Thanks to this group

Making some parts for connectors and of course the holes were too small, smaller than I designed. Downloaded the machine code again, no difference. No play in the carriage, belts looked good. Finally measured the bit. Well the SPE 1/8" 0 flute bit was only 0.105 in diameter!

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You are not the first person to find that an advertised bit diameter is not what is advertised. Is it possible that you mixed up the bit? I used a conversion online and .105 is about 2.6MM. That would seem an odd metric bit but maybe the factory mixed up the bit or it was just machined wrong.

I didn’t mix it up. I had a second bit still sealed in the container, it was also too small

Usual preface, I’m with PreciseBits so while I try to only post general information take everything I say with the understanding that I have a bias.

Extra preface. I’m not sure what method you used for checking the diameter so this might not apply and I’m not calling you out. However, I wanted post this as most will think to use calipers or the like to check a tool’s diameter.

You can only measure a single flute tool with an optical system (swept diameter) or by the cut (sort of). They can’t be measured with calipers. This is because material has to ground away behind the flute or the unground material will rub/grind into the cut. Another way to put this is if a single flute did measure the cutting diameter from the edge of the flute to 180° from the edge it would constantly be slamming smooth carbide into the material before the flute cut again.

In general it’s hard to correctly measure any tool accurately with calipers as the cut diameter is always going to be dictated by the largest flute (from center). That’s on top of possible damage or the change in cutting diameter due to runout. Although, those aren’t the tools fault.

More on this if interested:
Actual size of the #102 endmill

To be clear I’m not saying that an incorrect sized tool isn’t your issue. Just that it’s hard to measure a single flute cutter. Probably the easiest way without special equipment is to cut a slot and measure the width of the cut. That will be added to by runout and substracted by material compression. But it’s a reasonable way to do it.

Hope that’s useful. Let me know if there’s something I can help with.


I understand that it is hard to measure the diameter of the bit. I went down this path because I was cutting a .63" hole in aluminum and it turned out to be only about .605. Going back and saying my bit was .105, the hole became big enough. SO, in my mind that was the issue.


That’s a more or less valid way to get there. It’s going to include the tool and machine deflection. But it’s almost certainly not all deflection as that would be ~0.0125" and if you had that much in aluminum it would almost certainly snap the bit. At the very least it would leave a horrible cut. If you did a spring/clean up pass conservative enough then it’s probably almost all the tool.

Just as clarification of my previous statement. I wasn’t calling you out. Just wanted to make sure that people reading this didn’t run to calipers to check something they couldn’t.