain highly competitive.

Materials technology: Machining the latest materials-high-temperature alloys, ceramics, and nonmetals-requires machine tools with high stiffness and superior control and resolution. As this technology accelerates, it is leaving behind those grinding machines that have been on the shop floor for years and are now either technically or economically unable to machine these latest materials.

Continuous-dress capability: Continuous-dress creep-feed grinding (CDCF) is an additional need for US industry, beyond CNC creep-feed and CNC surface grinding. Other than companies such as Brown & Sharpe, Roberts, and Gallmeyer & Livingston in the US; CFG and CDCF machine-tool expertise lies in Europe with companies such as Elb, Maegerle, and Hauni-Blohm. The Japanese are closely following the creep-feed process with Niigata and Okamoto offering machines capable of CFG and ceramic grinding.

Controls and automation

Control systems need to be developed to perform complex multiaxis contouring of shapes on CFG machines. Very high resolution is required. Contouring has opened a new market for CFG, but needs the development of user-friendly controls. For CFG to machine a wide variety of materials and profiles, it will need machines designed for faster and easier setups and economical small-lot production. Today's just-in-time concepts are completely different from those for the CFG machines of even a short time ago.

Even in Europe, makers of CFG machines still rely on the traditional surf ace-grinder approach to their machine designs; i.e., a century-old concept of manual operation. A major disadvantage of CFG today is that cut time is such a small portion of floor-to-floor time.

Automation of part loading/unloading, and wheel and dresser changing is of paramount importance. A newer surface grinder concept is needed that is uncompromising in capitalizing on the potential of the creep-feed process. The move is away from dedicated automated grinding cells (ably suited for producing turbine blades in high volume) to more flexible systems that allow economical production of medium to small batches of a wide variety of workpiece shapes and materials.

Machineability research

Research in creep-feed machineability is presently being conducted in the US, and accelerated to encompass wider fields of materials, grinding wheels, dressing systems, and cutting fluids. With so little CFG expertise in the US, the process desperately needs a source of definitive machineability data, and users need confidence in what is very much a foreign process-in both senses of the word.

Process adaptive control is not presently a reality for CFG. Although machining to a predetermined algorithm is possible, true adaptive control is beyond the realm of present-day technologies.

Opportunity knocks

With these industry needs, there is enormous potential for the US machine-tool industry to build a new generation of CFG equipment. With little to be gained from equaling the competition, the effort should be to surpass it. A new approach would not only offer a competitive alternative, but boost export sales. This calls for a cooperation between user and machine builder, and input from independent sources to keep the design universal and not particular to one industry or application.

A concept I have long proposed for a whole new generation of grinding machines is based on bringing the part to the wheel, instead of the wheel to the part. This, after all, is how our ancestors sharpened their knives and tools: they held them against a stable, spinning wheel. They never attempted to do it the other way around!

My approach incorporates a wheelhead and dressing system more rigid and vibrationally stable than any existing production machine. Theoretical stiffness is in the order of 6 million lb/in. The principle of this patented design is a stationary, dual-supported grinding wheel. A special hydrostatic bearing allows the wheel and dresser to be changed easily, automatically, and accurately without sacrificing the mechanical stiffness of the system. The result is a single grinding machine capable of flat, form, contour,, cam, and OD cylindrical grinding. This concept of versatility, stiffness, and stability would take abrasive machining into the next generation.

By Dr Stuart C. Salmon
President
Advanced Mfg Science & Technology
Cincinnati, OH

President
Advanced Manufacturing Science and Technology
Cincinnati, OH

GRINDING IS ONE OPERATION WHERE today's technologies are impacting what used to be considered a finishing process and adding the flexibility to do more operations over a variety of processes.

In order to take advantage of what is becoming available in grinding technology and to justify its purchase, it is critical to know the actual cost of manufacturing a part. Competitive companies are no longer purchasing grinding machines to replace worn out grinders to perform the same operation. They are evaluating manufacturing methods and adopting new practices to achieve higher quality parts at lower piece part costs.

A creep-feed grinding operation, for example, could replace a number of milling and broaching operations. Apart from the improvement in workpiece quality, the abrasive machining process will off-set the cost of capital equipment, consumable tools, resharpening, inspection and inventory of cutters, fixture cost, tool changeover and part handling times, and post-process deburring/finishing operations.

New bond systems.
Superabrasives, particularly CBN (cubic boron nitride), appeared in the late 1960s. Great strides have been made since then to improve bond systems and techniques to eliminate or at least better control wheel trueing and dressing operations. Resin bond diamond and CBN wheels, which require a certain amount of finesse to prepare for grinding, are facing strong competition in the new vitrified and electroplated wheels.

Vitrified bond superabrasive wheels are similar to the vitrified bond conventional abrasive, aluminum oxide and silicon carbide wheels. Wheel preparation is simplified, in that traditional wheel trueing methods, like crush form dressing and single point diamond dressing, can be applied to true and form the wheel periphery. However, all vitrified superabrasive wheels are not so easy to use, as a certain amount of peripheral wheel conditioning (sticking with an aluminum oxide stick) may be required prior to grinding.

The vitrified bond superabrasive wheels available today provide better chip clearance and better cutting fluid application over resin and metal bond wheels. Vitrified superabrasive wheels are particularly suited to, and are finding the most application in, ID (internal diameter) grinding where the wheel wear and redressing time associated with conventional wheels drastically affects the productivity and precision of the process.

Resin and metal bond superabrasive wheels can have very strong grain retention properties, yet to achieve that strength, they are very closed in structure and therefore limit the efficiency of the process.

New abrasives.
Not all grinding will be with superabrasives in the future. Particularly when batch quantities are low and a variety of form profiles are required, economic justification can be made for the use of conventional grinding wheels. The advent of ceramic abrasives, like Cubitron by The 3M Company (St. Paul, MN) and SG (Seeded-Gel) Abrasive by Norton (Worcester, MA), has greatly enhanced the wear characteristic of the grains so that they stay sharp and cool cutting, extending the life of coated abrasive media (in the case of Cubitron), and extending the time between wheel dressings (as with Norton's SG abrasive).

Conventional aluminum oxide is fused from bauxite. After crushing in a ball mill, the result is a very random shape grain with a very brittle structure. This is in contrast with the very dense and hard structure of the SG abrasive. However, a balance has to be struck between the properties of the SG grain and the "fracturability" and ease of bonding of the conventional fused alumina. This is why SG wheels are being produced with a variety of percentage mix of both the SG and alumina grain.

An exciting aspect in the manufacturing process of the SG abrasive is that the shape and aspect ratio of the grains can be controlled. Perhaps we might one day see a grinding wheel made of grains which are the same size and shape as well as oriented in the wheel to perform at maximum efficiency.

Machining the materials of tomorrow.
Ceramic materials are the greatest challenge to the machining industry today. Grinding, using diamond grinding wheels, is the only economically viable method for the precision machining of ceramic materials in their sintered state. Ceramic materials are generally very hard and prone to edge chipping. The mechanism for machining ceramics is different from machining metals.

Ceramic materials are machined in a manner which resembles the chipping of ice from a driveway. The surface is pitted with craters the size of which depends on the size of the grain being used. The diamonds on the grinding wheel periphery act more like a series of hammer blows to the surface rather than as cutting edges causing chips to form.

Work that is being conducted on laboratory research machines in Japan has shown that, with very small infeeds and a very rigid machine, ceramic materials can be machined within their plastic regime. This yields excellent structural surfaces with high surface integrity.

Abrasive machine tool design.
High precision and the ability to produce consistently high quality workpieces in state-of-the-art materials is a technological challenge. To perform this economically is another challenge and is directly related to the employment of the latest technology.

Lawrence Livermore National Laboratories (Livermore, CA) is synonymous with ultra-precision machining. Its search for an ultra-precision grinding machine took it to Italy where it was able to commission the most accurate creep-feed grinding machine ever built. Cimat-Camut produced the Gamma 625 machine. Standing on a giant block of stone and in an environmentally controlled room, this machine surpassed the expected values of slideway straightness, squareness, positioning and stiffness expected by the engineers at Lawrence Livermore.

Flexibility and versatility have been built into the Ceratech T-25 grinding machine by Mazak (Florence, KY). This fully CNC machine has been specifically designed for machining ceramics. It has automatic wheel changing capability from a magazine of wheel heads, wheel position sensing and an unusual method for trueing metal bond diamond grinding wheels. The CNC control is used to turn the inverse shape of the grinding wheel profile in a graphite electrode, on the machine. That electrode is then used to EDM the profile into the metal bond diamond grinding wheel. This concept allows virtually any shape to be formed into the grinding wheel in as short a time as it takes to turn the electrode.

Equipment is changing.
Advanced Science and Technology has developed a machine that looks nothing like a grinding machine, but the advantages easily outweigh its peculiar appearance. Basically, it has a stationary wheelhead concept, but supported equally from both sides of the grinding wheel. The wheelhead does not move; it is isolated and de-coupled from the workpiece manipulation system so that minimal process vibration is transmitted through the structure, providing more stability for high-speed grinding operations. The dynamic stiffness of the system will approach 10,000,000 lbs/in. How the workpiece is presented to the grinding wheel is what makes the machine a plane, cylindrical, or contour grinder.

A new era.
Electroplated superabrasive grinding wheels are taking us into high-speed grinding. High speed refers to the peripheral wheel speed. Conventional wheel speeds range from about 2,000 to 7,000 sfm. High speed grinding is in the range of 20,000-50,000 sfm. At these speeds, stock removal capability increases, wheel life may be extended and part surface integrity can be improved. The electroplated grinding wheel is a metal hub with a single layer of superabrasive plated to an accurately machine rim. The wheel needs no dressing, only trueing on the machine spindle. As the electroplated wheel is basically a metal disc, it can withstand very high rotational speeds without bursting.

Competitive advantage.
These new abrasives technologies should be viewed as a significant element in an approach to strategic manufacturing. Advancing abrasives technology, plus grinding machine tool development and refinement, can be coupled to provide a serious competitive advantage in material removal operations. The enlightened grinding practitioner will keep a close eye on both machine tool and abrasives technology to turn their rapid advancement to individual and corporate advantage.

The use of creep-feed grinding has grown dramatically to meet the need for greater production efficiency. Of the three basic types, one or more may be just right for you.

By PAUL K. GIBREE
Product Engineer, Creep-Feed Grinding
Abrasives Marketing Group
Norton Company, Worcester, Mass.

Creep-feed grinding is an abrasive machining process mainly used to produce slots or intricate forms in difficult-to-grind materials such as prehardened tool steels and high-temperature nickel-base aerospace alloys. The process utilizes a grinding wheel to impart forms previously associated with milling or broaching operations into a workpiece in one pass, at full depth of cut and very slow table speeds. The use of creep-feed grinding has grown to meet the need for increased productivity and greater production efficiencies.

Creep-feed grinding has two main advantages over machining. First, it can work materials that are difficult and costly to shape by other methods. Second, it is easier to modify the form on a grinding wheel than it is on a broach or milling cutter, enabling rapid design alterations and changeovers.

The process also has several advantages over conventional reciprocating surface grinding:

More actual grinding time. Time lost with the wheel not in contact with the workpiece while reversing the table in conventional grinding, can exceed the actual time required to grind the part.

Less tendency to chatter. The increased depth of cut associated with creep-feed grinding produces a grater interface between the wheel and the workplace. This increased interface, combined with slower table speeds, has a tendency to stabilize any vibration generated during the grinding process.

Increased form-holding characteristics. The wheel enters the workpiece slowly and only once, generating complete form that equalizes the load over the entire wheel face. Entering slowly and only once eliminates the shearing of abrasive particles that occurs as the wheel repeatedly strikes the edge of the workpiece in conventional reciprocating grinding.

Less thermal damage. In conventional surface grinding, with higher depths of cut and increased spindle and table speeds, heat is generated (and transferred into the workpiece) in impulses. But in creep-feed grinding, the heat is a constant moderate influx distributed over a much grater area. The result is that a greater volume of the workpiece material is heated to lower average and maximum temperatures. Although the maximum temperature may reach a point high enough to cause thermal damage ahead of the grinding wheel, the disturbed material will be removed during, the grinding process.

Types of Creep-feed Grinding

Three basic types of creep-feed grinding are used in U.S. industry today: Pseudo creep feed, true creep feed, and continuous-dress creep feed. Each method is utilized for specific grinding applications.

Pseudo creep-feed grinding is used for workplaces with narrow cross sections. The piece is ground at full depth, but because of the narrow cross section the full length arc contact experienced in true creep-feed grinding is not generated.

The narrow cross section of the workplace allows the use of conventional grinding machines with hydraulic drives. Table speeds, although slow by conventional reciprocating standards, do not require the precise mechanical drives of true creep-feed grinders. Pseudo creep-feed grinding can provide greater productivity than conventional reciprocating grinding, but it cannot compete with true creep-feed grinding.

Wheels used for pseudo creep-feed grinding need not be as highly porous as true creep-feed wheels, because the coolant application and swarf removal requirements are not as demanding. In some cases, conventional wheels will perform acceptably, although very porous wheels will provide the best performance. Wheel speeds for pseudo creep-feed grinding are typically in the area of 6500 sfm.

True creep-feed grinding, utilizing machines especially designed for the process, offers high metal-removal rates with full-depth-of-cut, one-pass grinding. It offers great potential for increased productivity and accuracy. The workpiece can start out as hardened blank stock, be fixtured only once, and end up as a finished part. The process also offers improved dimensional stability and freedom from adverse thermal effects in the workpiece.

True creep-feed grinding maximizes the length of arc of contact between the wheel and the workpiece. For this reason it demands a specially designed grinding wheel and machine tool specifically built for creep-feed grinding. In pseudo creep-feed grinding, a surge in the table can cause the wheel to exit the part; in true creep-feed grinding, a table surge can actually cause the wheel to burst.

Because of the increased area of contact, wheels for true creep-feed should be softer than conventional wheels. In addition, high metal removal rates and increased demand to transport coolant into the grinding interface require as open a wheel structure as possible.

Rigidity is essential to creep-feed grinding machines. They must withstand increased forces resulting from crush forming the grinding wheel for close-tolerance, form-grinding repeatability. Wheel speeds should be variable, while table speeds should be mechanically driven to ensure stick-free, slip-free operation.

In continuous-dress grinding, the wheel is sharpened and profiled while actively grinding the workpiece rather than between grinding cycles. This type of grinding can provide greater metal removal rates than those of true creep feed. More important, continuous-dress grinding increases form-holding and dimensional stability.

Continuous-dress grinding requires specially designed machines. They must have all the attributes of true creep-feed grinding machines and also be equipped with compensating-speed wheel spindles. These are necessary to automatically increase the speed of the wheel as its diameter decreases during operation. The compensating spindles ensure that the grinding wheel operates at a constant surface speed.

The rate at which the dressing device is fed into the wheel and the rate at which the wheel is fed into the workplace must also be perfectly synchronized to compensate for wheel wear, otherwise it will be impossible to grind the workpiece parallel.

The dressing operation resharpens dull abrasive grains or releases them from the bond system. Selection of the type of diamond roll dressing device to use----hand-set or reverse-platted---depends upon the desired form, grit size, and wheel grade. Although diamond roll dressing will not produce as aggressive a wheel as will crush-truing, continuous dressing will maintain the wheel at a constant percentage of its full potential. This produces steadier and lower average grinding forces, resulting in more efficient use of abrasive materials and shorter cycle times.

In review, creep-feed grinding can provide significant productivity improvements without requiring investment in specialized machinery. The process can be implemented on conventional machines for workpieces that have narrow cross sections.

True creep-feed grinding requires specially designed machinery, but can provide high metal-removal rates while producing a workpiece of better quality. The process is especially beneficial in applications where close tolerances and repeatability are important.

Continuous-dress creep-feed grinding offers the highest metal removal rates and the best form-holding and dimensional stability. The grinding system must be carefully controlled, however, to ensure successful operation.

All of these grinding techniques require consideration of the total grinding system. Factors that can affect system performance and productivity include workplace fixturing; wheel types and speeds; infeed rates; coolant placement, volume and pressure; truing and dressing systems;and, most important, the grinding machine itself. Careful coordination of these elements into a total creep-feed grinding system can yield substantial quality and productivity benefits for manufacturers of difficult-to-grind components.

By producing groove-type shapes and contours to 1/2 in. deep or more in a single pass, machine cycle time is cut and manufacturing costs are reduced

Creep-feed grinding, a process of grinding finish shapes in solid soft or hard workpieces in one pass, is commonly used in Europe but relatively new to the U.S. industry. Benefits from the process include up to 50 percent reduction of cycle times, excellent repeatability, up to 300 percent more wheel life and improved surface finish compared to conventional reciprocating grinding operations. Additionally, the process does not induce subsurface stresses in workpieces.

The major benefits from creep-feed grinding are gained when shapes or contours a minimum of .015 in. deep are required; the deeper and more sophisticated the shape, the larger the benefits.

The Process. In principle, creep-feed grinding is similar to climb milling. The machine table feeds in the direction of the grinding wheel rotation; this motion tends to exert downward pressure on the workpiece. The grinding wheel is set to full depth and the table feeds at a precisely controlled rate ranging from 2 to 60 ipm. A particular operation's feed rate is dependent on a part's material, hardness, corner radius required, type of grinding wheel and depth of cut; tougher materials require a slower feed rate.

The Machine. A heavy-duty, rugged machine is necessary for production creep-feed grinding operations. The table feed must be variable, precisely controlled and without backlash. The machine should feature a crush or diamond-roll dresser and provide a minimum of 50 gpm of coolant at 30 psi to the grinding wheel-part contact area. The machine should provide full hp without vibration and a dc spindle drive is essential to provide constant peripheral surface speed.

Typically, creep-feed grinding made its U.S. debut in contract shops. Abrasive-Form Inc., Roselle, IL, pioneered the process in the U.S. in 1970. In addition to several other machines, the shop operates 11 large creep-feed grinding machines on a two-shift basis and produces millions of parts annually for the automotive, business machine, aircraft, electronic and medical industries.

John Stevenson, Abrasive-Form's vice president, reports: "Eleven years of production experience has revealed that the process offers substantial manufacturing benefits for roughing or finishing an unlimited variety of simple or sophisticated contours. Presently, the process is an efficient method that competes favorably with milling and broaching operations without some of their inherent drawbacks.

"Creep-feed grinding can produce almost any shape from the solid, in hardened or soft workpieces, in a single pass, in minutes. In comparison, conventional milling or broaching is used to rough-machine a shape in a part, which then is deburred as required, heat treated and finish ground on a reciprocating machine. Therefore creep-feed grinding not only eliminates rough machining operations, but also contributes to minimal material handling and in-process inventory. It also avoids the manufacturing problems associated with warpage from heat treating operations.

"For example, creep-feed grinding can produce a .300 in. deep form in a 6-in. long part within a three-minute maximum cycle. In comparison, a reciprocating grinder, using a .002-in. downfeed per pass in each direction, would require an about 20-minute cycle.

"Innovative tooling and machine setups further contribute to maximum productivity from creep-feed grinding operations. For instance, dual shuttle fixtures or multiple part holding fixtures are used to achieve maximum machine utilization. Wide grinding wheels, capable of grinding parts in a side-by-side arrangement, also are utilized to provide maximum machine uptime.

"However, the Swiss-made Magerle grinding machines are the real key to our successful creep-feed grinding operations. The extremely rugged machines, specifically built for creep-feed grinding, feature automatic sequencing of crush dressing cycles and size control which ensures consistent production of parts within a.0002-in. depth. The dc spindle drive also provides constant surface speed which contributes to long wheel life and high quality surface finishes.

"Usually, open structure aluminum oxide grinding wheels are used for creep-feed grinding operations. These porous grinding wheels allow room for metal chips and carry coolant to the wheel/ work contact area, and the wheel life ranges up to 300 percent longer than comparative conventional grinding operations. The climb grinding process tends to tear out dull, used abrasive grains and expose new sharp grains which contributes to a free-cutting action and improved surface finishes. The combination of high volume coolant and constant contact pressure with the part also contributes to extended wheel life," Stevenson adds.

Production Operations. The majority of production parts creep-feed ground at Abrasive-Form are about 3 in. long; however, parts up to 47 in. long have been produced. Generally, production parts are ground in batch lots and most of the business is repeat orders.

Actual examples of the benefits provided by creep-feed grinding include productivity improvements, reduced manufacturing costs and many more. In one particular case, overall productivity increased 40 percent when creep-feed grinding replaced milling a slot in a steel investment casting. The process change also contributed to a substantial quality improvement and eliminated a burr problem which had plagued the milling operation.

The operation involves creep-feed grinding an about 5/32-in. deep x 1/2-in. wide form with two vertical ribs in the 4-in. long cast parts. The fixture holds two pairs of parts in a side-by-side arrangement which produces four finished parts per machining cycle. Last year, 40,000 of these parts were produced for the customer.

Another example of creep-feed grinding reveals the economic advantage of the process in combination with a special 2-in. wide crush-dressable diamond grinding wheel. The German-made grinding wheel has .400-in. depth of crush-dressable diamond abrasive material and has been in use for several months.

The operation involves creep-feed grinding 50 V-grooves per inch, in two directions, to provide sharp pointed teeth similar to a double-cut file, in 1/2-in. wide x 4-in. long carbide strips. The grinding setup utilizes two vacuum chucks which each locate six carbide strips at opposing 45-degree angles from the table travel.

For a tooth creep-feed operation, six carbide strips are loaded on each vacuum chuck and the operator pushbutton-actuates the grinding cycle. The machine automatically completes one grinding pass, the table returns to its start location and indexes Outward to position the grinding wheel for cutting another group of V-grooves. The cycle then repeats until the teeth have been ground for the entire length of the carbide strips. The operator then changes the strips between chucks and repeats the cycle.

Another example of production creep-feed grinding involves the familiar "Christmas tree" or root section, holding a .0005-in. tolerance on turbine engine blades. Although productivity comparisons are considered classified the customer reports a significant improvement in quality and repeatability.

The creep-feed grinding operation produces two finish ground turbine blades per cycle. A pair of inconel blades loaded horizontally into a hydraulic chuck is located and clamped on gage points. Pushbutton-actuated, the creep-feed grinding cycle finishes one side of the blades' root section and the table returns to its start position. The operator manually indexes the chuck 180 degrees and repeats the machine cycle.

Regarding the future for creep-feed grinding, Stevenson says: "Because of the benefits it provides, creep-feed grinding will soon make significant inroads into the domestic manufacturing industry. In our experience, it is almost always the most efficient and economical process to produce grinding contours deeper than .015-in. on a production basis.

Vearl A. Williams