CARRIERS OF THE GRITS

Historically, Diamond grit was first used as a free grit mixed with olive oil on the rotating scaife. Tradition has it that 1

Carat of fine Diamond powder should be mixed with a dozen drops of olive oil. Some advocate castor oil and others ‘oil of brick’ (formed by dropping a hot brick in olive oil - it has an evil odor).

The same material was traditionally used on the Diamond saw as well as on the scaife. There are still many firm advocates of this ancient, well-tried method. Within recent years there has been a big move over to the use of what are called lapping compounds to replace the free grit in oil. In these compounds graded Diamond particles are evenly distributed, either in a special grease or in a chemical compound. The compound is supplied in a dispenser resembling a syringe, with piston, and exact amounts can be applied at will.

Two main kinds of vehicle for particles are available. The one is an oil-soluble compound which contains the coarser grades of Diamond. This is used for preliminary roughing and major removal of gross material. The second has the Diamond grit in a water-soluble vehicle compound and is meant for the finer grades, to be used for finishing off and ultimate fine polishing. About a dozen graded sizes of particle from 90 microns to 1/10 micron are available commercially. Furthermore, the compounds are offered with different degrees of Diamond concentration. The vehicle compounds are selected as to viscosity and adhering properties also. It is common practice to include a coloring dye (blue, green, brown, etc.) as a coding recognition method for identification of particle sizes held in suspension. These pastes have incorporated in them both wetting agents and plasticizers which function so as to keep them uniform in mix and in consistency. There is, of course, trade secrecy in this sort of thing. Some compounds are based on soaps rather than oils.

Next to purely freely suspended particles come those cases where particles are rubbed into the porous holes in cast iron, i.e. the traditional scaife, and here too the newer compounds replace the olive oil. Tough sharp blocky particles are best. Most important are the truly bonded wheels, which consume most of the grit used and which we shall now consider. Resinoid bonds are an unexpected development. There

are certain machine tools, especially those made from the important grinding material tungsten carbide, which are best shaped with a Diamond-bonded wheel but only by application of moderate pressures. By incorporating the Diamond particles into a wheel of a hard resinoid material, it is found that such a wheel can give a little and so the Diamond is then cutting with application only of medium pressure. For such wheels friable Diamond particles are desirable, embedded in the selected resins. They have considerable application in engineering and are widely used.

Next in grade of performance are those bonded wheels in which the Diamond particles are embedded in either copper or in softish bronze. Such wheels are good for machining glass and ceramics.

Finally, for high-pressure work such as sawing, drilling, grinding, etc., bonding in iron or even in hard carbides is desirable.

A great deal of research has gone into all of these methods; Machine-tool makers offer extensive advice in their catalogues as to which type of bond is best for which type of work to be done.

Bonds are not only used as wheels. They are also used as saw blades for roughing stone and marble blocks, shaping concrete, and so on. This is always treated as high-pressure work, and bonds are selected accordingly.