A lathe is a machine tool
, specifically designed to help in the manufacture of three-dimensional items which have circular symmetry
, and, as a special case, things which have circular symmetry, except for a helical pattern (such as a screw thread) cut along the axis of symmetry.
Lathes come in a variety of shapes and sizes, but common to them all are the following elements:
Tailstock with centre
Separately, a tool to cut the workpiece is also necessary.
The power unit is usually an electric motor, but in some old-fashioned machines could be a treadle powered by the feet of the operator. It is designed to generate rotational motion with a reasonable torque, which is subsequently transferred to the headstock.
The headstock is where the mechanical power comes out of the machine. It is often a pipe (called a mandrel) with a screw thread cut into it, although sometimes the pipe is not threaded, but has a long taper. The operator can attach a variety of workholders into the headstock, either on a thread, or on the taper joint. The thread is a much more secure fitting, but where the torque is low, or where a quick change mechanism is required, the taper fitting is often chosen. For a threaded fitting it is important that the workholder cannot come unscrewed during machining, and the thread direction is chosen to ensure that the chuck tightens on the headstock during machining, rather than the opposite.
The work holder is usually, but not always a chuck. A three-jaw chuck is used to hold workpieces which already have rotational symmetry, such as round or hexagonal bars. The distinguishing feature of the three-jaw chuck is that all three jaws move together in a radial direction as the chuck key is turned to adjust the jaw diameter.
Where the workpiece is irregular (such as a casting, or forging) then a four-jaw chuck is normally used. Each jaw can be adjusted independently of the others, allowing the operator to line up the rotational axis with a specific feature on the workpiece, such as a boss or bearing housing.
Before any cutting takes place, the workpiece must be supported at the end away from the headstock, ensuring that the workpiece does not move in the chuck during machining. This is where the tailstock comes in. It is a large solid lump of metal, fitted to the machine bed, and lined up precisely with the axis of rotation. It can accept a variety of centres. The fitting arrangement of tailstock and centre is similar to that between the headstock and chuck: it is either a screw fitting, or, more usually a taper-fit.
The centre can be a rotating centre, or simply a sharp point, called a static centre. Rotating centres are preferred where loads and torques are relatively high, but static centres can be used on wood or when turning small piece of aluminium, for example.
The bed connects the headstock and tailstock, and provides a rigid, secure reference point for the tool holder and other parts of the machine. If high precision work is needed, then the bed needs to be very rigid and dimensionally stable. For lower precision work, the bed can be lighter or less rigid.
The tool holder is usually fixed to the bed, but in woodworking lathes, it is often just a rest so that the operator can move the wood cutting tools around easily and conveniently. Wood working lathes therefore require more skill to drive than metal-cutting lathes.
The tool holder usually has a mechanism which allows the operator to move the tool radially inward, to remove more metal from the workpiece, and another mechanism to move the tool parallel with the machine axis, to create a cylindrical shape along the length of the workpiece. Moving these two mechanisms together allows any shape to be achieved.
For wood the tool is usually made of tempered steel. For aluminium, tool steel is preferred, but for steel, or other hard materials, the tool is usually made from a synthetic ceramic, such as silicon nitride. They are often in the shape of a triangle, with rounded corners, and each corner can be used as a separate tool. Ceramic tools remain sharp for a long time, and can be run very hot. The energy required to remove large amounts of steel from a big workpiece is very substantial, and the cutting area of the tool is typically only a few millimetres in size. All the energy of new surface creation is focussed in this small tool area, and it results in extremely heavy wear and high temperatures at the tool.
The rotational speed of the chuck is governed by the ideal linear cutting speed of the material. A large workpiece will have a slower rotational speed than a small workpiece of the same material. Each type of metal has its own ideal cutting speed.
Where complex shapes need to be cut, hand control is often inadequate, especially where many identical parts need to be manufactured. A new generation of machine tools has therefore emerged, called CNC (Computer-numerically-controlled) machines, in which the motions of the headstock, tool and other parts of the machine are all under computer control. The tool paths are usually worked out off-line (often with dedicated software) and pre-programmed into the machine controller.
Although the lathe is still used in many heavy engineering shops, and, together with the pillar drill, is the workhorse of the hobbyist, its use in general industry is declining with the introduction of so-called machining centres. These typically hold the workpiece in a chuck, but the tool is like a mill tool, which spins like a drill, but can be used to cut axially and laterally, like a router. The tool is held in a robot arm, and can be used to cut complex shapes and patterns in the workpiece.