Turning

Turning is the art of shaping various materials in the lathe. If a piece of wood or metal is rotated about an axis, while a cutting tool is held against its surface, portions will be removed until the material is reduced to a circular form. A lathe provides the means for giving this rotary motion to the work. In a modern lathe there is a steel spindle or mandrel, provided with a pulley to which motion is communicated from a fly-wheel driven by a crank and treadle or by steam or other power. This mandrel is fitted in bearings witht he greatest nicety, as it is of importance that it shall be able to revolve freely, but be quite unable to move in any other way. The end near the "nose," or screw to which the work is fixed by means of "chucks," runs in a steel collar, and has a conical shoulder to stop end play. In small lathes the other end is supported on a pointed screw, working in hole in the mandrel, while larger machines usually have a second collar, and a flat-ended screw, working upon the end of the mandrel. In either case the end-shake can be adjusted to be as small as possible. The mandrel and its supports - or the "headstock" - is secured to an iron bed, usually consisting of two bars of iron fixed together at the ends, and accurately made flat and straight. A back centre can slide along the bed and be clamped in any position; it has a steel point, adjustable by means of a screw, which can be used to support the end of long pieces of work. At rest, whose top is shaped like the letter T, cam also be fixed to the bed in any convenient position. A "chuck" is used to secure the material to the mandrel, and of these there is an endless variety, adapted to various classes of work. The speed of rotation is a matter of importance. For soft wood it can hardly be too rapid, while for metals it must be comparatively slow; and it is, of course, the peripheral speed which is of consequence, so that the speed of the mandrel must depend upon the diameter of the work. In order that the speed may be adjusted, grooves of various diameters are made on the mandrel and driving pulleys, and for heavy work a "back-geared" headstock is used. In this case the pulley runs freely upon the mandrel, and is connected to it by means of a counter-shaft and gear wheels, so that the speed is much reduced.