A collet is a workholding device for bar stock or workpieces that must be held to a higher degree of accuracy than a standard chuck can accomplish. The device forms a collar around an object, and exerts a strong clamping force when tightened – usually by a tapered outer collar.

Hardinge, a machine tool builder, is also the world’s largest manufacturer of collet, with multiple designs for all brands of machines, including custom engineered solutions.

Hardinge detailed recently that collet blocks can be used horizontally or vertically on mill tables, jig grinders, flat or angled fixture plates, tombstones, or rotary trunnions. They can be grouped effectively, hydraulically or pneumatically, for high-production multi-part machining. Part gripping sizes range from 0.008 in. (1C collet) up to 6 in. (step chuck) in diameter. Hex, square and custom-shaped collets are available to hold a variety of shapes.

The company offers several models of collet blocks that use standard collets — 1C, 3C, 5C, 16C, 3J, 22J, 35J, and FlexC 42 and 6 5mm.

The diameter of a part or range of parts that will dictate the collet size required. The part size and the machining process that you will perform will determine the required gripping force. Higher gripping forces will be needed for heavy rotation, tapping, threading, or off-center work. Hardinge offers consultation to help machinists determine that size of a collet block needed, whether a high-force or low-force collet block is required, and whether hydraulic or pneumatic operation is recommended.

“Clamp only what is required,” according to Hardinge, “and be sure to operate at the pressure required for each individual machining application.”

Most shops prefer to use pneumatic-operated collet block systems, because the shop air is available and easy to connect, eliminating the need for plumbing accessories. If shop air is not available a small compressor will work as well. Filtered air is always recommended. A simple pneumatic arrangement will consist of a sub plate to mount the collet blocks on and a control valve to clamp and unclamp the part. Use one valve to clamp and unclamp all parts simultaneously or configure multiple valves to control each block or row of blocks independently.

Some Hardinge collet block models will plumb together with connector bushings for internal plumbing. Collet block models that don't use connector bushings will have to be plumbed externally.

Hydraulically operated collet block systems provide a higher clamping force. A hydraulic collet block arrangement will require a hydraulic source. If a machine has no hydraulic source, Hardinge offers an inexpensive air-over-hydraulic pump and accessories for a compact setup. Uses a 2-position, 4-way control valve to clamp and unclamp all parts simultaneously, or to configure multiple valves to control each block or row of blocks independently. The control valve would mount on the fixture along with the collet blocks and connect to the pump outside of your machine.