Characteristics of the headstock of a centre lathe

The lathe is a machine tool that can carry out different machining jobs: threading, cutting, straight turning, drilling, roughing and grooving workpieces of revolution. It works when the workpiece rotates, held in the lathe headstock or chuck, while the cutting tools approach its surface, removing swarf in the process.

How a lathe works

The lathe is a very versatile machine thanks to the large number of jobs it can do, which is why it is found in most machining workshops. But how does this tool work? The lathe has a headstock or chuck to which the workpiece is held, through the various clamping and tooling systems. It is what makes the workpiece rotate.

Then there are the cutting tools, suited to the different processes the lathe carries out. These are mounted on a carriage that travels along guideways, in a movement parallel to the axis of rotation of the workpiece (Z axis). On this same carriage there is another that moves in a radial direction to the workpiece (X axis). Some lathe models have a third slide or compound rest, which tilts to machine tapered workpieces.

Lathes can work many different materials, such as steel and cast iron among the hardest; they can also turn bronze, brass and even plastics such as nylon.

Parts of the lathe

The lathe is made up of four main assemblies:

• Bed, which supports the other parts of the lathe. It consists of guideways along which the main carriage and the movable headstock or tailstock travel.
• Lathe headstocks. The lathe has a set of headstocks, the movable one and the fixed one. The fixed headstock is permanently attached to the bed on the left-hand side. This is where the spindle is located, which turns through a pulley system. It is responsible for holding the workpiece and making it rotate.
• Tailstock, made up of two parts. The lower part sits on the guideways and the upper part sits on the first. These parts are adjusted with screws. This is where the tailstock spindle is located, along with the live centre of the lathe.
• Carriage assembly. This is the part that moves and controls the various cutting tools. The carriage is made up of five parts: the saddle, which slides on the guideways; the apron, attached to the saddle; the cross slide, which sits on the main carriage; the compound rest on the cross slide; and the tool post.

Types of lathe headstock

Movable headstock

The movable headstock or tailstock of the lathe is made up of two parts. The first is the support, placed on the guideways of the machine, along which it can be moved or locked. The other part sits on top of the first and houses a spindle operated by a crank to carry out the longitudinal movement of the tailstock. The spindle applies the right pressure on a blind tapered hole called the centring point.

This lathe headstock has the particular feature that the tailstock can be moved and locked in different positions and can support the outer edge of the workpiece to be machined. The handwheel of the headstock allows the tailstock to advance or retract on the body of the headstock.

The movable headstock is used for the machining of large workpieces, but it also acts as a toolholder, where various cutting elements, such as drill bits, are stored.

The movable headstock is made up of the following elements:

• Pedestal
• Locking nut
• Handwheel
• Barrel
• Sleeve
• Sleeve set screw
• Tailstock centre

Fixed headstock

This lathe headstock, called the fixed or speed headstock, as its name suggests, does not move, but is permanently attached to the left-hand side of the bed of the machine. The main function of the headstock is to hold the workpiece and make it rotate for the machining process. The fixed headstock is made up of a series of gears or pulleys that drive the workpiece and the other feed units; the fixed headstock contains a motor, a spindle and three different selectors: speed, feed unit and feed direction.

In the fixed headstock, the spindle turns through a system of gears and pulleys. In addition, a series of clamping systems and tooling are added to it to hold the workpiece correctly.

Depending on the speed change used, the fixed headstock can be:

• With a pulley group. Used less and less on modern lathes, it is made up of a set of pulleys of different diameters.
• Single-pulley, where the movement is transmitted from the motor through a single pulley and the spindle speed is obtained through gears or variable-speed drives.
• Direct drive through the motor, which has an internal variable-speed drive and is able to transmit the movement to the main spindle.