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Correct use and assembly of collets and toolholders

In this article we will discuss the correct use and assembly of collets and tool holders. Making sure that these basic procedures are followed is important in maintaining good results and preventing breakage.

Typical Toolholder & Collets Assembly


This image shows a typical toolholder with covernut and collet assembly. All these items have a direct bearing on the quality of your work and the performance of your tools. Assembly and maintenance of these critical components is paramount .


Remember that these items are under enormous and constant pressures and forces. They spin at high velocities, excert high axial forces, are subject to high heat buildup and vibration, so you must make sure that they are treated accordingly.


Failure to do so not only affects the quality of your work, but can lead to additional wear and tear on other parts of your machinery, such as spindle bearings and clamping systems.


Collets and toolholders must be rated to match your machinery. If you are setting spindle rotation speeds of 24,000rpm, collets and toolholders must also be rated to these speeds. Items that don't have matching speed ratings will invariably introduce vibration on to your cutting process as they are not usually balanced for these speeds.


How to assemble a collet

  • Select the correct collet for the tool shank. You should always select a collet that gives the least amount of compression to clamp the tool. e.g. Use a 5-6mm collet to grip a 6mm shank tool and a 4-5mm collet to grip a 5mm shank tool.
  • Make sure collets are clean prior to assembly
  • Check collets for wear, dents, corrosion or other damage. Replace if any of these are present.
  • Check collets for "bell mouthing" where the fron to the collet bore wears and prevents proper gripping or the tool shank.
  • Make sure covernuts are clean prior to assembly
  • Check covernuts for wear, dents, corrosion or other damage. Covernuts are especially susceptible to damage because of the action of tightening spanners. Replace if any of these are present.
  • Check the mating surface of the covernut and collet. These should be clean and blemish free.
  • Inspect the thread of the covernut. Make sure there are no sharp edges that can possibly cause the nut to seize on to the thread of the spindle or toolholder.
  • Make sure toolholder are clean prior to assembly.
  • Check toolholders for wear, dents, corrosion or other damage. Especially inspect the polished taper. If this is damaged, continued use will cause damage to the mating taper in the spindle. Replace if any of these are present.
  • Check the "pull stud" on the toolholder for excessive wear or damage. Replace as necessary
  • Double check all mating surfaces!


Correct Collet Assembly


The diagram above shows how to assemble a collet, toolholder and tool.


Collet Insert


Some collets, such as RDO types, require that the collet be inserted and removed from the covernut using a sideways action as shown here.


Insert one edge of the collet into the covernut and then lock it in to place by excerting sideways pressure.


To remove the collet the procedure is the reverse.


If you have these locking type collets you must ensure they they in fact do lock and hold the collet. If the collet can "fall out" when the nut is inverted, then either the collet or nut will need replacing.