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Grinding Techniques: In Thousands Of Years, The Basics Of Grinding Haven't Changed

Grinding is one of the oldest methods known for shaping and sharpening objects. It was first used in prehistoric times to make weapons and other tools by rubbing wood, stones, bones and eventually metal against hard, abrasive rocks.
In thousands of years, the basics of grinding haven't changed. We're still rubbing an abrasive against metal to change its shape or polish its surface - which probably explains why many people still call grinding wheels "rocks." But rocks they aren't.
A modern grinding wheel is a precision cutting tool that contains a sophisticated mix of various abrasives and other ingredients that are carefully blended and processed to deliver specific cutting qualities. Rebuilders today use a variety of abrasive wheels rotating at high speeds to grind crankshafts, camshafts, flywheels, clutch plates, valve faces, stems and seats, and dozens of other parts.
The basic techniques for grinding and resurfacing haven't changed a great deal over the years, but recently the pace of evolution has quickened as the need for faster production speeds, higher quality finishes and increased productivity have taken on increased emphasis and importance. In a highly competitive market, rebuilders need every advantage they can get, so abrasives are getting better and better, and more application specific.
There's a much broader spectrum of abrasives available than ever before, including a new generation of blends and superabrasives that may revolutionize grinding. These new superabrasives might aptly be called "jewels" rather than rocks because of their high performance (and price!).

Superabrasives

As tooling speeds increase, along with the demand for increased productivity, less downtime and higher quality finishes, superabrasives such as diamond and cubic boron nitride (CBN) may find a growing range of applications in the aftermarket - at least that's what some people are saying.
"We see diamond and CBN emerging as superabrasives because of their excellent cutting properties," said Jerry Qualiana of K-Line, Holland, MI. "We're doing a lot of research and development work on future abrasives because the aftermarket needs unique products.
Qualiana says that diamond and CBN could play an increasingly important role in more grinding applications, as they already do now in cylinder honing and resurfacing, but the high cost of these materials is still hard for many rebuilders to justify. So there hasn't been much demand for them yet.
Scott Biesanz at Goodson Shop Supplies, Winona, MN, said superabrasive grinding wheels are available today, but are considered too expensive by the typical aftermarket buyer. A typical 28" diameter, 1" thick general purpose crankshaft grinding wheel that costs about $300 (jobber) in a conventional abrasive might cost twice as much in a superabrasive. "The higher cost of superabrasives can be easily justified in high volume production and manufacturing applications, but the typical aftermarket user doesn't see the cost benefit," said Biesanz.
According to Cincinnati Milacron, a manufacturer of grinding abrasives, superabrasives can provide exceptional wear ability, reduce grinding time and help eliminate common problems like burn, part-to-part variations, and excessive downtime due to wheel dressing.
Diamond, the hardest material known, is three times harder than its conventional abrasive counterpart, silicon carbide. That's why a diamond is used to dress a conventional grinding wheel. Diamond also has a thermal conductivity that's about six times that of silicon carbide and aluminum oxide, which permits high speed grinding speeds without excessive heat buildup in the workpiece.
CBN is second in hardness only to diamond; it is 2.5 times as hard as its conventional counterpart, aluminum oxide. CBN can withstand higher temperatures than diamond (2,500° F versus 1,300° F for diamond), and is also a good conductor of heat, having a thermal conductivity about four times higher than silicon carbide and aluminum oxide.
Less expensive than diamond, CBN is a popular superabrasive in industrial and manufacturing applications today because of its ability to grind hard steels, superalloys, cast iron and powdered metal. It is generally superior to both diamond and aluminum oxide for grinding hard steels (except tungsten carbide, which diamond handles best), and is used for production grinding of bearing bores, crankshafts, camshafts and many other parts that require high quality, precision finishes including fuel injection components.

One of the qualities that has made CBN so popular in such applications is its ability to operate at higher than normal wheel speeds - up to 6,500 to 20,000 surface feet per minute (SFPM). Vitrified CBN wheels show increased grinding efficiency at higher wheel speeds, yet can also handle speeds as low as 2,500 SFPM in applications that require small grinding wheels.
The SFPM speed of a wheel can be calculated by multiplying 3.1416 times the diameter of the wheel (in inches) times the rpm and dividing by 12.
Cincinnati Milacron says CBN really excels in camshaft and crankshaft grinding, as well as centerless valve grinding. In one case study the company undertook, changing to CBN abrasive for valve stem and seat grinding showed a measurable improvement in seat roundness, stem roundness and straightness. Cycle times were reduced to six seconds per valve seat and 2.55 seconds for the valve stems. The need for wheel dressing was also reduced, with an average of 800 parts per dress on seats and 1,000 parts per dress on stems. With higher wheel speeds, the parts per dress on stems increased to 4,000.
In another case study, a CBN grinding wheel was used to grind the lobes on camshafts. Changing to the superabrasive reduced the need to dress the grinding wheel from once every 50 cams to once every 130 cams. CBN also eliminated any trace of burning and produced more consistent finish and overall quality.

Blends & bonds

One thing that anyone who has looked through a shop supply catalog lately will notice is the wide variety of grinding wheels and abrasives that are available. The days of choosing between a green (silicon carbide) wheel for grinding cast iron, and a white (aluminum oxide) wheel for grinding hard steel are gone. Today, there's a full spectrum of abrasives for virtually every conceivable need. Colors range from white to gray to green to black to rose to pepper.
Take valve seat refinishing stones, for example. In one supplier's catalogs, there are six different stone types from which to choose: general purpose gray stones for rough or finish grinding cast iron seats, ruby stones for general refacing as well as for hardened seats, blue stones for high performance beryllium seats, pepper stones for nickel chrome seats, white stones for stellite seats, and fine grit gray stones for general purpose finish refacing.
But the color of the wheel is only a small part of the story. There are a lot of variables that determine the cutting properties of any given abrasive, including the type of abrasive (aluminum oxide, silicon carbide, CBN or diamond), the hardness and friability of the abrasive, the grain or grit size of the abrasive, the type of bond material that binds the abrasive together (vitrified or resinoid), the amount of bond in proportion to the abrasive (the "grade" of the wheel), and the relative spacing of the abrasive particles with respect to the bond (the "structure" of the wheel).
On every grinding wheel are manufacturer's markings that indicate the type of abrasive, grit size, grade, structure and bond type. Lower grit numbers indicate coarser grit size while higher numbers indicate a finer grit size. A finer grit size provides the best finish and also allows better penetration when grinding hard metals. Coarse grit is generally better for unhardened metals and for grinding applications that require large areas of contact between the wheel and workpiece. Most automotive grinding wheels use grit sizes between 24 and 80.Grade ratings refer to the amount of bond material in the wheel, and are designed by letters A through Z. "A" represents the softest end of the spectrum while Z represents the hardest wheels. Soft wheels work well on hard metals, but wear quickly. Hard wheels are better on softer metals, and generally produce a finer finish and longer life.
The structure rating of a wheel, which ranges from 0 (most dense) to 12 (open) indicates the spacing of the abrasive grains in the wheel. The structure of a given wheel tends to be standard for a given grit size and grain, but some have more open spacing to provide cooler, faster cutting action in dry grinding applications.
The color of the wheel depends on the type of abrasive in the wheel, as well as the impurities mixed with it when it is manufactured. Pure aluminum oxide is very friable, which makes it well suited for grinding tool steels and other heat sensitive steels.
There are also semi-friable versions of aluminum oxide. Some may contain a sodium impurity which gives it a unique pinkish beige color and helps it fracture more easily. Others may contain a chromium impurity (pink color) which makes it better suited for grinding heat sensitive alloy steels and chrome alloys. Combinations of pure aluminum oxide and various semi-friable versions may offer improved grinding performance for applications where burning with pure aluminum oxide is a problem.
According to Steve Rupe, an engineer with Radiac Abrasives, an abrasives manufacturer in Salem, IL, there's much greater use of mixtures than ever before. "The old standard catalog products are not the answer to today's grinding needs," said Rupe. "Customers today want products that are specifically engineered to their particular application.
"Abrasive manufacturers are offering more customized products than ever before to satisfy the demands of the market. By adjusting the various proportions of different aluminum oxides in a wheel, we can significantly alter its cutting characteristics."
Rupe said that increasing the percentage of friable aluminum oxide in a grinding wheel helps keep temperatures down to reduce burning. It also increases the cutting action of the wheel as well as the wear. Adding more semi-friable aluminum oxide to the mix improves durability and wheel life, but the wheel won't cut as freely or run as cool. Rupe said that most of the blending that's being done is with different grades of aluminum oxide. There aren't many instances where silicon carbide is being mixed with aluminum oxide.
Another area where a lot of work is being done to improve and customize grinding wheel performance is on the bond. The bond is the material that holds the abrasive grit together. The bond itself does no cutting, but holds the grit until it tears away to expose fresh grit underneath. The secret to good grinding performance, therefore, is designing the bond so that it holds the grit until the grit dulls before it allows it to break away.
If the bond is too strong and holds the grit too long, the wheel may become dull, glazed, cut poorly and burn the metal. It may also clog up with debris. On the other hand, if the bond is too weak, the wheel will wear too quickly resulting in short wheel life.
The strength of the bond also affects the surface finish on the metal. A strong bond will produce a finer surface finish while a weaker bond will leave a rougher finish. So by using various bond strengths and mixes of abrasives, a wheel manufacturer can design a wheel to match virtually any application.
Most automotive grinding wheels have a vitrified bond because it is strong, rigid and can withstand high temperatures, oil and water. A vitrified bond is a glass-like ceramic material made of clay, fluxes and other ingredients. During the manufacturing process, the bond material is carefully mixed with the abrasives in measured proportions. The resulting paste is then pressed into a mold (called "greenware"), and fired in a high temperature (more than 2,300° F) kiln for 24 to 180 hours depending on the size, density and ingredients in the wheel. The length of the baking cycle is critical to achieve complete vitrification and a fully cured bond.
Resinoid bonds, by comparison, use an organic resin (powdered phenol formaldehyde) to hold the abrasive particles together. This type of bond provides better mechanical and thermal shock resistance than vitrified bonds, and is often used with diamond abrasives for cut-off grinding, flute, tap and drill grinding. Resinoid wheels are used primarily for rough grinding or where wheel speeds are high. But resinoid bonds are limited to low temperature (less than 350° F) applications. Resinoid bonds can also soften by prolonged exposure to water.
Metal bonds are also used in certain applications, but primarily with diamond for special applications like cylinder honing. The metal bond in this case helps absorb and dissipate heat generated by the diamond.

Selecting the wheel

One thing most suppliers agree upon is that the automotive market requires unique abrasives. Generic general purpose abrasives have their place, but today's generation of specialized abrasives can do a far superior job in most cases. But choosing the "right" abrasive for a particular application takes product knowledge and a familiarity with what's available.

"A lot of people who are grinding today are not aware of all the improvements that have been made in recent years in grinding technology. These people could easily improve their productivity by taking a closer look at what's available and choosing wheels that are much better suited to their needs," said Radiac's Ron Reed."Take crankshaft grinding. We have a new bond system for our premium 43A line of abrasives that is specifically designed for this market. The wheel requires less dressing and won't load up like an ordinary wheel."
Jack Roche of Bay State Sterling, a Westborough, MA-based abrasives manufacturer, said the type of grain that's used in a grinding wheel is dictated by the metal you're grinding. "When grinding a soft metal, you want a grain that's less friable. For harder metals, you want a grain that is more friable. But you also have to consider the bond and structure of the wheel before you can make a specific recommendation."
Goodson's Biesanz says customers want performance at a price, but don't always know how to pick the best wheel for a particular application. "The general rule is that the harder the material you're grinding, the softer the wheel should be, he said. "But when the wheel heats up, the bond goes away and the grain opens up. The wrong wheel can load up, burn the metal and fishtail. So we often help customers solve a grinding problem by recommending a wheel that better suits their needs."
K-Line's Qualiana says, "Today's smaller displacement, higher revving engines have parts that are different from those in the typical 350 Chevy V8. They have smaller, harder valves that require a different composition grinding wheel. The same goes for crankshafts. Steel cranks need a different grinding wheel than ordinary cast iron cranks.
"It's the same with flywheels. Flywheel resurfacing is a big business today and it's profitable, too. But you need one type of abrasive for cast iron and another for steel. So when choosing a grinding wheel, you have to look at all the variables, not just the hardness of metal. You have to consider the type of equipment that's being used, grinding speeds, surface finish and so on before you can determine the best wheel to use."
Randy Neal at Atlas Shop Supplies in Norcross, GA, says he sells predominantly Norton wheels (made by Norton Co., Worcester, MA). "They must have 200 different combinations of abrasives available," said Neal. "We try to target our mix of wheels to what our customers really want. Industrial users have formulas for determining an exact abrasive to use, but that's pretty uncommon in the aftermarket.
"If you were going by technical correctness, you might need 60 different wheels to cover all the different crankshaft applications. But the typical shop only has four or five different wheels with little or no variation in grits. That's real world. So we try to help them pick a wheel that will give them the best results."

Common grinding mistakes

Using the wrong type of grinding wheel for a particular application can cause any of a number of problems as well as reduced wheel life. But the number one mistake that many users don't realize they're making is failing to dress their grinding wheel properly.
A crankshaft grinding wheel should always be dressed with coolant, never dry. Diamonds can't take a lot of heat and can burn or become dull if no coolant is used during the dressing procedure. Dry dressing can shorten the life of the diamond by a factor of 10! Coolant flow should be continuous. Stopping the flow can cause the diamond to overheat and shatter when the coolant starts flowing again.
The diamond should be sharp. Once it gets a .030" flat or a dome, it needs to be reset or replaced (some suppliers can reset a diamond to save the cost of having to replace it). Premium grade dressing diamonds have more facets per stone than standard grade diamonds. They keep their sharpness longer to provide a better dress and longer life.
The diamond also needs to be sized to the wheel that's being dressed. For a typical crankshaft grinding wheel, a one-and-one-half to two carat diamond is generally recommended.
The traverse speed of the diamond across the wheel has a major affect on wheel finish. Too rapid a speed causes a rough finish that makes the wheel act more aggressive (cuts faster). Yet a fast dress can actually help a hard wheel act softer and cut better. A slow dress has a tendency to close the wheel face and produce a very smooth surface. Though this may be necessary if you're trying to achieve a fine finish on the workpiece, it reduces the wheel's cutting ability.
How often the wheel needs to be dressed depends on the type of grinding that's being done and the quality of the finish that's required. For rough grinding, the wheel usually needs to be dressed less often. But for high quality finish grinding, more frequent dressing is a must.
Clues that may indicate a need to redress the wheel include changes in color, chatter marks on the workpiece, a change in the sound of wheel while it is grinding, an increase in the power (amps) needed to drive the wheel, and/or the beginning of a deformation in the workpiece causing an irregular shower of sparks.
Dressing a wheel more frequently will obviously shorten its life. Even so, a wheel should not require more than one or two passes at a depth of .001". A deeper dress can actually damage a wheel by fracturing the bond, causing the wheel to break down more quickly.
Ideally, a crankshaft grinding wheel should be dressed after every use (especially if sweep grinding). This would, in the opinion of some experts, eliminate most of the burnish, shape and taper problems that are frequently encountered by crankshaft grinders. At the very least, most of these experts recommend dressing the wheel after grinding two or three crankshafts.
Another mistake that's often made is using the wrong type of coolant, or no coolant at all. Virtually every supplier and manufacturer we interviewed recommended wet grinding over dry grinding. The primary reason for using coolant is to keep the metal surface from getting too hot, which can cause burning, cracking, poor surface finish and distortion (out-of-round). Coolant also helps flush away swarf to keep the grinding wheel from loading up. Coolant also helps lubricate the wheel so it provides a more uniform cut, and rust-inhibitors in the coolant help protect metal surfaces from corrosion and discoloration.
Either synthetic or oil-based coolants can be used, but many prefer synthetics because they won't spoil over time. It's important to make sure the coolant that's being sprayed on the workpiece is actually getting between the wheel and metal. A large grinding wheel can have a surface speed that exceeds 70 mph. At such speeds, the wheel tends to drag along a surface layer of air that forms a bubble which blows coolant away from the grinding area. The use of an air scraper on the wheel ahead of the coolant nozzle can break up this air bubble to help keep the coolant where it belongs.

Grinding safety

A variety of publications covering grinding procedures and safety are available from the Grinding Wheel Institute, Cleveland, OH, (216) 899-0010. Publications cover mounting procedures, general safety recommendations, safety requirements and standards for grinding wheels, even cost reduction principles in grinding.

Some general safety precautions include:

• Vitrified grinding wheels can be cracked by mishandling, mechanical or thermal shock. Because of the high speed at which they rotate, cracks may cause a wheel to explode. Wheels should always be inspected prior to use. Tapping a vitrified wheel lightly on the side should produce a ringing sound. If the wheel doesn't ring, it's cracked and must be replaced.

• Eye protection should always be worn by the operator.

• Wheels are designed to operate within certain speed limits. Never exceed the maximum spindle speed rating for the wheel.

• Wheels should be trued and balanced when first installed, and rebalanced as needed to compensate for wear. If a wheel cannot be rebalanced because of wear, it should be replaced.

• All wheel and machine guards must be in place before grinding. The operator should never stand in a direct line with the wheel when the equipment is first started.

• When making contact between the grind wheel and part, contact should be made gently without bumping or gouging.

• Grind only on the face of a straight wheel, never the side.

• Never force grind so that the motor slows noticeably or the work gets hot.