How Collets Work

The principle of collet workholding provides a powerful gripping force. As you take a slotted sleeve ( or bushing) with a tapered interior diameter and draw it up / over a tapered nose, this sleeve will expand on diameter.

Also, as it does this, it draws the part back so that it can be squared against a datum surface (workstop). This creates a wedging action where not only are you gripping on the diameter generating radial force but also on the back surface generating longitudinal force. By doing this you create greater grip and drive force.

Optimum conditions for this are a qualified bore or diameter and a good location surface. However variations in these conditions can be addressed.

MAXIMUM EXPANSION

The chart belows shows the maximum expansion figures for standard single taper collets, and the short and long series of standard double taper collets. While the below maximum expansion figures carry a small safety factor, and while all collets are made from high grade materials, they will be less apt to be damaged if the maximum expansion figures are not exceeded.

MAXIMUM EXPANSION CHART FOR STANDARD SINGLE AND DOUBLE TAPER COLLETS HARDENED STEEL
(Dimensions shown below are in inches)

LOAD CLEARANCE

For convenience in loading parts on chucks, the O.D. of any collet, when in the relaxed position, must be somewhat smaller than the low limit of the bore in the work part. The chart below shows recommended load clearances for the various size standard collets. In some instances where parts are to be loaded automatically, or for some other reason an unusual amount of load clearance is required, collets can, in most instances, be ground to meet the customer’s requirements.

LOAD CLEARANCE CHART FOR STANDARD COLLETS

Clearances shown above are for manual loading conditions.
When Autoloading of parts is taking place, it is recommended
that the factory be consulted regarding
required expansion to grip parts.

HOW TO SIZE A COLLET

When sizing a collet we need to be aware of the necessary Expansion and Load Clearance, as well as the need for Overtravel that is outlined above. There are limitations to these and how they are applied when we look at alternately slotted collets. For example, let us look at a Speedgrip No.ST-2 standard, single taper collet.

If this is to be used to chuck a part with a 1" bore with a tolerance of +.005" / -.000" . We know that Load Clearance is .003", Expansion required in the bore is .005" and Overtravel is .005". The total of these are .013". As we look at the maximum expansion of a No.ST-2 we see that it is .014" which means this collet can accommodate this part. If larger load clearance is required or part tolerance is wider overexpansion can result in breakage. Also if you sacrifice on your overtravel, grip force is compromised.

Speedgrip places various diameters of standard collets, that are left slightly oversize on the O.D., in stock. Upon receipt of an order, collets are drawn from stock and ground to suit the customer's requirements. Customers should have grip diameter and part tolerance available when placing orders.

COLLET SIZING FORMULA:

Load Clearance + Part Tolerance + Overtravel =/< Collet Expansion