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Expanding Collets

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5c, 16c, AND 3J Expanding Collets

Dunham Tool manufactures high quality 5C, 16C, and 3J Expanding Collets along with other special sizes with diameter capacities of 1/4" to 6" and expandable lengths from 1" to 3". These Expanding Collets, patented in 1958, are machinable by the customer. The Dunham Tool expanding collet is designed for use with standard air collet or hydraulic cylinders or other collet fixtures. Expanding Collets hold on the inside diameters or inside dimensions of workpieces.
The Dunham Tool Expanding Collet provides an expanding member to locate and hold work from the inside, using your present 5C, 16C, and 3J equipment - lathes, milling machines, grinders, fixtures, etc. with no extra attachments. Accuracy within .0005" is not unusual on concentricity and the Expanding Collet enables you to hold close shoulder tolerances.


The Dunham Tool Expanding Collet is inserted into the spindle of your machine and it's threaded draw bar until the 5C or 16C taper seats. The conventional movement of the draw bar will expand and release the soft slotted member.

The soft mandrel may be machined to fit any I.D. within the range of the collet, i.e. from 1/4" to 6".

The solid taper on the collet, once seated, will not draw back into the spindle. Thus a machined shoulder will hold the width dimensions within .001". The Dunham Tool Expanding Collet may be removed and replaced in a machine without affecting its accuracy as with any standard collet. If accuracy to within .0005" is desired, a slight truing cut will suffice. The Expanding Collet can operate with the spindle moving.

Dunham Tool expanding collets may be transferred to any machine using standard 5C collets. Therefore, it may be switched from machine to machine for use in turning, grinding, machining, drilling, milling, etc., saving valuable time.

Features and Benefits

  • Requires no special adapters
  • Rigid one piece body / mandrel construction
  • Actuated by machine draw tube or collet fixture
  • Available from 1/4” to 6” diameter
  • Available from 1” to 3” expandable length
  • Machinable by customer to desired diameter
  • No draw back gives positive end length
  • Special expanding collet configurations available
  • Internal collet chucking
  • Economical and accurate


                           Construction Diagram for 5C Expanding Collet

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