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Rh them put together, to mill out irregular curves and grooves, and so save a vast amount of labour formerly required from the fitters. Slide bars, piston rods, and many other parts are ground to size on grinding machines by means of emery or corundum wheels revolving at high speeds. The work is all done to limit gauges, and can be finished within 0·0005 in. if required.

When a large quantity of similar pieces are to be made, they are machined in “jigs” or “fixtures.” These are attachments bolted to the machine, and made to suit the particular pieces to be machined. The latter are secured in or by the jig, which locates their position in such a way that the machining is done accurately without preliminary marking-out of holes and contour lines. If only one or two engines of the same class are being built, it does not pay to make expensive jigs and fixtures, and the older methods of marking off and machining are then employed. The object of the modern improvements is to reduce the cost of the work, and at the same time to improve its accuracy and quality.

Pistons, and Piston Rods. The pistons are usually of cast iron, but are now frequently made of stamped steel, which has the advantage of enabling a lighter piston to be used owing to the greater strength of the material. The latest practice is to machine the pistons on a heavy turret lathe, in which a number of tools are fixed in a revolving turret. When one operation