For general work a diameter of 21⁄2 inches will serve well, the thickness being about 5⁄16 inch or 3⁄8 inch. The speed of a wheel of this diameter varies in practice from 3,000 to 4,500 revolutions per minute, but either too fast or too slow a speed will cause the wheel to burn the cutter, and the same thing will occur if the cutter is fed too fast to the wheel, or if too deep a cut is taken. The finishing cut should obviously be very small in amount, especially in cutters of large diameter, for otherwise the wear in the diameter of the wheel will sensibly affect the teeth height, those last ground being the highest.
Chapter XXIII.—EMERY WHEELS AND GRINDING MACHINERY.
Emery Wheels and Grinding.—Emery grinding operations may be divided into four classes as follow:—
1st. Tool or cutter grinding, in which the emery wheel is used to sharpen tools which, from their shape, were formerly softened and sharpened by the file, already largely treated in the preceding chapter.
2nd. Cylindrical grinding, in which both the work and the emery wheel are revolved, as has been explained with reference to grinding-lathes.
3rd. Flat surface grinding, in which the emery wheel takes the place of the ordinary steel cutting tool; and
4th. Surface grinding, in which the object is to remove metal or to smoothen surfaces.
The distinctive feature of the various makes of solid emery wheels lies in the material used to cement the emery together, and much thought and experiment are now directed to the end of discovering some cementing substance which will completely meet all the requisite qualifications. Such a material must bind the emery together with sufficient strength to withstand the centrifugal force due to the high speeds at which these wheels must be run to work economically; and it must neither soften by heat nor become brittle by cold. It must not be so hard as to project above the surface of the wheel; or in other words, it should wear away about as fast as does the emery. It must be capable of being mixed uniformly throughout the emery, so that the wheel may be uniform in strength, texture, and density. It must be of a nature that will not spread over the surface of the emery, or combine with the cuttings and form a glaze on the wheel, which will prevent it from cutting. This glazing is, in fact, one of the most serious difficulties to be encountered in the use of emery wheels for grinding purposes, while it is a requisite for polishing uses, as will be explained farther on. Many of the experiments to prevent glazing have been in the direction of discovering a cement which would wear away under about the same amount of duty as is necessary to wear away the cutting angles of the grains of emery, thus allowing the emery to become detached from the wheel, rather than to remain upon it in a glazed condition.