Cutting accuracy issues

I’m new to the CNC and recently purchased a Shapeoko 5, 4x2. For whatever reason, the machine is not very accurate. I make small .5 to .75" diameter pockets and they always are too big by .015" to .025. I have also made a bit holder with rectangular slots and the are .020" bigger than i created.

Im positive i have the correct bit as it does it for 1/8" and 1/4" endmills. I have the carbide 201 and a whiteside RD1600. Im using the correct diameter as well

Any ideas on how i can make this machine more accurate?

What kind of spindle do you have? If it’s the palm-router type, have you measured the run-out on it?

Also, are you sure the machine has been trammed properly? An out-of-vertical bit will create a slightly oval shaped cut for a circle.

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It took me a while and i finally figures it out! In the bit set up screen there is an option called “finish allowance” and it was set to .015 for the 1/8" bit and .020 for the 1/4" bit. I put them both to zero and it works perfect.

The finish allowance setting is under the 3D section so i didnt think is pertained to me since im only 2D.


I thought those parameters only applied in 3D roughing toolpaths.

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The Finish Allowance in the 3D section should only apply to 3D roughing. And from my experience it doesn’t even get used. 3D roughing sets the finish allowance to 0.0197" (0.5mm) no matter what.

I just created a small pocket path using the default 0.012 finish allowance, and then edited the tool to use 0.000 finish allowance. The toolpaths are identical.

Have you calibrated the machine? If you move the tool a known distance, then measure it, did it actually move that exact distance? Check all 3 axes.