Square Box not Square

So I calibrated the belt tension for ALL axis. Made a square box and lid. My square box not square 1.77" x 1.72".

What else to look for …

Did you calibrate for steps / mm?

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Yes, I cal’d in mm …

If the movement is consistent, then the problem comes to some aspect of cutting — climb vs. conventional milling, the machine getting pulled into, or deflected out of the cut, &c.

Leave a roughing clearance, take a finishing pass, and look at how the machine performs when cutting.

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This makes sense because on the inside corner (1 out of 4 corners), the bit digs in deep into the cut thus not making a straight line.

Will try that … Finish Pass.

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This is an interesting answer… you went through the process of entering a number that isn’t 40 into the mdi interface to set the calibration of belt stretch against length?

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It’s always the INSIDE corners, top right and bottom left of the square that the bit digs in to the side walls. I went back and checked the calibration and found it off. Redid the calibration and still the same issues. Placed the calibration back to 40 and still the same.

I currently do not have a stable table at this time so I am wondering if that is also my issue when the steppers change directions the table moves causing the momentum to dig into the corners.

Perhaps I need to try a slower speed and feed from the default with #201 bit.

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I saw the stock move … need more clamps on all 4 corners of the stock in order to keep the stock from sliding.

Newbie oops … learning curve.


Securing the stock fixed my issues.

My square box has a loose fit, how do I get a tighter fit between the upper lip and bottom lip or recessed?

Is there a way to add a correctional factor? The OD of the bottom lip is 1.445" while the ID of the upper lip is 1.484".

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I ended up increasing the bottom lip by 0.02" x 0.02". Now my square box and lid are a perfect fit.

Question: How to fix this 0.04" error?

Thanks to ALL that responded.

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You may want to check if maybe you don’t have too much tool deflection ? this can show up as slightly larger/smaller pieces than expected. I have covered it (not well enough) here, but first you could use @gmack calculator and figure out how much deflection you have for your setup/cutting parameters

Or if you don’t feel like learning about this worksheet just yet, just retry your cut with:

  • the endmill stickout reduced to a minimum
  • reduced depth per pass
  • or even better, a roughing pass and then a separate finishing pass

Also check for runout. Can you see your bit wobble when you manually spin the router? You can rig an indicator to measure runout if you want to really know.


In order for the machine to cut squarely we have to have a bunch of things lined up:

  • machine must be square
  • spindle and endmill must be plumb and have minimal deflection
  • stock must be level and secured in place so that it won’t move
  • toolpaths must match desired result for the selected endmill and its runout/deflection
  • feeds and speeds must match machine/material/endmill

It’s best to not assume anything when troubleshooting, and to start from the beginning and check each thing per https://docs.carbide3d.com/general-faq/machine-operating-checklist/

The great thing about CNC is that given a correctly prepared file, and a properly set up machine which matches the file, if nothing goes wrong in the cutting, a part will be made correctly.

The awful thing about CNC is that the file must be correctly prepared, the machine properly set up to match the file, and nothing go wrong in the cutting, in order for a part to be made correctly.


I have taken ALL of your advice and GREATLY appreciate your help and input.

In CC, how do you setup a final pass. I’m a bit confused here.

Do you create a tool path (1st pass) just short of the target, a 2nd tool path on target (final pass).

Disable all the final pass, save the g-code, run CM. Repeat for final pass.

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See: Adding geometry to cut as a pocket with a finishing pass

Newbie question: How does one actually edit the default Grbl code once the calculation to calibrate for belt stretch has been completed? Is this done in Carbide Motion or elsehwere?

Make sure your controller is communicating with CM.

In CM …

Settings Tab > Click “Open Log”.

This will open your browser and you will see the codes streaming. Note, you will have to scroll to verify the values for $100, $101 and $102.

Click on the MDI Tab

Enter the command $$ > click Send.

Look in the log and verify that $100, $101 & $102 are at a default value of 40.

Enter the following values in the MDI.
$100 = X (where X is your new calculate values)
Click SEND

Repeat for $101 & $102, verify on the Log that the new values have be entered correctly based on your calculations in mm (millimeters).

NOTE: CM will default back to default values if you ever change machine type by clicking “SEND CONFIG DATA”. This is in the settings Tab in CM. Type $$ in the MDI tab to verify your new values are current.

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Sounds simple enough, Gerard. Thanks.

For further details see: https://docs.carbide3d.com/shapeoko-faq/how-to-calibrate-the-machine-for-belt-stretch/

Thanks Will. That is a good article, but it was unclear to me how to actually edit the Grbl in Carbide Motion. Thanks to all that helped. Seems pretty clear now.:grinning:

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