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Precision grinding: tips on selecting the right wheel.

Recent advances in grinding-wheel technology--especially in superabrasives--are significantly changing tool selection criteria. Until recently, conventional wheels with vitrified bonds were the mainstay of production grinding, while superabrasives, such as cubic boron nitride (CBN), were used only in the toolroom. As CBN wheels are introduced to production applications, manufacturing engineers are finding them capable of substantially improving productivity and lowering grinding costs.

For these reasons, CBN is replacing many conventional abrasive wheels. Although this trend is accelerating, superabrasives will never completely eliminate conventional wheels; the costs is significantly less than superabrasive wheels, so they will continue to be cost effective in many applications.

On grain and bond

The basic difference between conventional and superabrasive wheels is in the abrasive grain. Conventional wheels are made from aluminum oxide, silicon carbide, or high-performance abrasives such as zirconia alumina. Superabrasive wheels are usually made of CBN, a synthetic grain that is two to three times harder and more durable than conventional abrasives, Figure 1.

Bonds hold the abrasive grains together in the wheel. The two basic bonds used in conventional wheels are organic and vitrified. Bonds for superabrasive whels include organic, vitrified, metal, and electroplated. Organic bonds are primarily resins, fired at fairly low temperatures. Vitrified bonds are made of glaceous particles, fired at higher temperatures, forming a glass-like structure.

There are two types of metal bonds. One consists of a thin layer of abrasive particles that is electrically bonded to a metal form. The other has multiple layers to a thickness of 1/16" to 1/8".

In conventional wheels, organic bonds are used for roughing applications such as in foundry work, while vitrified bonds are for precision grinding. The distinctions are more complicated with CBN wheels. Generally, vitrifid wheels are used for all ID grinding, and resin-bond wheels are used for cylindrical and surface work. Metal-bond wheels are used in various high-production applications, and plated wheels are generally used for roughing. For specific applications, see Figure 2.

Conventional vs

superabrasive wheels

Factors ranging from work material to type of grinder must be considered when selecting a wheel. Conventional abrasives are used on all materials, while CBN is primarily for ferrous metals, 45 Rc and up.

Conventional abrasive wheels are the weakest link in a grinding system, so the tool can compensate for much of the variability in the machine and setup. While this compensation may reduce wheel life, the lower cost of conventional wheels makes them a sensible choice in applications where machinery is highly variable.

With superabrasive wheels, the entire grinding system must be considered. Successful machining requires a good machine with moderate rigidity, fine (0.0001") incremental infeeds for trueing, as well as proper coolant type and delivery.

While superabrasive wheels are more expensive than conventional types, total cost accounting--i.e., measuring total cost of the grinding system--can result in lower overall cost with superabrasives. In other words, even though CBN costs more, it can reduce total grinding cost 30 to 50 percent by improved performance.

Here are the major benefits of each wheel type: Conventional wheels--lower wheel cost, easier to shape, and more adaptable; CBN wheels--lower total cost/part, shorter grinding cycles, less metallurgical damage, better part geometry, more consistent parts, and fewer rejects.

CBN trueing and dressing

Until recently, high machine rigidity was considered essential to successful CBN grinding. Recent research, however, shows that rigidity isn't as important as managing the forces in the grinding system, primarily during trueing and dressing operations. In fact, 20-year-old conventional grinders have been successfully converted to CBN grinding.

The key is improved trueing and dressing systems, often using rotary diamond dressers that traverse the wheel instead of plunging into the wheel's full width. Lowest trueing forces are achieved with rotary diamond dressers.

Figure 3 compares trueing forces between conventional vitrified wheels and CBN wheels. A trueing infeed of 0.001" requires a force of 5 lb with a conventional vitrified wheel. At the same infeed rate, a resin CBN wheel requires 35 lb. By reducing infeed to 0.0002", however, trueing force can be maintained at 5 lb.

Power requirements follow a similar pattern. New trueing techniques enable reducing force and power previously required for CBN grinding.

As in any grinding operatioin, optimum metal-removal rates require a good coolant and delivery system. Conventional wheels often use water-soluble oil or synthetic coolants mixed at 20:1 or 40:1. For CBN, straight oil is generally best; however, heavy-duty water-soluble oil mixed at 10:1 is adequate in many applications.

In general, CBN wheels are preferred on hard-to-grind materials and to achieve precision tolerances. In the bearing industry, for example, many producers used CBN. In addition to improving productivity, CBN also provides greater consistency from part to part, which is especially important in products such as bearings.

The automotive-component industry also is switching to CBN as new designs require smaller engine components and harder materials. In fuel-injection systems, for example, small ID CBN wheels provide higher grinding productivity than conventional wheels. CBN is being used to grind even steel camshafts.

Because of higher productivity and improved part consistency, most conventional precision grinding will convert to CBN. But, conventional wheels always will have a place--in rough grinding, for example. They are reshaped easily and will continue, at least in the near future, to be preferred for intricate form work as well. Nevertheless, product development eventually will make it economically justifiable to put such shapes into CBN wheels.

Conventional wheels also will remain strong in applications involving softer materials, such as cast iron, and for small lot runs, because they're more adaptable than CBN to reshaping.

For more information about CBN wheels, circle E18.
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Title Annotation:includes the related article - good grinding tip sheet
Author:Schwarz, Jeff
Publication:Tooling & Production
Date:May 1, 1985
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