The Powerful Potential of Peel Grinding
A grinding wheel "lathe"
By Robert Aronson
Why consider peel grinding? Among the reasons: high precision; ability to make complex, small features; works well on hard materials conventional machining can't handle; and the falling cost of CBN.
Peel grinding is one of those techniques that has been around for some time--usually applied to rather specialized tasks. But, changing industry needs have recently given it more prominence.
Basically, peel grinding uses a narrow grinding wheel, usually 0.25" (6.4 mm) or less in width, to grind a part. Because the process is much like cutting with a conventional lathe, it is a major competitor to hard turning.
The advantages of peel grinding include:
- Dramatically reduced cycle time.
- Combined operations.
- Decreased throughput time. Only software needs to be changed to accommodate different parts, as opposed to tooling changes.
- Ability to grind dissimilar metals on one platform.
- Complex shapes can be formed without a profile wheel. The thin wheel functions like a single-point tool under CNC control. Therefore it's simple to change profiles without changing wheels.
Grinding-wheel manufacturers also see a positive side to peel grinding. Mike Hitchiner, Vitrified Product Manager St. Gobain (Romulus, MI) notes, "Since China has become a major supplier of CBN grit, the prices have dropped for this material," notes Hitchiner. "Therefore, wheel prices are lower, and this encourages more use of peel grinding.
"In addition, more economical wheel bond technologies have made it practical to use peel grinding. For our wheels, the bonding material is mainly vitreous but with some use of metal. Typically the wheels are 350-450-mm diam with widths of 10-15 mm. The new bond technology also helps make peel grinding cost effective.
"At the same time, US manufacturers are being influenced by international companies that use the process with oil lubricants. Lubrication is a big factor. In the US, most manufacturers were going to water-based fluids because of environmental issues. But Europe stayed with oil, which can improve wheel life from three to 20 times. Now the US is slowly migrating back to oil even though with oil you have to have mist-extraction devices."
Automotive is one of the major growth industries for peel grinding because of the growing interest in transmission manufacturing. Precision requirements are greater both because of increased complexity of six-speed transmissions and the need for greater efficiency and quieter running. There is particular emphasis on gear grinding.
"Finish with peel grinding is in the 0.24-0.4 Ra range, and because of the cutting speed this process can be quite a bit faster than conventional grinding," says Hitchiner.
The physics of peel grinding also brings advantages. "You concentrate all the grinding force in one small area with peel grinding," says Hitchiner. "This tends to drop the forces on the part as compared to straight plunge grinding. Therefore, you don't use the same horsepower and part changeover is easier.
"With force concentrated like that, you can cut a lot of material with a small wheel. The actual power required per unit volume of metal removed goes down by a factor of two or three. So you can remove the same amount of metal with a 10-hp [7.5-kW] spindle in peel grinding as you can with a 20 or 30 hp [15 or 22.5-kW] spindle in plunge grinding operations with a much wider wheel. The larger wheel needs more power to turn and has more coolant drag.
"In selecting a machine for peel grinding, you want overall high stiffness and a good bearing system. Wheel speed should be in the 90-140 m/sec range with rpm in the 1000 to 10,000 range," he concludes.
"Two features have changed in peel-grinder performance in the last few years: increased surface speed, to accommodate greater production speed, and shorter dressing intervals, because of improved wheels," explains Denis Fritz, vice president and COO, Erwin Junker Machinery Inc. (Elgin, IL).
"But the amount of improvement can't be linked to a specific number. Any production estimate is product-specific. But we have seen cases of 75,000 to 100,000 parts per wheel.
"We have machines with a B axis and up to three spindles. This provides a lot of versatility. So a single machine can, for example, provide OD peel grinding, a snap-ring slot, and special grooves. We recently introduced a machine with two independent wheelheads that enables the user to grind two crankshaft pins at the same time.
"Another advantage is that we can grind lead free. (Lead free means that there is no directional pattern left by the wheel, this is important for seal diameters, as lead would carry oil out through the seal).
"It's important to keep the grit as sharp as possible, but dressing frequency is not commonly done on a fixed interval. It's usually necessary to evaluate a part's surface. When grit gets dull, it rubs rather than cuts, and this is obvious from a look at the part.
"One of our clients, a drill manufacturer, formerly made his products in a five-step operation that included profiling, tapering, cutoff, centerless, and chamfering. We were able to do the same job in one grinding machine, using two wheels with a single-setup operation.
"In another case, a client was responsible for making 250 different transmission shafts. He formerly did this with 10 machines. We replaced them with two of ours. Part change only required a readjustment for shaft length and calling up a different program.
"It is sometimes a tossup between hard turning and peel grinding, but hard turning cannot achieve diameter tolerances better than 10µm holding a high Cpk.
"We have noticed that US manufactures still hesitate to use CBN. They are content to stay with what they are familiar with. But in the rest of the manufacturing nations, this market is taking off. In some cases, India is surpassing China in some areas of technology. US manufacturers have to wake up and start looking at the new technology, or they are going to be replaced," Fritz concludes.
Flexibility is one of the main attractions of peel grinding, and greatly responsible for its growing popularity. This is particularly true among manufacturers who are faced with reduced lot size and the need to minimize changeover time.
"Another key advantage is that there are no driver dogs," explains Nelson Beaulieu, United Grinding Technologies (Miamisburg, OH). "Typically, this is a ring mounted on one end of the part. You use only center pressure to drive the components. Male or female centers can be used, and for some specific applications it's possible to use synchronous workheads and tailstocks to drive the parts. This eliminates a lot of up-front cost when trying to orient a part. There is no workholding prerequisite.
"For our machines, we recommend sintered, galvanic bonded wheels. Only about 25% of applications use vitrified bonded superabrasives. Wheel speed is typically in the 80-110 m/sec range.
"We offer a number of peel grinding machines. For example, our Studer line is available with linear or digital drives, and can carry a variety of powered wheels, in either a single fixed wheelhead design or using a turret style on a twin-wheelhead platform.
"Offering a platform with the ability to use one or two wheelheads and a turret gives the customer maximum flexibility by providing the ability to call up subroutines and apply a grinding wheel that is specifically matched to a batch of components or shapes.
"One of our fastest growing applications is peel grinding of multiple-diameter shafts, such as those in automotive transmissions. They can be ground in a single setup using a single or multispindle machine. Typically, when grinding at a speed of 3 mm/sec lateral feed, we can remove 1 mm of material at full depth. Often we replace plunge grinding or conventional machining where interrupted cuts and multiple diameters eat up a lot of cutting tools and setup time.
"Our newest machine is the Kairos, which combines conventional part turning and peel grinding. It's particularly effective when working with sintered metal parts. This material tends to pit, and you get a feathering on a face or an edge with conventional turning."
Cutting-tool manufacture is a key peel-grinding application for Rollomatic (Mundelein, IL). "We notice a strong trend to carbide tooling and small parts that require a diamond wheel," says Eric Schwarzenbach, president. "RPM is also up to 8000 and wheel surface speed is now in the 16,000-20,000 fpm [4876-6096 m/min] range.
"We have found an increasing number of applications for our machines in small [1/4-3/8" or 6.35-9.5-mm] diam injection molds, because of the precision and automation we offer. Parts can be ground automatically on one machine instead of multiple machines with multiple setups.
"To accommodate the newer feeds and speeds, we have modified our machines. Our newest model will have an increased surface speed capability in the 12,000-20,000 fpm [3658-6096 m/min] range. Prebalancing is essential for operation at those speeds.
"With standard abrasive wheels, you don't need those speeds because of their larger diameter [around 16-20" or 400-500 mm]. Their surface speed is high. With diamond and CBN wheels, maximum diameter is about10" [250 mm]. The reason is cost. It would be very rare for someone to buy a 20" [508 mm] diamond or CBN wheel.
"One of our key features is pinch grinding, a process in which we 'pinch' the workpiece between a roughing wheel and a finishing wheel. This allows the user to rough and finish in the same setup. Metal-bond wheels work better than resin bond for roughing, while we suggest a polyimide resin wheel for high-gloss finishing.
"The machine uses a V-block shank-guidance system, instead of a tailstock holder, which is a Swiss-turn-like 'through-feed bushing.' This provides constant support until the workpiece form is completed. It also simplifies part handling for automated work.
"The corner area of the peel-grinding wheel is critical in peel grinding. You want to minimize the contact area to be able to cut small features and to minimize heat generation. Our wheels are relieved 10° radially and 3° axially, so that the wheel does not drag and a good surface finish is achieved. Any new wheel has a true single-point edge, but this quickly wears into a radius. With our wheels, that's about 0.001" [0.03 mm], which is a small area of contact."
The two major markets for peel (and pinch) grinding are grinding components and cutting tool blanks. Pinch grinding is suitable for grinding products such as precision punches, micro pins, and mold components. The purpose is to have minimal contact between the wheel and the workpiece in order to reduce the introduction of heat.
With pinch grinding, part deflection is minimized. It can do both roughing and finishing operations simultaneously. Examples are the Rollomatic ShapeSmart 148P4 or NP4, both of which are cylindrical grinders that utilize pinch grinding. The pinch grinding process with a roughing and finishing wheel is patented by Rollomatic.
Europeans have widely accepted peel grinding," explains Denny Rowe, Weldon Solutions (York, PA). "Now US manufacturers are starting to show an interest in peel grinding in two primary areas:
- As an alternative to conventional formed-wheel plunge grinding to reduce setup time by eliminating grinding wheel and dresser-roll changes between parts.
- As an alternative to hard turning for improved statistical control, lower tooling cost-per-piece, and as a better way to handle interrupted cuts as on keyways and slots that can be difficult to address with single-point turning tools."
Peel grinding with vitrified CBN abrasives, has also been proven to be much more productive on difficult to cut materials such as Inconel and soft stainless. This is accomplished by "scrubbing" the wheel during the grind cycle with high-pressure wheel-cleaning systems.
Weldon began its peel grinding effort in 1997 while working with the aerospace industry. "We found that the peel-grinding process was perfectly suited for grinding complex forms in exotic, tough materials that tended to 'load' the grinding wheel when using conventional abrasives and plunge-grind processes," says Rowe. "For example, removing up to 0.030" [0.8 mm] stock from the diameter of an Inconel shaft in a conventional process would require that the cycle be interrupted twice to dress the wheel for every part. Peel grinding with vit-CBN allowed us to grind up to 20 parts without dressing.
"This same process was found to be successful for high-stock-removal applications in hard steel such as 52100 and D2, allowing very short cycle times without burning or cracking the surfaces. We have found that high-speed CBN peel processes grind much cooler and with greatly reduced forces."
Weldon is currently developing peel-grinding processes for mild-steel automotive components to take advantage of the quick changeover from part to part and long-term tooling cost savings for the customer.
Weldon has concentrated on developing these processes using water-soluble coolant. Although CBN wheel life is better when using straight oil coolant, the poor heat dissipation properties of oil make it difficult to hold the close tolerances required in grinding applications. "Water-soluble coolant allows us to maintain high levels of part quality at faster rates, which more than offset the tooling costs. We have also found that the US customer base is hesitant to accept the environmental and safety problems associated with straight oil processes," Rowe states.
Requirements for successful peel grinding:
- Machine tools must have a higher static and dynamic stiffness than that typical of conventional grinders designed for use with aluminum oxide abrasives.
- Grinding spindles must be designed to handle wheel speeds up to 140 m/sec.
- Rigid electric rotary dressers are required for wheel conditioning.
- Size-control gaging (post-grind) with feedback to the machine control is required primarily where automation is used for machine tending.
- Coolant temperature control is required for maximum process stability."
- Acoustic emissions sensor systems are required for automatic "touch dressing" in increments of approximately 1 µin. (2.5 µm).
- High-pressure wheel scrubbers are required to prevent wheel "loading" that can be detrimental to CBN grinding wheels.
This article was first published in the February 2006 edition of Manufacturing Engineering magazine.