BONDED WHEELS AND THEIR APPLICATIONS

In this section we shall only briefly indicate some of the types of bonded wheels made and some of their applications in industry. Indeed, applications are so numerous that there exists an extensive literature dealing with this alone - with the theory, the practice, the speeds of operation, general utility, and with the economics of use.

The history of the development of bonded wheels is curious. It is generally stated that they first came on the market in 1932, but a type of bonding is far older than that. Already, as we have seen, Pliny in a.d. 77 was describing a kind of

bonded tool. Long after this, indeed early in the 19th century, there are also exact descriptions in the literature of their use. Pritchard, who in 1824 described the manufacture of Diamond lenses, ground these by beating coarse Diamond particles with n punch into a hemispherical iron cup. He used various grades of Diamond for different finishes, and his tool was certainly a kind of bonded wheel. By 1840 it was common practice to hammer Diamond grit into a copper disc (a penny!) and use this for shaping sapphire plates for watch bearings. By 1870, dentists were using Diamond-impregnated drills for rapid boring of teeth.

Thus, even if bonded wheels did not come on the market in a big way till 1932, they are certainly much older than that. The early modem bonded wheels were not made by beating powder in, but by transforming an intimate mix of Diamond grit and carrier material into a solid wheel. The diamondiferous material was usually concentrated round an edge region only. Soon after, wheels were made by powder-metal techniques. The Diamond powder is mixed with a metal powder (say iron, copper, cobalt) and this, when heated under pressure, 'sinters' lo a solid mass. These wheels have long economical life. Two developments following this were the art of what is called vitrified bond and the later extensive use of Diamond bonded by sintering into a hard carbide.

The most recent type of bond involves a coating called metal ‘cladding’. This is used on complex-shaped tools. The Diamond grit is applied to the outside of the tool and then this is coated over with, say, nickel. The coating is thick enough to help the grit bind to the base metal. Metal cladding is considered by some technologists to be an important advance. If the Diamond is clad first, then when it binds in the matrix die metal film assists a tighter binding through some chemical action. It is also surmised that metal on the surface of the grains assists heat conduction; and the cooler the Diamonds arc kept, the better they function. All in all, it is reckoned by some that performance and lifetime of wheels can be doubled by introducing metallic cladding.

The bonding techniques are used over an exceptionally wide range of tools. Indeed, the number of bonded tools made, of different kinds, run into many thousands. Whole ranges of drilling tools, cutting tools and files are available. Prices are not excessive, depending on tool size and concentra­tion of Diamond.

Recently, bonded wheels have been used in connection with profile grinders, a subject which will occupy us in the next chapter.