Why are my CNC bits breaking?

Every so often, I’m asked the question “Why are my CNC bits breaking?” and then the process of elimination begins. There are an array of issues that could be causing this to happen. Here are the most common:

1. Incorrect feed/speed
2. Dull bit
3. Bad vacuum/hold-down
4. Debris in collet/tool holder
5. Bad collet

Incorrect Feed/Speed

First off, the feed/speed or feed rate and spindle speed are often the culprit. Bits can break at too high or too low of both the feed rate and spindle speed. It’s important to find the correct mix of feed/speed that meets your quality requirements. The easiest solution for this is to call me at (616) 502-7277. On a daily basis, I’m trouble shooting and offering starting points on how to cut a specific material for my customers.

Dull Bit

Most operators know this, but a dull CNC bit is far more likely to break than a new sharp one. Instead of the dull bit cutting through the material, the pressure is pushing through it until it no longer can. In this case, a smaller diameter tool will break before a larger one will. Pay attention to the cut quality and sound coming from the tool when a new tool is cutting vs. a used one. This way you can spot a dull tool before it breaks or sacrifices too much quality.

Poor Vacuum/Hold-Down

Bad material hold-down can very easily break a bit. Vibration or material movement is the enemy. The hold-down could be bad because of vacuum problems, lack of milling the spoil board, vacuum pressure leaking out the side of material, etc. Do whatever you have to do to keep material held in place. Some people use tape, down-spiral bits, or vacuum enhancers to further decrease movement. Again, do everything in your power to keep material held in place.

Debris In Collet/Tool Holder

If tool holders and collets are not properly cleaned, debris can build up and cause the tool holder to be off balance. When your CNC machine is cutting material out of balance, you face a decrease in tool life, spindle life, and cut quality. This can easily be a reason why your bits are breaking. Remember that collets and tools holders are consumables and should be replaced in a reasonable time-frame. See a recent article, I wrote, with more detail on tooling maintenance HERE.

Bad Collet

Like I mentioned above, collets are consumables. Depending on use, collets have a 90 day life. When you break a bit near the collet, they should also be replaced because they can damage the collet to where it no longer holds a bit correctly. Think of a collet as a spring. If a spring is compacted together for 90 straight days, it will no longer be the spring it once was. Therefore, your CNC bits are not being held as tight and balanced as they need to be. Find collets on our webstore HERE

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