Lathe
A lathe is a machine tool that rotates a workpiece about an axis of rotation to perform various operations such as cutting, sanding, knurling, drilling, deformation, facing, threading and turning, with tools that are applied to the workpiece to create an object with symmetry about that axis.
Lathes are used in woodturning, metalworking, metal spinning, thermal spraying, reclamation, and glass-working. Lathes can be used to shape pottery, the best-known such design being the potter's wheel. Most suitably equipped metalworking lathes can be used to produce most solids of revolution, plane surfaces, and screw threads or helices. Ornamental lathes can produce more complex three-dimensional solids. The workpiece is usually held in place by either one or two centers, at least one of which can typically be moved horizontally to accommodate varying workpiece lengths. Other work-holding methods include clamping the work about the axis of rotation using a chuck or collet, or attaching it to a faceplate using clamps or dog clutch. Lathes equipped with special lathe milling fixtures can be used to complete milling operations.
Examples of objects that can be produced on a lathe include screws, candlesticks, gun barrels, cue sticks, table legs, bowls, baseball bats, pens, musical instruments, and crankshafts.
History
The lathe is an ancient tool. The earliest evidence of a lathe dates back to 4th century BC Egypt an is a depiction in the tomb of Petosiris. There is also tenuous evidence for a turned artifact at a Mycenaean Greek site, dating back as far as the 13th or 14th century BC.Clear evidence of turned artifacts have been found from the 6th century BC: fragments of a wooden bowl in an Etruscan tomb in Northern Italy as well as two flat wooden dishes with decorative turned rims from modern Turkey.
During the Warring States period in China,, the ancient Chinese used rotary lathes to sharpen tools and weapons on an industrial scale.
The first known painting showing a lathe dates to the 4th century BC in ancient Egypt. Pliny later describes the use of a lathe for turning soft stone in his Natural History.
Precision metal-cutting lathes were developed organically during the lead up to the Industrial Revolution, with many inventors contributing. They were critical to the manufacture of mechanical inventions of that period. There are many claims in the literature for the person who invented the precision lathe.
Some of the earliest examples include a version with a mechanical cutting tool-supporting carriage and a set of gears by Russian engineer Andrey Nartov in 1718 and another with a slide-rest shown in a 1717 edition of the French Encyclopédie. The slide-rest was a particularly important development because it constrains the motion of the cutting tool to generate accurate cylindrical or conical surfaces, unlike earlier lathes that involved freehand manipulation of the tool.
By the 1770s, precision lathes became practical and well-known. A slide-rest is clearly shown in a 1772 edition of the Encyclopédie and during that same year a horse-powered cannon boring lathe was installed in the Royal Arsenal in Woolwich, England by Jan Verbruggen. Cannon bored by Verbruggen's lathe were stronger and more accurate than their predecessors and saw service in American Revolutionary War. Henry Maudslay, the inventor of many subsequent improvements to the lathe worked as an apprentice in Verbruggen's workshop in Woolwich.
During the Industrial Revolution, mechanized power generated by water wheels or steam engines was transmitted to the lathe via line shafting, allowing faster and easier work. Metalworking lathes evolved into heavier machines with thicker, more rigid parts. 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, which often was coupled with computers to yield computerized numerical control. Today manually controlled and CNC lathes coexist in the manufacturing industries.
Design
The most common design is known as the universal lathe or parallel lathe. Other general designs include the frontal and vertical lathe, and others.Components
A lathe may or may not have legs, which sit on the floor and elevate the lathe bed to a working height. A lathe may be small and sit on a workbench or table, not requiring a stand.Almost all lathes have a bed, which is almost always a horizontal beam, although CNC lathes commonly have an inclined or vertical beam for a bed to ensure that swarf, or chips, falls free of the bed. Woodturning lathes specialized for turning large bowls often have no bed or tail stock, merely a free-standing headstock and a cantilevered tool-rest.
At one end of the bed 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 an interior Morse taper on the spindle nose by which work-holding accessories may be mounted to the spindle. Spindles may also have arrangements for work-holding on the left-hand end of the spindle with other tooling arrangements for particular tasks. end, or may have a hand-wheel 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 or gear drive from a power source such as electric motor or overhead line shafts. In most 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, to an entire gear train similar to that of a manual-shift automotive 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 sliding it to the required area. The tail-stock 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 types of tooling. Its most common uses are to hold a hardened steel center, 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 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 usually another slide called a compound rest, which provides two additional axes of motion, rotary and linear. Atop that sits 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 tool-post, at the top of which is a horizontal tool-rest. 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, 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. In the alternative, faceplate dogs may be used to secure the work to the faceplate.
A workpiece may be mounted on a mandrel, or circular work clamped in a three- or four-jaw chuck. For irregular shaped workpieces it is usual to use a four jaw chuck. These holding devices mount directly to the lathe headstock spindle.
In precision work, and in some classes of repetition work, cylindrical workpieces are usually held in a collet inserted into the spindle and secured either by a draw-bar, 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 preferred, as this ensures that the position of the workpiece does not move as the collet is tightened.
A soft workpiece 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°. 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 running or live center, as it turns freely with the workpiece—usually on ball bearings—reducing the frictional heat, especially important at high speeds. When clear facing a long length of material it must be supported at both ends. This can be achieved by the use of a traveling or fixed steady. If a steady is not available, the end face being worked on may be supported by a dead half center. A half center has a flat surface machined across a broad section of half of its diameter at the pointed end. A small section of the tip of the dead center is retained to ensure concentricity. Lubrication must be applied at this point of contact and tail stock pressure reduced. A lathe carrier or lathe dog may also be employed when turning between two centers.
In woodturning, one variation of a running 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 to a precise angle, then lock it in place, facilitating repeated auxiliary operations done to the workpiece.
Other accessories, including items such as taper turning attachments, knurling tools, vertical slides, fixed and traveling steadies, etc., increase the versatility of a lathe and the range of work it may perform.