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the discipline of the shop. Again, where a tool-room is kept, all tools being attended to by skilful mechanics ensures their accuracy in shape, rake, and temper. Lastly, because where tool fitters are employed, dupli

FIG. 188.-Examples of gearing change wheels.

cates of all tools are kept, so that there is only a minimum of time lost by the machines standing, i.e. simply while the tools are changed.

FIG. 189.-Group of tools ground.

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CHAPTER X.

GRINDING WHEELS AND MACHINERY.

THE old method of grinding by using wheels of grit or sandstone is gradually, but surely, giving place to the use of emery, or corundum wheels. It would be impossible even to refer to all the uses to which grinding wheels may be put, seeing they are alike indispensable in the tool room, the machine shop, and foundry fettling shops.

Special machines are made to carry these comparatively small discs, and most of them are made to work automatically and by hand. Fig. 190 is a grinding machine by Messrs. Smith and Coventry, Manchester. Fig. 190A shows a machine by Luke and Spencer, of Manchester. This machine carries a very hard grinding wheel, and is

FIG. 190.

used exclusively for sharpening tools. A universal grinding machine is represented on Fig. 191, made by Messrs. Brown and Sharpe, of America, details of this machine and some of its uses will be considered later.

Emery Wheels.-Rock emery stone, which is imported, is crushed by machinery. The finely divided grains are afterwards compressed with glutinous matter into a variety of shapes of wheels or bars to suit different purposes. The wheels are graded as coarse, medium, or fine; they are also classed according to their size, and the nature of the metal they have to work upon. Some are intended to work dry upon cast iron, brass, etc., while with others, made to work upon cutting tools and hardened portions of machinery, it is customary to use water. these were used dry, the steel tools would be immediately softened, and their temper destroyed. These wheels are much superior to grindstones, especially in small tool grinding. They cut more quickly and smoothly and their side faces may be used as well as their periphery. Screw-cutting tools, which would have to be softened and filed, can readily be ground with emery wheels and their temper and form maintained.

If

Another important use is the grinding of engine connecting rods after hardening; this will be treated later.

It is well known to all engaged in the manufacture of high-class

machines and instruments of precision that the lathe is incapable of producing accurate work even in the softer metals, and in operating upon hardened surfaces it fails altogether.

upon methods which will produce better work than the lathe. The only successful method employed at the present time to meet this want is to use the lathe as a roughing tool, to bring the work approximately

It is very important to have journals and other wearing surfaces smooth and true when they are finished. If they are imperfectly made their tendency is to become worse by wear. In fact, success in the manufacture of machine tools demanding accuracy depends largely

to the desired size, and then to finish by grinding with emery wheels in a suitably designed machine.

The introduction of hardened spindles (Fig. 192), bearings, etc., into

FIG. 191B.-Universal grinding machine, end view.

lathes, milling machines, drilling machines, etc., which are most valuable, would be impossible without the use of suitable grinding