Troubleshooting Bit Breakage

Many different things could cause a bit to break inside the collet. July 21, 2006

Does anyone have suggestions as to what may have caused the shank of a solid carbide spiral bit to shatter inside of the collet? The bit was a 1/2 inch diameter, 2 inch cut length upcut. We were cutting 250 ipm at 18,000 rpm. The cut was the first of three to go through a 1 11/16 thick oak panel.

Forum Responses
(CNC Forum)
From contributor M:
For that thickness of solid, I use either a 5/8" or 3/4" diameter 3 flute upshear or rougher. Your feed speeds may have been a little high for that depth of cut. With a cut length of 2 inches, you probably get more tool deflection than you want for a diameter that small. Depending on the vibration from the work piece, I would not be surprised by tool failure. Remember that the ability to hold the work piece rock solid without vibration is half the battle.

From contributor F:
Make sure that the spiral cut in the shank is not below the collet surface. Sometimes with longer bits, an operator may try to put it deeper in the collet, thinking there will be less tension, but this will leave a weak area. Also, your tool selection should work fine, as we routinely cut solid wood with a 1/4 upshear at 18,000 rpm and 200 ipm 3/4" at a time. However, there is no more than 1.75" total bit protruding from the collet.

From contributor B:
Is this a cutting process you've done successfully before and the bit failed this most recent time? Or is it the first time you've made a 1 11/16" deep cut in oak in this manner? I think some more specific guidance can be given if you let everyone know.

From contributor G:
I would say vibration. The collet could have been dirty, thus becoming unbalanced, or your work piece could have lost its hold down.

From contributor I:
It could simply have been a defective tool, too.

From contributor E:
Your rpms are too high and/or feed speeds too slow. I assume that it is an upshear. Slow the rpms down to 14K at that speed. Traditionally, if the tool breaks in the collet, the collect can be dirty with chips or simply worn, and a new one should be installed. The correct type of tool would be a necked down version. Meaning, most of the cutting edge should be engaged in the cut. This will assist the tool from vibration breaks, which has occurred.

From the original questioner:
Thank you all for your responses. I find all of your opinions helpful. I realize that we have many different issues to address. I think the main cause of this particular situation was a worn collet. It is possible there was a bur inside the collet that caused the bit to break when it was tightened.