Shapeoko 3 XXL not cutting to size

I know that this has probably been asked many times before, but I am not finding an understandable answer. I am try to cut a part to a specific size, but I keep coming out with a .010 to .012" undersize part.

I have cut a test coupon 3" x 5" and after cutting it measures .010" to .012" undersize. I then cut a coupon 6" x 12" and it also comes out .010" to .012" undersize. I have checked carbide create multiple times and it is programed for a .062" dia. bit. I have measured multiple times the actual dia. of the bit that I am using. Once again .062" dia. Carbide motion shows panel sizes of 3 1/16" x 6 1/16" and 6 1/16" x 12 1/16" respectively. I suspect I could create a bit size to adjust the outlines to the correct size, but that doesn’t seem to be the correct thing to do.

Can anyone tell me what I might be doing wrong?



Have you tried cutting a slot and measuring it?

Have you calibrated your belts for belt stretch?

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Multiple .125" slots are measuring .140".

I have looked at the referenced “Belt Stretch” article, but it doesn’t make sense to me. It says to machine a right triangle feature, but then talks about 3 holes. And then it talks about making changes to Grbl. How do I access Grbl? Is there a good video that shows the process.



Three holes at the points of a right triangle.

Grbl is accessed under the MDI tab in Carbide Motion when connected to the machine.

Being as the amount of error is the same even after doubling the size of the object, I wonder, “How are you setting your toolpath?” on the object line, or as an outside contour cut?

Have you taken a caliper to your endmill? They are often not to size.

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Will Adams-Thank You

MindlessCorpse-Outside contour

SLCJedi-Yes, size is correct

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This has a bit on it:

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Never take a caliper to the cutting edge of your endmill, aside from being unreliable, it damages the cutting edges. See this thread for more discussion, particularly from someone who knows a great deal better what they’re talking about than me.


Ladies and Gentlemen and Children in the Choir:

I have found the problem and the problem is us.

I took the time today, started with a clear head. No distractions and low and behold, I found the problem. I normally look for the simplist solution first when things aren’t going correctly. And typically 90 % of the time the simplest solution is the correct solution. Well, I checked and confirmed that the bit size in the program and the bit size in machine did not match. For some reason the bit size in Carbide Create was showing as .031, which is a size I never use and it isn’t even selectable in the end mill table. After I corrected the problem, all is well with the world.

I want to thank everyone for their help and suggestions. Although they weren’t the answer in this case, they have made me aware of how to fine tune my machine in the near future.

Again Thanks



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