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Metal lath12/17/2022 This conversion ratio is often built into the quick change gearboxes. Multiples of 3 and 7 giving a ratio of 63:1 can be used to cut fairly loose threads. To accurately convert from one thread form to the other requires a 127-tooth gear, or on lathes not large enough to mount one, an approximation may be used. The leadscrew will be manufactured to either imperial or metric standards and will require a conversion ratio to be introduced to create thread forms from a different family. For screw cutting, a half nut is engaged to be driven by the leadscrew's thread and for general power feed, a key engages with a keyway cut into the leadscrew to drive a pinion along a rack that is mounted along the lathe bed. Some lathes have only one leadscrew that serves all carriage-moving purposes. This ratio allows screwthreads to be cut on the workpiece without the aid of a die. This provides a constant relationship between the number of turns the spindle makes, to the number of turns the leadscrew makes. Tumbler gears (operated by H5) are provided between the spindle and gear train along with a quadrant plate that enables a gear train of the correct ratio and direction to be introduced. These intermediate gears allow the correct ratio and direction to be set for cutting threads or worm gears. Both the feedscrew and leadscrew (H7) are driven by either the change gears (on the quadrant) or an intermediate gearbox known as a quick change gearbox (H6) or Norton gearbox. These gears are located in the apron of the carriage. The feedscrew (H8) is a long driveshaft that allows a series of gears to drive the carriage mechanisms. In both instances the level is used as a comparator rather than an absolute reference. It is advisable also to use such a level along the bed to detect bending, in the case of a lathe with more than four mounting points. A precision level is a useful tool for identifying and removing any twist. There is no need to make the machine exactly horizontal, but it must be entirely untwisted to achieve accurate cutting geometry. When a lathe is installed, the first step is to level it, which refers to making sure the bed is not twisted or bowed. "V" and combination beds are used for precision and light duty work, while flat beds are used for heavy duty work. Types of beds include inverted "V" beds, flat beds, and combination "V" and flat beds. The leadscrew of accurate pitch, drives the carriage holding the cutting tool via a gearbox driven from the headstock. The carriage travels by means of a rack and pinion system. This is facilitated by hardened and ground bedways which restrain the carriage and tailstock in a set track. The bed is a robust base that connects to the headstock and permits the carriage and tailstock to be moved parallel with the axis of the spindle. A fully 'geared head' allows the operator to select suitable speeds entirely through the gearbox. Later machines use a gear box driven by a dedicated electric motor. On older machines ('50s) the spindle was directly driven by a flat belt pulley with lower speeds available by manipulating the bull gear. This end of the spindle usually also has an included taper, frequently a Morse taper, to allow the insertion of hollow tubular (Morse standard) tapers to reduce the size of the tapered hole, and permit use of centers. The spindle runs in precision bearings and is fitted with some means of attaching workholding devices such as chucks or faceplates. This reduces preparation and waste of material. The main spindle is generally hollow to allow long bars to extend through to the work area. The headstock is required to be made as robust as possible due to the cutting forces involved, which can distort a lightly built housing, and induce harmonic vibrations that will transfer through to the workpiece, reducing the quality of the finished workpiece. The headstock (H1) houses the main spindle (H4), speed change mechanism (H2, H3), and change gears (H10). Headstock with legend, numbers and text within the description refer to those in the image This helps ensure the components manufactured on the machines can meet the required tolerances and repeatability. Better machines are solidly constructed with broad bearing surfaces ( slide-ways) for stability, and manufactured with great precision. These machines consist of (at the least) a headstock, bed, carriage, and tailstock. The design of lathes can vary greatly depending on the intended application however, basic features are common to most types.
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