Machinery's Handbook, 31st Edition
GRINDING WHEELS 1299 both operational conditions and process objectives. With variations from such average values, the composition of the grinding wheels must be adjusted to obtain optimum re- sults. Although it is impossible to list and to appraise all possible variations and to define their effects on the selection of the best suited grinding wheels, some guidance is obtained from experience. The following tabulation indicates the general directions in which the characteristics of the initially selected grinding wheel may have to be altered in order to approach optimum performance. Variations in a sense opposite to those shown will call for wheel characteristic changes in reverse. Conditions or Objectives Direction of Change To increase cutting rate Coarser grain, softer bond, higher porosity To retain wheel size and/or form Finer grain, harder bond For small or narrow work surface Finer grain, harder bond For larger wheel diameter Coarser grain To improve finish on work Finer grain, harder bond, or resilient bond For increased work speed or feed rate Harder bond For increased wheel speed Generally, softer bond, except for high- speed grinding, which requires a harder bond for added wheel strength Dressing and Truing Grinding Wheels.— The perfect grinding wheel operating under ideal conditions will be self sharpening, i.e., as the abrasive grains become dull, they will tend to fracture and be dislodged from the wheel by the grinding forces, thereby exposing new, sharp abrasive grains. Although in precision machine grinding this ideal sometimes may be partially attained, it is almost never attained completely. Usually, the grinding wheel must be dressed and trued after mounting on the precision grinding machine spin- dle and periodically thereafter. Dressing may be defined as any operation performed on the face of a grinding wheel that improves its cutting action. Truing is a dressing operation but is more precise, i.e., the face of the wheel may be made parallel to the spindle or made into a radius or special shape. Regularly applied truing is also needed for accurate size control of the work, particularly in automatic grinding. The tools and processes generally used in grinding wheel dressing and truing are listed and described in Table 1. Table 1. Tools and Methods for Grinding Wheel Dressing and Truing Designation Description Application For interrupted or coarse work surface Harder bond For thin walled parts Softer bond To reduce load on the machine drive motor Softer bond
Preferred for bench- or floor-type grinding machines; also for use on heavy portable grinders (snagging grinders) where free-cutting proper ties of the grinding wheel are primarily sought and the accuracy of the trued profile is not critical. Usually hand held and use limited to smaller-size wheels. Because it also shears the grains of the grinding wheel, or preshaping, prior to final dressing with, e.g., a diamond.
Freely rotating discs, either star-shaped with protruding points or discs with corrugated or twisted perimeter, supported in a fork-type handle, the lugs of which can lean on the tool rest of the grinding machine. Made of silicon carbide grains with a hard bond. Applied directly or supported in a handle. Less frequently abrasive sticks are also made of boron carbide.
Rotating Hand Dressers
Abrasive Sticks
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