Capstan Lathe

Overview

A capstan lathe is a small-part production lathe closely related to the turret lathe, but arranged for short, fast cycles on smaller bar stock. The difference is in how the turret moves. On a turret lathe the turret rides a saddle that traverses the whole bed; on a capstan lathe the turret sits on a short quick-return slide carried by a saddle that is clamped in a fixed position. The operator pulls a capstan wheel to feed the turret over its short stroke, then lets it snap back, and the turret indexes to the next tool. This short-stroke, quick-return action gives very fast cycle times on small components such as screws, pins, terminals, nipples, and bushings, which is what the machine was made for.

The machine is built on a compact Bed & Ways, with a Geared Headstock driving the bar spindle, a fixed Fixed Turret Saddle carrying the Quick-Return Capstan Slide and its Capstan Turret, and a Cross Slide & Tooling for forming and parting. A Bar Feed Magazine keeps the spindle supplied with fresh stock.

Headstock and bar handling

The Geared Headstock drives a hollow Bar Spindle bored to pass the bar, through a Speed Gearbox off the Main Drive Motor. Work is gripped by the Collet Workholding: a Draw Tube pulls a Spring Collet into its taper to clamp the bar concentrically, and a Collet Closer opens and closes it between parts. After each finished part is cut off, the Bar Feed Magazine takes over: a sprung Bar Pusher advances the bar through the spindle until it meets the Bar Stop on the turret, presenting the right length of stock for the next part with no measuring.

The fixed saddle and quick-return slide

The Fixed Turret Saddle is what distinguishes the machine. It is clamped to the bed by Saddle Clamps and does not move during the cycle, acting only as a rigid bridge for the slide. Riding on it is the Quick-Return Capstan Slide, a short ram driven by the Capstan Wheel through a Pinion & Rack. The operator pulls the wheel to feed the turret into the work, and at the end of the cut a Quick-Return Spring snaps the slide back. Because the stroke is short and the return is quick, the operator can run through six operations in seconds. The fixed saddle limits the machine to shorter parts than a saddle-type turret lathe, which is the trade made for speed and simplicity.

The capstan turret

The Capstan Turret holds six tools on its Turret Head, each face bored for a Turret Tool Holder such as a drill, a die head, a knee tool, or a box tool that turns a diameter. As the slide returns, the Indexing Mechanism rotates the head to the next station. A Geneva Cam produces the stepped motion and a spring Index Plunger seats each face accurately, so every tool returns to the same position. The sequence around the six faces is planned so the part is built up in one pass of the turret: center, drill, turn, thread, and so on.

Cross slide and parting

The Cross Slide & Tooling works the diameter while the turret works the end. A Front Tool Post holds a forming or chamfer tool, and a Rear Tool Post holds the parting blade that cuts the finished part off the bar at the end of the cycle. Forming tools on the cross slide let a complex profile be produced in a single plunge, which keeps the cycle short.

Stops, auxiliaries, and use

Repeatability comes from the Turret Length Stops, a Stop Drum with one Length Stop Screw per station setting how far each tool advances. A Lubrication System system oils the slides and headstock, a Coolant System system floods the cut, and the Electrical Control cabinet carries the motor controls and an operator Operator Pendant. Castings are gray iron, ways and the turret index parts are hardened. The capstan slide and its pinion and rack are hardened and ground because they cycle constantly, and the return spring is sized to snap the slide back briskly without slamming the stops.

The capstan lathe suits high-volume runs of small turned parts from bar, the kind of work once done in their thousands for fasteners, electrical hardware, and instrument components. Typical products are screws, studs, terminals, contact pins, gland nuts, and small bushings, each built up in one pass of the six-station turret and parted off by the rear tool post. The economics favor it wherever the part is small enough to fit the short stroke and the run is long enough to justify tooling the turret: setup takes time, but afterward the cycle is fast and the operator's role is reduced to feeding bar and watching the stops. The machine is the manual ancestor of the automatic screw machine, and it remains a clear illustration of how preset tooling, collet loading, and per-station stops turn a plain lathe into a repeatable production machine.