Introduction of Lathe Machine
Lathe machine is known as the mother of machine tools, as it was the first machine tool that led to the invention of other machine tools.
A lathe machine is a machine tool that rotates a workpiece around axis of rotation to perform various operations like threading, cutting, sanding, knurling, drilling, deformation, facing, and turning, with tools that are applied to the workpiece to create an object with symmetry about that axis.
Lathe machines are used in woodturning, metalworking, metal spinning, thermal spraying, parts reclamation, and glass-working. Lathes are also 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. Lathes can produce three-dimensional solids of incredible complexity. The workpiece is usually hold in between one or two centers, . Other work-holding methods include clamping the work about the axis of rotation using a chuck or collet, or to a faceplate, using clamps or dog clutch.
Now a days manually controlled and CNC lathes coexist in the manufacturing industries.
Components of lathe machine
Lathes usually have a bed that is a horizontal beam. In woodturning lathes designed for turning large bowls, the bed and tail stock usually do not exist. Instead, the headstock stands free and the tool rest is cantilevered.
It has a headstock at one end. It has precision spinning bearings on the headstock. Within the bearings is a horizontal axle with an axis parallel to the bed, which is called the spindle. Spindles are hollow and have internal Morse taper on the nose so that work-holding accessories can be attached to the spindle. A spindle may have work-holding arrangements on the left hand end of the spindle along with other tooling arrangements, or may have a hand-wheel or other accessory mechanism on the outboard end. The workpiece is moved by spindles that are powered.
Foot power is applied to the spindle through a treadle and flywheel, or through a belt or gear drive from a power source such as a motor or overhead line shaft. In most modern lathes, the power source is an integral electric motor, normally either in the headstock, to the left of the headstock, or beneath the headstock, concealed in the stand.
Headstocks often include components that convert motor speed into spindle speed. Different types of speed-changing mechanisms are used for this, such as cone pulleys or step pulleys, cone pulleys with back gears, or gear trains similar to those in conventional automobile transmissions. The speed of some motors can be controlled electronically via a rheostat, eliminating the need for cone pulleys or gears.
The tailstock, sometimes called the loose head, can be positioned at any convenient place on the bed by sliding it to where it is needed. Rather than rotating, the barrel of the tailstock slides in and out parallel to the axis of the bed and directly in line with the spindle of the headstock. In most cases, the barrel is hollow and contains a taper to facilitate the gripping of various types of tools. Common applications include holding a hardened steel center, allowing long thin shafts to turn, or holding drill bits for drilling holes axially into a work piece. However, they are often not the only uses for these plates.
Metalworking lathes have a carriage (consisting of a saddle and apron) and a cross-slide, a flat piece that sits crosswise on the bed and can be cranked at an angle to the bed. There is usually another slide that sits atop the cross slide, called a compound rest. This slide provides two additional axes of motion, rotary and linear. A toolpost sits atop it, on which a cutting tool is mounted, which removes material from the workpiece. There may or may not be a leadscrew that moves the cross-slide along the bed.
There are no cross-slides on woodturning and metal spinning lathes; instead, there are banjos, which are flat pieces that sit crosswise on the bed. It is possible to adjust the position of a banjo by hand; no gearing is necessary. 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 of Lathe Machine
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. Alternatively, 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 jaw universal -or four-jaw independent chuck. For irregular shaped workpieces it is usual to use a four jaw (independent moving jaws) 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.
Running center (top); dead center (bottom)
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 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 (stationary) 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 will 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, 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.