A lathe is a machine tool which spins a block of material to perform various operations such as cutting, sanding, knurling, drilling, or deformation with tools that are applied to the workpiece to create an object which has symmetry about an axis of rotation.
Lathes are used in woodturning, metalworking, metal spinning, and glassworking. Lathes can be used to shape pottery, the best-known design being the potter's wheel. Most suitably equipped metalworking lathes can also be used to produce most solids of revolution, plane surfaces and screw threads or helices. Ornamental lathes can produce three-dimensional solids of incredible complexity. The material is held in place by either one or two centers, at least one of which can be moved horizontally to accommodate varying material lengths.
Examples of objects that can be produced on a lathe include candlestick holders, cue sticks, table legs, bowls, baseball bats, musical instruments (especially woodwind instruments), crankshafts and camshafts.
History
The origin of turning dates to around 1300BC when the Egyptians first developed a two-person lathe. One person would turn the wood work piece with a rope while the other used a sharp tool to cut shapes in the wood. The Romans improved the Egyptian design with the addition of a turning bow. Early bow lathes were also developed and used in Germany, France and Britain. In the Middle Ages a pedal replaced hand-operated turning, freeing both the craftsman's hands to hold the woodturning tools. The pedal was usually connected to a pole, often a straight-grained sapling. The system today is called the "spring pole" lathe (see Polelathe). Spring pole lathes were in common use into the early 20th Century. A two-person lathe, called a "great lathe", allowed a piece to turn continuously (like today's power lathes). A master would cut the wood while an apprentice turned the crank.
During the industrial revolution, mechanized power was applied to the lathe via steam engines and line shafting, allowing faster and easier work. The design of lathes diverged between woodworking and metalworking to a greater extent than in previous centuries. Metalworking lathes evolved into heavier machines with thicker, more rigid parts. The application of leadscrews, slide rests, and gearing produced commercially practical screw-cutting lathes. Between the late 19th and mid 20th centuries, individual electric motors at each lathe replaced line shafting as the power source. Beginning in the 1950s, servomechanisms were applied to the control of lathes and other machine tools via numerical control (NC), which often was coupled with computers to yield computerized numerical control (CNC). Today manually controlled and CNC lathes coexist in the manufacturing industries.
Parts
A lathe may or may not have a stand (or legs), which sits on the floor and elevates the lathe bed to a working height. Some lathes are small and sit on a workbench or table, and do not have a stand.
Almost all lathes have a bed, which is (almost always) a horizontal beam (although some CNC lathes have a vertical beam for a bed to ensure that swarf, or chips, falls free of the bed). A notable exception is the Hegner VB36 Master Bowlturner, a woodturning lathe designed for turning large bowls, which in its basic configuration is little more than a very large floor-standing headstock.
At one end of the bed (almost always the left, as the operator faces the lathe) is a headstock. The headstock contains high-precision spinning bearings. Rotating within the bearings is a horizontal axle, with an axis parallel to the bed, called the spindle. Spindles are often hollow, and have exterior threads and/or an interior Morse taper on the "inboard" (i.e., facing to the right / towards the bed) by which accessories which hold the workpiece may be mounted to the spindle. Spindles may also have exterior threads and/or an interior taper at their "outboard" (i.e., facing away from the bed) end, and/or may have a handwheel or other accessory mechanism on their outboard end. Spindles are powered, and impart motion to the workpiece.
The spindle is driven, either by foot power from a treadle and flywheel or by a belt drive to a power source. In some modern lathes this power source is an integral electric motor, often either in the headstock, to the left of the headstock, or beneath the headstock, concealed in the stand.
In addition to the spindle and its bearings, the headstock often contains parts to convert the motor speed into various spindle speeds. Various types of speed-changing mechanism achieve this, from a cone pulley or step pulley, to a cone pulley with back gear (which is essentially a low range, similar in net effect to the two-speed rear of a truck), to an entire gear train similar to that of a manual-shift auto transmission. Some motors have electronic rheostat-type speed controls, which obviates cone pulleys or gears.
The counterpoint to the headstock is the tailstock, sometimes referred to as the loose head, as it can be positioned at any convenient point on the bed, by undoing a locking nut, sliding it to the required area, and then relocking it. The tailstock contains a barrel which does not rotate, but can slide in and out parallel to the axis of the bed, and directly in line with the headstock spindle. The barrel is hollow, and usually contains a taper to facilitate the gripping of various type of tooling. Its most common uses are to hold a hardened steel centre, which is used to support long thin shafts while turning, or to hold drill bits for drilling axial holes in the work piece. Many other uses are possible.
Metalworking lathes have a carriage (comprising a saddle and apron) topped with a cross-slide, which is a flat piece that sits crosswise on the bed, and can be cranked at right angles to the bed. Sitting atop the cross slide is a toolpost, which holds a cutting tool which removes material from the workpiece. There may or may not be a leadscrew, which moves the cross-slide along the bed.
Woodturning and metal spinning lathes do not have cross-slides, but rather have banjos, which are flat pieces that sit crosswise on the bed. The position of a banjo can be adjusted by hand; no gearing is involved. Ascending vertically from the banjo is a toolpost, at the top of which is a horizontal toolrest. In woodturning, hand tools are braced against the tool rest and levered into the workpiece. In metal spinning, the further pin ascends vertically from the tool rest, and serves as a fulcrum against which tools may be levered into the workpiece.
Accessories
Unless a workpiece has a taper machined onto it which perfectly matches the internal taper in the spindle, or has threads which perfectly match the external threads on the spindle (two things which almost never happen), an accessory must be used to mount a workpiece to the spindle.
A workpiece may be bolted or screwed to a faceplate, a large flat disk that mounts to the spindle. Alternatively faceplate dogs may be used to secure the work to the faceplate.
A workpiece may be clamped in a three- or four-jaw chuck, which mounts directly to the spindle or mounted on a mandrel.
In precision work (and in some classes of repetition work), cylindrical workpieces are invariably held in a collet inserted into the spindle and secured either by a drawbar, or by a collet closing cap on the spindle. Suitable collets may also be used to mount square or hexagonal workpieces. In precision toolmaking work such collets are usually of the draw in variety, where, as the collet is tightened, the workpiece moves slightly back into the headstock, whereas for most repetition work the dead length variety is preferered as this ensures that the position of the workpiece does not move as the collet is tightened, so the workpiece can be set in the lathe to a fixed position and it will not move on tightening the collet.
A soft workpiece (wooden) may be pinched between centers by using a spur drive at the headstock, which bites into the wood and imparts torque to it.
A soft dead center is used in the headstock spindle as the work rotates with the centre. Because the centre is soft it can be trued in place before use. The included angle is 60 degrees. Traditionally a hard dead center is used together with suitable lubricant in the tailstock to support the workpiece. In modern practice the dead center is frequently replaced by a live center or (revolving center) as it turns freely with the workpiece usually on ball bearings, reducing the frictional heat, which is especially important at high RPM. A lathe carrier or lathe dog may also be employed when turning between two centers.
In woodturning, one subtype of a live center is a cup center, which is a cone of metal surrounded by an annular ring of metal that decreases the chances of the workpiece splitting.
A circular metal plate with even spaced holes around the periphery, mounted to the spindle, is called an "index plate". It can be used to rotate the spindle a precise number of degrees, then lock it in place, facilitating repeated auxiliary operations done to the workpiece.
Modes of use
When a workpiece is fixed between the headstock and the tailstock, it is said to be "between centers". When a workpiece is supported at both ends, it is more stable, and more force may be applied to the workpiece, via tools, at a right angle to the axis of rotation, without fear that the workpiece may break loose.
When a workpiece is fixed only to the spindle at the headstock end, the work is said to be "face work". When a workpiece is supported in this manner, less force may be applied to the workpiece, via tools, at a right angle to the axis of rotation, lest the workpiece rip free. Thus, most work must be done axially, towards the headstock, or at right angles, but gently.
When a workpiece is mounted with a certain axis of rotation, worked, then remounted with a new axis of rotation, this is referred to as "eccentric turning" or "multi axis turning". The result is that various cross sections of the workpiece are rotationally symmetric, but the workpiece as a whole is not rotationally symmetric. This technique is used for camshafts, various types of chair legs.
Varieties
The smallest lathes are "jewelers lathes" or "watchmaker lathes", which are small enough that they may be held in one hand. The workpieces machined on a jeweler's lathes are metal, jeweler's lathes can be used with hand held "graver" tools or with compound rests that attach to the lathe bed. Graver tools are generally supported by a T-rest, not fixed to a cross slide or compound rest. The work is usually held in a collet. Three spindle bore sizes to receive the collets are common, namely 6, m/m , 8;mm and 10 m/m these dimensions refer to the straight shank body size of the collet. The term W/W refers to the Webster/Whitcomb collet and lathe , this lathe was invented by the American Watch Tool Company of Waltham Ma. The name Webster/ Whitcomb was the last names of the founders of that Company. Most of the lathes that are commonly referred to as watchmakers lathes are of this design. Most of the watchmakers lathes built anywhere in the world after the introduction of this lathe in 1888 were copies of this lathe design. In 1909 the American Watch Tool company introduced the Magnus type collet ( the 10 m/m body size collet)using the same basic lathe , originally as the Webster/Whitcomb Magnus. In 1918 the American Watch Tool Company was purchased by the former superintendent of the American watch Tool Company Frederick William Derbyshire , the names Webster/Whitcomb and Magnus are STILL trade names of F.W.Derbyshire, Inc. the company founded by Frederick Derbyshire in 1911 and these collets are still correctly produced ( nominal thread size 7m/m 6.95 m/m x .625 m/m pitch NOTE many poor quality copies reduce the pitch diameter and run 40 threads per inch) by F.W. Derbyshire, Inc. today Two patterns of bed are common: the WW (Webster Whitcomb) bed, a truncated triangular prism (found only on 8 and 10 m/m watchmakers lathes); and the continental D-style bar bed (used on both 6 mm and 8 mm lathes by firms such as Lorch and Star). Other bed designs have been used, such a triangular prism on some Boley 6.5 mm lathes, and a V-edged bed on IME's 8 mm lathes.
Lathes that sit on a bench or table are called "bench lathes".
Lathes that do not have additional integral features for repetitive production, but rather are used for individual part production or modification as the primary role, are called "engine lathes".
Lathes with a very large spindle bore and a chuck on both ends of the spindle are called "oil field lathes".
Fully automatic mechanical lathes, employing cams and gear trains for controlled movement, are called screw machines.
Lathes that are controlled by a computer are CNC lathes.
Lathes with the spindle mounted in a vertical configuration, instead of horizontal configuration, are called vertical lathes or vertical boring machines. They are used where very large diameters must be turned, and the workpiece (comparatively) is not very long.
A lathe with a cylindrical tailstock that can rotate around a vertical axis, so as to present different facets towards the headstock (and the workpiece) are turret lathes.
A lathe equipped with indexing plates, profile cutters, spiral or helical guides, etc., so as to enable ornamental turning is an ornamental lathe.
Various combinations are possible: e.g. one could have a vertical CNC lathe (such as a CNC VTL), etc.
Lathes can be combined with other machine tools, such as a drill press or vertical milling machine. These are usually referred to as combination lathes.
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