Evaluating your CNC Rotary Toolholders for Wear & Fretting

by Bernard Martin

A worn out toolholder will not provide good accuracy and will quickly wear out your cutting tools. Worn tooling can also cause poor surface finish, and may even damage your spindle.  Learning where to look and what to look for will alert you to potential problems before they become catastrophic. 

Taper Wear & Fretting

Check the taper for signs of wear or damage where it contacts the spindle mouth. Any problems with the taper will have a direct effect on machining accuracy. If there are any imperfections on the taper, the toolholder should not be used. ​

If noticeable marking is evident on the taper (see photo next page) a condition called fretting may be occurring.

Fretting happens when two steel parts (holder and spindle mouth) are rubbing against one another.

Typically, fretting starts just above the gage line on the toolholder which is the edge of the spindle

Fretting is caused by imperfect mating between tooholder taper and spindle, creating vibration and heat which develops the fretting. It is visible as small copper colored pits or marks on the taper. This is evidence that the toolholder is becoming worn. Fretting is easily mistaken for rust, but it is not. ​
​Once noticeable fretting develops the toolholder should be replaced. New toolholders that quickly develop fretting, or toolholders that stick in the spindle, may indicate a spindle that needs to be reground.

T.I.R. (total indicator runout) is the measurement of axial deflection of the cutting tool in the toolholder assembly. Techniks toolholders are manufactured to minimize runout and extend cutting tool life.

A worn taper causes an out-of-round condition when the holder is inserted into the spindle. This is called “runout”. Runout causes premature tool wear and excessive vibration during operation.
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This vibration can cause poor surface finish which may require additional operations to correct. Vibration can also cause heat build-up in the spindle and damage spindle bearings.

​See our video below to learn how to check toolholder T.I.R. “How to check Runout”.

This video will help you learn how to check runout (total indicator runout or T.I.R.) to check your CNC machine tool holders performance.

Good T.I.R. (less than .0002") keeps cutting tools cutting smoothly and prevents cutting tools from wearing out prematurely.

​Tool holders do wear out over time and if good T.I.R. cannot be achieved either the collet, nut, or holder may need replaced.