Inside and outside cuts don't match

Hi everyone, I have a wood worker upgraded to the 50 inch rail, I’m having a problem with cuts from inside to outside, I have cut a 6 inch circle on the inside and then cut the same circle on another piece of material on the outside and my thinking is the one circle should fit inside of the other cutout. It’s not and I don’t know why? I’m using carveco maker. I have not made any changes to the settings other than the the changes needed for the 50 inch upgrade, my machine is one the original buildbotics controllers and I have had the machine for several years and just now diving into using it. Total old newbie. any assistance would be great, thank you in advance.

Im not sure how far you’re out but you will need some tolerance if for no other reason than wood will move.

Hey Steve,

this is a frequently appearing phenomenon, however it can have many causes. Some were already discussed in the forum. A few possible causes that come into my mind would be:

  • Has your bit really the diameter that the specs say?

  • Workpiece moves, or is bent during milling (possibly due to clamping)

  • Wood dilates or changes its shape after some material has been removed by milling

  • Backlash of the ball screws

  • Temperature change under workload may affect bit diameter

  • Temperature change of workpiece under load (heat)

  • Bit torsion or bending under load (the longer the bit, the more bending of bit – always choose shortest possible bit)

  • Torsion or bending of the machine under load (the duller the bit, the more the torsion) – e.g. torsion of Z slider with milling motor (the more the spindle protrudes from Z assembly, the longer the lever, the more the bending under load)

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Thanks for getting back to me, the bit is a .25 compression bit I think is called a Jenny, I using ca and blue tape for hold down, material I cut the test on is .75 mdf, the tool path is set to cut .1 per pass so don’t think it’s bit torsion. I was wondering if it could be something in the post processor, I currently have it set for the inch/ no tool change bb ?

Hey Steve,

besides mechanical load depending on plunge rate, or other things like material, dullness, climb or conventional cutting – relevant for bit torsion or bending of the bit under load is the distance between the end of collet and the tip of bit, or better said, the absolute length of the cutting part (flutes), no matter how deep your plunge rate is.

I don’t know. But if you suspect it converts units wrongly you got to tell this to the post processor author.

I would track down all possible causes one by one.

People who had such issues often took longer to track the cause down :frowning:

Hi Steve, as usual, Aiph5u provided a ton of good info. How bad are we talking on this fit? Is it like the parts don’t even look like they’re supposed to fit together? Or is it like a ‘maybe I should try a mallet’ kind of fit?

I’d like to add a couple more that might not have been said explicitly.

  • are you cutting the id and od at nominal? If so, you will definitely need a bit of an offset for that to fit. Also on the profile cuts in carveco, there may be an unintended allowance/offset that was selected (hopefully not in the opposite direction).

  • in addition to the bit deflecting, it could be the squareness of the bit to your table and/or work. A slight angle on your edge faces could prevent a good fit.

  • how rough is your cut edges? Is there chatter or other noise? Could be an issue with the type of bit, or how it’s run (cut speed, rpm, chip load, spindle power, etc).


Maybe you should mirror one of your cuts because maybe the circle is not a perfect circle/ That is what we do when doing inlays.

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First I would to thank everyone for their suggestions and advice, I rewrote the file smaller,3 inches this time, and reran the cuts and they turned out exactly as they should, tight but it was what I was expecting out of the cut. Difference in cut was mirroring the vector instead of copy and paste to another model. not sure if that was the problem or not, exact same hold down method and bit, no difference in cut strategy. This is a great machine and is only limited by my knowledge and experience, that being said this forum is a great resource and I’m grateful for all the help! Thanks again