A metal lathe or metalworking lathe is a large class of lathes designed for precisely machining relatively hard materials. They were originally designed to machine metals; however, with the advent of plastics and other materials, and with their inherent versatility, they are used in a wide range of applications, and a broad range of materials. In machining jargon, where the larger context is already understood, they are usually simply called lathes, or else referred to by more-specific subtype names (toolroom lathe, turret lathe, etc.). These rigid machine tools remove material from a rotating workpiece via the (typically linear) movements of various cutting tools, such as tool bits and drill bits.
The design of lathes can vary greatly depending on the intended application; however, basic features are common to most types. These machines consist of (at the least) a headstock, bed, carriage, and tailstock. Better machines are solidly constructed with broad bearing surfaces (slide-ways) for stability, and manufactured with great precision. This helps ensure the components manufactured on the machines can meet the required tolerances and repeatability.
Headstock with legend, numbers and text within the description refer to those in the image
The headstock houses the main spindle, speed change mechanism, and change gears. 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 main spindle is generally hollow to allow long bars to extend through to the work area. This reduces preparation and waste of material. The spindle runs in precision bearings and is fitted with some means of attaching workholding devices such as chucks or faceplates. 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. On older machines (’50s) the spindle was directly driven by a flat belt pulley with lower speeds available by manipulating the bull gear. Later machines use a gear box driven by a dedicated electric motor. A fully ‘geared head’ allows the operator to select suitable speeds entirely through the gearbox.
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. This is facilitated by hardened and ground bedways which restrain the carriage and tailstock in a set track. The carriage travels by means of a rack and pinion system. The leadscrew of accurate pitch, drives the carriage holding the cutting tool via a gearbox driven from the headstock.
Types of beds include inverted “V” beds, flat beds, and combination “V” and flat beds. “V” and combination beds are used for precision and light duty work, while flat beds are used for heavy duty work.
When a lathe is installed, the first step is to level it, which refers to making sure the bed is not twisted or bowed. There is no need to make the machine exactly horizontal, but it must be entirely untwisted to achieve accurate cutting geometry. A precision level is a useful tool for identifying and removing any twist. 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. In both instances the level is used as a comparator rather than an absolute reference.(about abdulrehman)
Feed and lead screws
The feedscrew is a long driveshaft that allows a series of gears to drive the carriage mechanisms. These gears are located in the apron of the carriage. Both the feedscrew and leadscrew are driven by either the change gears (on the quadrant) or an intermediate gearbox known as a quick change gearbox or Norton gearbox. These intermediate gears allow the correct ratio and direction to be set for cutting threads or worm gears. Tumbler gears 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. This provides a constant relationship between the number of turns the spindle makes, to the number of turns the leadscrew makes. This ratio allows screwthreads to be cut on the workpiece without the aid of a die.
Some lathes have only one leadscrew that serves all carriage-moving purposes. 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.
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. 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. Multiples of 3 and 7 giving a ratio of 63:1 can be used to cut fairly loose threads. This conversion ratio is often built into the quick change gearboxes.
The precise ratio required to convert a lathe with an Imperial (inch) leadscrew to metric (millimeter) threading is 100 / 127 = 0.7874… . The best approximation with the fewest total teeth is very often 37 / 47 = 0.7872… . This transposition gives a constant -0.020 percent error over all customary and model-maker’s metric pitches (0.25, 0.30, 0.35, 0.40, 0.45, 0.50, 0.60, 0.70, 0.75, 0.80, 1.00, 1.25, 1.50, 1.75, 2.00, 2.50, 3.00, 3.50, 4.00, 4.50, 5.00, 5.50 and 6.00 mm).
Carriage with legend, numbers and text within the description refer to those in the image
In its simplest form the carriage holds the tool bit and moves it longitudinally (turning) or perpendicularly (facing) under the control of the operator. The operator moves the carriage manually via the handwheel or automatically by engaging the feed shaft with the carriage feed mechanism . This provides some relief for the operator as the movement of the carriage becomes power assisted. The handwheels on the carriage and its related slides are usually calibrated, both for ease of use and to assist in making reproducible cuts. Calibration marks will measure either the distance from center (radius), or the work piece’s diameter, so for example, on a diameter machine where calibration marks are in thousandths of an inch, the radial handwheel dial will read .0005 inches of radius per division, or .001 inches of diameter. The carriage typically comprises a top casting, known as the saddle , and a side casting, known as the apron .
The cross-slide (3) rides on the carriage and has a feedscrew that travels at right angles to the main spindle axis. This permits facing operations to be performed, and the depth of cut to be adjusted. This feedscrew can be engaged, through a gear train, to the feed shaft (mentioned previously) to provide automated ‘power feed’ movement to the cross-slide. On most lathes, only one direction can be engaged at a time as an interlock mechanism will shut out the second gear train.
The compound rest (or top slide) is usually where the tool post is mounted. It provides a smaller amount of movement (less than the cross-slide) along its axis via another feedscrew. The compound rest axis can be adjusted independently of the carriage or cross-slide. It is used for turning tapers, to control depth of cut when screwcutting or precision facing, or to obtain finer feeds (under manual control) than the feed shaft permits. Usually, the compound rest has a protractor marked in its base , enabling the operator to adjust its axis to precise angles.
The slide rest (as the earliest forms of carriage were known) can be traced to the fifteenth century. In 1718 the tool-supporting slide rest with a set of gears was introduced by a Russian inventor Andrey Nartov and had limited usage in the Russian industry. In the eighteenth century the slide rest was also used on French ornamental turning lathes. The suite of gun boring mills at the Royal Arsenal, Woolwich, in the 1780s by the Verbruggan family also had slide rests. The story has long circulated that Henry Maudslay invented it, but he did not (and never claimed so). The legend that Maudslay invented the slide rest originated with James Nasmyth, who wrote ambiguously about it in his Remarks on the Introduction of the Slide Principle, 1841; later writers misunderstood, and propagated the error. However, Maudslay did help to disseminate the idea widely. It is highly probable that he saw it when he was working at the Arsenal as a boy. In 1794, whilst he was working for Joseph Bramah, he made one, and when he had his own workshop used it extensively in the lathes he made and sold there. Coupled with the network of engineers he trained, this ensured the slide rest became widely known and copied by other lathe makers, and so diffused throughout British engineering workshops. A practical and versatile screw-cutting lathe incorporating the trio of leadscrew, change gears, and slide rest was Maudslay’s most important achievement.
The first fully documented, all-metal slide rest lathe was invented by Jacques de Vaucanson around 1751. It was described in the Encyclopédie a long time before Maudslay invented and perfected his version. It is likely that Maudslay was not aware of Vaucanson’s work, since his first versions of the slide rest had many errors that were not present in the Vaucanson lathe.
The tool bit is mounted in the toolpost which may be of the American lantern style, traditional four-sided square style, or a quick-change style such as the multifix arrangement pictured. The advantage of a quick change set-up is to allow an unlimited number of tools to be used (up to the number of holders available) rather than being limited to one tool with the lantern style, or to four tools with the four-sided type. Interchangeable tool holders allow all tools to be preset to a center height that does not change, even if the holder is removed from the machine.
Tailstock with legend, numbers and text within the description refer to those in the image
The tailstock is a tool (drill), and centre mount, opposite the headstock. The spindle does not rotate but does travel longitudinally under the action of a leadscrew and handwheel. The spindle includes a taper to hold drill bits, centers and other tooling. The tailstock can be positioned along the bed and clamped in position as dictated by the work piece. There is also provision to offset the tailstock from the spindles axis, this is useful for turning small tapers, and when re-aligning the tailstock to the axis of the bed.
The image shows a reduction gear box between the handwheel and spindle, where large drills may necessitate the extra leverage. The tool bit is normally made of HSS, cobalt steel or carbide.(about abdulrehman)
Steady, follower and other rests
A steady rest
Long workpieces often need to be supported in the middle, as cutting tools can push (bend) the work piece away from where the centers can support them, because cutting metal produces tremendous forces that tend to vibrate or even bend the workpiece. This extra support can be provided by a steady rest (also called a steady, a fixed steady, a center rest, or sometimes, confusingly, a center). It stands stationary from a rigid mounting on the bed, and it supports the workpiece at the rest’s center, typically with three contact points 120° apart. A follower rest (also called a follower or a travelling steady) is similar, but it is mounted to the carriage rather than the bed, which means that as the tool bit moves, the follower rest “follows along” (because they are both rigidly connected to the same moving carriage).Follower rests can provide support that directly counteracts the springing force of the tool bit, right at the region of the workpiece being cut at any moment. In this respect they are analogous to a box tool.
Any rest transfers some workpiece geometry errors from base (bearing surface) to processing surface. It depends on the rest design. For minimum transfer rate correcting rests are used.