GRBL MDI commands?

I am calibrating my SO3 to cut round circles instead of oval ones :/. I need to change the step size OF $100 and $101 can anyone tell me how to do this from the MDI.

Did you see these instructions on the C3D site ?
Once you determine the value you need, you just need to go in the MDI tab, type in
(this is an example, use whatever value you computed), and hit Send.
Do the same for $101.
These settings will stick (in the controller’s non volatile memory).


Hey Julien,

I finally figured this out. I want to override the default settings of 100 and 101. I enter the values I want and read back the log data and see the changes, but when I read back the config data the default setting are there so I am not really sure where i am effecting the change…how do I know my system config file is permanently changed?


Open the Log window. Go back to MDI and type $$ (then send) to print the current list of GRBL params. If you changed $100 or $101, you should see it reflected there.
Also, do NOT use the “send config data” button, this re-sends factory default GRBL params, it should only be done once upon setting up the shapeoko.
Going to sleep now, hopefully someone else will jump in with answers if you have questions


Sleep well, Thanks!!!

Hopefully you went through an X length and a Y length calibration test are are not just trying to make a circle round. If you dont do the calibration correctly, you might get that circle right and round but everything else you cut will be incorrect.

Hi Stacy,

I love taking the test first and failing then learning the lesson…paradox of life, hahaha!!!

I really enjoy getting better guidance from users than the vendor. Can you point me in the direction of the X length and Y length calibration procedure. I was given the belt tension procedure so that is what i am going through now.


Here you go. If you have trouble let me know. There may be some better guidance out there. Once you do it, it is just second nature. I check my machine every few months to make sure the belts havent stretched.

I have adjusted the 100 and 101 respectively and dimensions are consistent and pockets are square and circles are round. Square pockets better than I expected ±.002" That’s great!

The round pockets are very good ±.001 but the dimension is too big??? I went back and checked my CAD file and it is correct, cutter diameter is correct , tool comp is correct so I am at bit of a loss as to why linear interpolation is correct but circular interpolation is not.

Not sure how to proceed, any suggestions?


What do you mean by this? If I’m not mistaken, grbl doesn’t support cutter compensation.

What format is your CAD?
How big is too big? Are you measuring inside diameter or out?

If you give more detail, it’ll be easier to help diagnose the problem.

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Are you saying you are trying to cut say a 3" circle and you are getting consistently a 3.05" circle in every diameter across it? If that is the case you may be having some router collet runout. Give us your measurements of what you expected to cut and what you did cut to help a little more with diagnostics.


My cutter compensation offset , left /right / on center. I’m cutting pockets not bosses so these are inside dimensions. Too big is .040" on the round pocket. I use fusion 360 for design and manufacturing.

The GRBL issue has been resolved and unfortunately this topic is going off into a completely different one. Maybe I should just start a new topic?


I am cutting a 2" and 3" pocket. My measured dim is 2.04" and 3.04" across the XY axis. I also measured at several other locations and the pockets are very consistent, I’m actually very impressed.


I’ll put dial on the router taper and the end mill and check for run out. Excellent suggestion thank you.

Hi Nielferreri,

Tool comp is correct…I use fusion 360 to run all my design and manufacturing. Its NOT CREO so its very affordable for me. Anyway, I let fusion calculate tool comp for me depending on if I am cutting pockets or bosses.

So to make a long story even longer I just double checked tool diameter and tool compensation in fusion 360 and it was correct. I also checked my CAD file and I did in fact specify a 2" ID and 3" ID.


The standard router collet is for woodworking and not CNC production. You may want to buy some precise collets. They made a world of difference for me. I did have one Dewalt router that apparently the bearings were seriously out of round. I bought new bearings but never replaced them. Bought a new router instead and they have been fine. These are the ones I use.

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Hey Stacy,

Right on, thank you for the heads up on the precision collets. Just to be clear I am a non-atom splitter just tooling around in his garage, I do not do production CNC work.


Me either but I want things to be as close to accurate when I try to put them together. I hate to have to try to sand something to fit.

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