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OTHER FACTORS IN WHEEL SELECTION

Grinding is probably the most challenging part of machine tool operation because so many variables must be integrated to get first-class results. Four of these factors relate to workpiece materials, which are constantly being changed in the shop environment. Four other factors in wheel selection are relatively fixed.
Variable Factors Composition of the Workpiece For most steels and steel alloys, aluminum oxide grinding wheels are the usual
choice. With high-temperature alloys, particularly when the part will be operated under stress, cubic boron nitride can be far preferable. Since microcrystalline cubic boron nitride has become available there are economical applications for this abrasive in softer steels, particularly for form grinding. For grinding cemented carbides, diamond is greatly preferred because of its cool-grinding characteristics. (Diamond is
actually a better conductor of heat than copper.) The use of silicon carbide on cemented carbide tools often results in heat checking and resultant premature cutting tool failure. For most cast irons, silicon carbide is the wheel abrasive of choice, but for some roughing applications, aluminum oxide can be used. Whereas ferritic and martinsitic stainless steels are best ground with aluminum oxide, austenitic stainless steel (chromium–nickel) is better ground with silicon carbide. Silicon carbide is also useful in the grinding of bronze and other copper-based alloys and titanium.
Cutting Fluids For most toolroom grinding, a synthetic grinding fluid that avoids the use of soluble oil is generally preferred. But where heavy-duty grinding is done, soluble oils of differing characteristics are necessary. In demanding, high-removal-rate operations, such as thread grinding, sulfochlorinated straight oils may be needed to avoid damage to the workpiece. In these latter cases, extensive exhaust ducting with oil precipitation equipment is needed. Because the disposal of oils is an environmental hazard problem, great effort is being expended to develop effective substitutes for oil as a grinding fluid.
Material Hardness This factor is of concern in the choice of both grit size and grade. Generally, for soft, ductile materials, the grit is coarser and the grade can be harder. For hard materials, finer grit and softer grades are the rule. In the machine shop a coarse abrasive is considered to be in the range of 36 to 60, and a fine abrasive in the range of 80 to 100.
In this setting, grades F, G, and H are considered soft, and J, K, and L are considered relatively hard. Too coarse a grit tends to leave excessive scratching, which can be difficult to correct later. On hard material, coarse grit may in fact remove less stock than a finer grit because of lack of workpiece penetration.
Too soft a wheel may wear too fast to be economical; too hard a wheel may glaze quickly and cease to cut. In many cases, excessively hard wheels are selected when experimenting might show that a softer wheel would cut more freely, without significantly more wheel wear.
The structure of the grinding wheel also relates strongly to the material being ground. For soft, free-cutting materials that produce relatively large chips, an open structure is essential to provide space for the chip to be carried through and be ejected. Otherwise, the wheel will become loaded with workpiece material.
Workpiece Finish When production grinding is done on larger lot sizes, wheels need to be selected carefully for abrasive grit size and bonding material to perfectly match the finishing needs of the product. In the job shop setting, with typically small lot sizes (or even just one part), it is uneconomical to keep changing grinding wheels. In most such cases, a generalpurpose wheel is selected about midway between the usual roughing and finishing requirements.
The skill of the operator in dressing the wheel then becomes the controlling factor in the finish obtained. The same wheel can be dressed either for aggressive rough grinding or for gentle finish grinding, to make a relatively coarse wheel behave as a finishing tool. This
process is addressed in Unit 2.
Fixed Factors Horsepower of the Machine Machine horsepower tends to rise with each new generation of grinding machines.
With increased horsepower comes the need for increased machine rigidity to utilize the horsepower. Higher power and increased rigidity permit the selection of harder wheels to match the capacity to do more work in a given time. As the superabrasives become more widely accepted, specific machine tools are being designed for their use. These newer machines are more compact, have high spindle power,
require stiff spindle construction, and are typically designed to use smaller-diameter grinding wheels.
Severity of the Grinding In a production setting, great care should be taken in matching the characteristics of the grinding wheel to the specific job to be done. This can involve trials with wheels that come close to meeting the requirements before selecting the one best wheel. The Machining Data Handbooks are an excellent source of starting recommendations on grit size and wheel grade. In the toolroom setting, where grinding pressures are typically low, a friable aluminum oxide wheel of relatively soft bond hardness is the typical choice.
Area of Grinding Contact On any given design of grinding machine, the area of grinding contact remains essentially constant. The rule is finer grit sizes and harder wheels for small areas of contact, and coarser grit sizes and softer wheels for larger areas of contact. In the machine shop, this means grits mostly in the range 46 to 100, except for specialty work such as thread grinding, which may use wheels
as fine as 220 grit, or vertical spindle, rotary table grinding (often called Blanchard grinding), which often uses a grit size of 30, especially for soft steel applications. For horizontal spindle surface grinding the typical bond hardness selected lies in the I and J region, with the softer bond used for the harder material. For cylindrical grinding, with line contact with the wheel, J and K are the most common hardnesses.
Wheel Speed Wheel speed is usually fixed by the nature of the machine, but the operator has the responsibility to be sure that the mounted wheel is safe for the spindle speed of the machine. On vitrified wheels, most have a maximum speed of 6500 sfpm. (There is a trend toward higher-speed grinding, which requires machines with special interlocking safety systems to protect operators.) Organic bonded wheels (rubber, M12_KIBB5087_09_SE_C12.QXD 6/3/09 1:36 AM Page 605 resinoid, or shellac) can be rated at higher speeds, up to 16,000
sfpm, and experimental work is being done with plated superabrasive media with speeds in the region of 30,000 sfpm.
Use of Grinding Fluid The use of grinding fluid is a relatively minor factor in grinding wheel selection and typically results in only one step of grade change. With grinding fluids as opposed to dry grinding, a wheel one grade harder is typically chosen.
In any shop that has been operational for some time, there should be information on which wheels have worked best on specific jobs. If there are changes to be made, these should be in small steps, rather than making radical changes.
Because most shops handle a variety of work, it usually doesn’t pay to switch wheels all the time. A great deal can be done by the operator in his or her preparation procedures to adapt a general-purpose wheel to specific workpiece grinding. When heat-sensitive workpiece materials are being ground, careful matching of the grinding wheel to the work is essential to avoid metallurgical damage.

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