Correct use and assembly of collets and tool holders

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.

This image shows a typical tool holder with a cover nut 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 are paramount.

Remember that these items are under enormous and constant pressures and forces. They spin at high velocities, exert high axial forces, and 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 tool holders must be rated to match your machinery. If you are setting spindle rotation speeds of 24,000rpm, collets and tool holders must also be rated to these speeds. Items that don't have matching speed ratings will invariably introduce vibration into 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 front of the collet bore wears and prevents proper gripping of the tool shank.
• Make sure cover nuts are clean prior to assembly.
• Check cover nuts for wear, dents, corrosion or other damage. Cover nuts 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 cover nut and collet. These should be clean and blemish free.
• Inspect the thread of the cover nut. Make sure there are no sharp edges that can possibly cause the nut to seize onto the thread of the spindle or tool holder.
• Make sure tool holders are clean prior to assembly.
• Check tool holders 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 tool holder for excessive wear or damage. Replace as necessary
• Double check all mating surfaces!

 

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

 

Some collets, such as RDO types, require that the collet be inserted and removed from the cover nut using a sideways action as shown here. Insert one edge of the collet into the cover nut and then lock it into place by exerting sideways pressure. To remove the collet the procedure is the reverse. If you have these locking type collets you must ensure 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.