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.

A Lathe is a machine for revolving a piece of material so as to enable a cutting tool to shape it into a component of circular cross section or to perform a screw – cutting operation. The most widely used type is the center lathe, also known as the engine lathe (Fig.1), in which the work is held between centers or in a chuck. The rotational movement is imparted to the workpiece by the work spindle mounted in the headstock (at the left-hand end of the lathe in Fig.1).

The advancing adjustments movements of the slides can be performed by means of the so-called feed shaft, which receives its rotational motion from the work spindle. The feed shaft is provided with a worm which rotates with this shaft, but can slide longitudinally in relation to it. When the longitudinal feed motion is started (Position LF in Fig.3), the warm rotates the worm wheel, which in turn rotates the gear Z1 (Mounted on the same shaft). Z1 drives the gears Z2, Z3 and Z4, which engages with the rack that moves the saddle longitudinally. When the cross-feed motion is engaged (position CF in Fig.3), the gear Z2 – which can be swiveled – is brought into mesh with the gear Z5 instead of with Z3 and thus drives the shaft for moving the cross slide. The carriage can also be moved longitudinally by means of the lead screw, which is a long bar extending along the lathe and provided with a square screw thread. Two half nuts can be brought into engagement with the lead screw (Fig.2), so that the rotation of the latter imparts of longitudinal motion to the saddle. The lead screw is intended essentially for screw cutting.

At the opposite end of the lathe bed from the headstock is the tailstock, which can move along the guideways and clamped in any desired position. The center sleeve in the tailstock can be moved in the longitudinal direction of the lathe by means of a handwheel and screw spindle and can thus be brought toward the workpiece. The sleeve is provided with a taper socket to take a center or a boring or reaming tool.

With the belt drive (Fig.1) and the gear drive (Fig.2) the speeds of the two shafts are inversely proportional to the number of gear teeth. The shafts of a pair of meshing gears rotate in opposite directions. When an intermediate gear is introduced, the transmission ratio remains unchanged, but now the two shafts have the same direction of rotation. The cone-pulley transmission system given in Fig.3 permits selection of any of four different speeds by shifting the belt from one pair of pulleys to the next. This type of transmission is now virtually obsolete. In the gearbox (Fig.4) various pairs of gears are brought into mesh by shifting one of the two gears. As a rule, a whole range of gears is employed to provide a variety of speeds.

For instance, if three gears, each with two possible combinations with other gears, are employed, a total of 2 x 2 x 2 = 8 basic speeds will be available. Alternatively, the gears may be permanently in mesh and mounted loose on their respective shafts, but can be locked to the latter by means of multiple-disc clutches.

The outer element of the clutch forms an integral part of the gear and is provided on the inside with longitudinal grooves in which the outer discs can slide. When a coupling sleeve is actuated, the pack of disc is compressed and a frictional connection between the outer and inner discs is established, so that rotation motion is transmitted from one shaft to the other.

Adjustment of the speed of rotation to any desired value, so as to keep the cutting speed constant even when the cutting diameter continually varies, can be achieved by means of the PIV (positive infinitely variable) drive (Fig.1). This transmission system comprises a pair of radial-toothed conical pulleys on the driving shaft and on the driven shaft. A wide belt of special construction connects the two pairs of pulleys; it is provided with projecting elements which engage with the pulley teeth and thus provide a positive nonslip drive. Speed control is affected by the shifting of one pair of pulleys closer together and the other pair farther apart, and vice versa. In Fig.1a the speed of the driven shaft is lower than that of the driving shaft; the speed of the former is progressively increased by the shifting of its two pulleys farther apart, while those on the driving shaft are brought closer together; thus in Fig.1b the driven shaft rotates faster than the driving shaft.

Another type of variable-speed mechanical drive is shown Fig.2. The cone on the driven shaft can be shifted in the axial (arrowed) direction. Depending on its position, the contact diameter of the cone with the friction ring varies. The latter, which is driven by the cone, can perform a swiveling movement on the driven shaft, so that the friction surface is always in full contact with the driving cone. The transmission ration can be varied by shifting the cone axially to the left or right. When the cone is moved inwards (to the right in Fig.2), the speed of the driven shaft is increased, and vice versa. Fig.3 illustrates a so-called fluid drive.

Mounted on the left-hand shaft (the driving shaft) is the oil-pump rotor, rotated by an external power source (usually an electric motor). It rotates eccentrically in a movable casing. The space between the rotor and the casing is subdivided into compartments which increase and decrease in size consequence of the rotation of the rotor, so that oil is alternately sucked into them and then discharged into the inlet of the oil motor, whose rotor is mounted on the right-hand shaft (the driven shaft). The motor is very similar in construction to the pump. The oil delivered by the pump causes the rotor of the motor to rotate.

The greater the eccentricity of the pump rotor in relation to its casing, the higher the rate of delivery of the oil and the higher the rotational speed of the motor. When the rotor of the pump is shifted to the central position within its casing, delivery of oil ceases, so that the motor stops. When the pump rotor is shifted farther to the left (in Fig.3), the direction of flow of the oil is reversed, and the motor therefore also reverses its direction of rotation. The tank merely serves as a container for a reserve supply of oil. Speed control is therefore affected by shifting the position of the pump rotor.

Infinitely variable speed control by electrical- as distinct from mechanical or hydraulic– means is usually affected by means of variable–speed direct–current motors. A wide variety of machining operations can be performed on a lathe, requiring appropriate control of the speed of the work spindle and also of the feed – i.e., the advance of the cutting tool (Fig.1). For maximum efficiency it is necessary to adjust the feed correctly in relation to the size of the workpiece and the speed at which it rotates. For finishing cuts the speed is generally controlled by hand, but for roughing cuts and medium cuts an automatic feed is achieved by locking the saddle to the lead screw or to a separate feed shaft (if provided).

This shaft is geared to the work spindle to give the appropriate traverse motion to the saddle. Fig.2 shows a system of gears through which the feed shaft is driven by the work spindle. These gears can readily be exchanged to give various desired feed speeds. The reversing gear unit is shown in detail in Fig.4. The lever can be moved up or down to engage the required direction of rotation. When it is up, the transmission from Z1 to Z4 is effected through Z3 only; when it is down, the transmission is effected through Z2 and Z3, so that (as a result of the introduction of the additional gear Z2) the direction of rotation of the shaft II is reversed in relation to that of shaft I.

The Norton gearbox (Fig.3) contains a tumbler gear which is mounted on a movable lever and can be brought into mesh with any one of a number of other gears, permitting rapid change in the speed of the feed shaft. Another type is the driving-key transmission, in which a number of gears of varying diameter are fixed to a shaft and are permanently in mesh with gears that are freely rotatable on a second shaft. Sometimes the feed gear system on a lathe will comprise a driving-key system combined with a Norton gearbox and an additional sliding gear system similar to that for the main drive (Fig.4). The number of possible feed speeds will then be equal to the product of the number of possibilities provided by each of these transmission systems.