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156 
ROAD AND WORKSHOP by HANDYMAN
Page 48
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Grinding Equipment Part 1
A LTHOUGH now in regular use during I–I vehicle manufacture and repair, the grinding wheel was originally adopted mainly to deal with hardened work, or hard spots on normal work. It is true to say that in those days it did not lend itself to anything more.
The reason for this was simple enough: a lack of reliable wheels and know-how in their manufacture. And although grinding already had known advantages over the lathe, some time was to elapse before the wheel could be put to full use. Nevertheless, it was possible in those early days to work to really fine limits, together with a finish far beyond the scope of the lathe, and it was inevitable that some fast progress would be made with the manufacture of sound grinding wheels.
The operation of grinding metal is
nothing more or less than a cutting process, the abrasive grains being the cutters, each taking off a minute chip of metal in no way different to the milling cutter. The grinding wheel consists of two materials—the grains of grit which form the cutting faces, and the bonding material holding the grit together.
A wheel is identified by its grain and grade, the grain number indicating the mesh size of the screen selecting the material, whilst the grade of a wheel is designated by letters and refers to the hardness of the wheeL Grain sizes are numbered from 4 to 200 for general application, although finer grain material is available under the letters F, FF, or FFF.
The ideal grinding wheel is one which combines a grain that will fracture after the edge has become blunt (and at once present a new cutting face to the work) with a bonding material which will hold the grain until its edge has become dull, ' then release it.
Whilst there is a good, natural abrasive named corundum, which comes from Canada, many wheels are made from artificial abrasives. For one instance, the Norton company is able to make a full range of wheels from a compound they call alundum. , The Norton grade list starts in alphabetical order from the letter E, which indicates a soft wheel, and progressively hardens with each letter. For example, M is medium, U is hard and Y extremely hard.
The ability to deal quickly with metal
of any hardness has enabled the grinding wheel to steal a march over the lathe in the motor-vehicle industry. This wheel can serve the factory and the repair shop equally well, whether it be the complex crank grinder or the simple, twin-wheel bench machine.
Grinding wheels are mounted between flanges on a collet, and are either straightsided or recessed, the recessed pattern being used for crank-grinding. As a rule, a Woodruff key is fitted between spindle and collet to prevent the wheel coming loose should the spindle stop suddenly.
Where several wheels are used on the one machine, it is recommended that an equal number of collets are used. Each collet should remain attached to its wheel, as it is far easier and quicker to slip the complete wheel and collet off the spindle than to disconnect the wheel from the collet and—more important—this removes the need to true the wheel after each change.
In fitting the wheel on to the collet it is necessary to use rubber or other compressible washers just above the flange size. Do not use hard washers, as it is important that the pressure on the grinding wheel is equally distributed. From time to time a trainee grinder will complain that his wheel is running raggedly and vibrating, and it is a safe bet that he has oiled the tapered spindle nose. This is the cause of the vibration, as the collet is no longer rigid on the spindle. The collet and spindle should be clean and bone dry when mated together.