Help needed.... Trying to cut a perfect circle

So I thought I was performing a relatively simple task, however…

I am trying to cut round plugs that will be used to fill a hole created by a forester bit. The holes are anywhere from 7/8" - 1 1/4" in diameter. I noticed after cutting the plugs they are not perfectly circular. So my 7/8 (.825) plug has a diameter ranging from .846 - .896.

Does this sound right? I used a 1/4" end mill and left the default speeds and feeds.

Any suggestions would be appreciated.


Hi @joshdanamax

You should first check whether your belts are tight (enough), v-wheels too.
Then there is the possibility to calibrate the X/Y/Z movements (I have a section on this in the ebook) but this is usually not necessary, the “out-of-the-box” accuracy should be much, much better than 0.07", so a mechanical check is in order I think


I agree with Julien on checkiung the machine and making sure it’s all tight-and-proper

also have you seen How I Make REALLY Round Holes on the Shapeoko ? that sounds like very promising technique for what you want

another thing is that for max precision I would recommend first cutting “short” by say 0.5mm (0.02") and then do a 2nd pass just to shave off that last bit… that reduces forces on this critical part


The guys are spot on doing a mechanical check.

Dunno how often you plan on making these plugs, but I have an assortment of plug cutters, that really make quick work of it. You can knock out 50 in a few minutes, I like to cut a bunch of each size I use and just keep em in little bins. Even have one that cuts little wood balls,

Another thing to check once you get the mechanical issue sorted if you want tight tolerance fitment is the diameter of the endmill. Many come under sized or are metric sold as imperial.

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Cutting plugs with a CNC, to me, depending on the depth of cut there may be some deflection in the wood unless you keep material at the bottom of the stock for stability and cut the bottom off.
This is my brain on Retirement so I may be incorrect.

Although I thought I was incorrect once but was only mistaken. :sunglasses:

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OK… Thank you to all who replied… I have tried some suggestions and have some more info to share.

I checked belts and they did need to be tightened slightly which I did. I also checked V-wheels and they too needed adjustment. Also, One of them was crack which I replaced.

I did another test… I cut out a 1" plug (.5 radius) and left about .2 inch on the bottom of the black to make sure the plug itself was not loose as it was being cut. The result was just as bad as before tightening belts and adjusting v-wheels. The diameter ranges from .905 to .945. So… obviously not a perfect circle, but also not close to 1". Any thoughts on where to go from here?

The plugs I am looking to cut are 1"-1.25 inch in diameter and about 1" deep. Are you aware of cutters that can do this? I have the smaller ones that you would use on like a hardwood floor, but I haven’t seen large ones like described above. Ill check amazon now. :slight_smile:

One more mechanical thing to check is the set screws on the belt pulleys.

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Allright so if th belts and v-wheels are good now, next step is to check the X and Y movements without cutting anything.

Try and measure (with a caliper, or just a ruler) how far the machine actually moves when you jog along X it by, say, 10 inches

Do the same measurement while jogging it along Y axis a certain distance.

Also, double-check the value of your $100 and $101 GRBL parameters (by sending “$$” from the MDI command line, with the Log window open)

We’ll investigate from there.

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They have carbide tipped plug and tenon cutters from 3/8" to 3" in 1/8" increments. Definitely worth the $. Put them on a drill press


This looks perfect. Ill give them a shot… appreciate the help!

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Thank you Julien for sharing CNC A to Z, been buried looking into this for several hours today… going to the garage now …lol


@Julien is on the right track going toward belt calibration. If you’re at least cutting the desired shape (e.g. it’s 0.905in all the way around vs. an ellipse) then this is just a matter of:

  • calibrating your machine so that machined size = real world size
  • accepting the calibration offset and scaling your design (i.e. trying to machine a 1in circle yields 0.95in, so machine a 1 * 1/0.95 = 1.053in circle. (This is along the lines of what @fenrus posted with @RichCournoyer 's technique, and where I got the idea in the first place.)

I’d ask about work holding, thickness, and bit type as well. Up vs. downcut will produce different forces, and if you’re using tape + glue on a thick piece, you could easily be getting plenty of shift (though I’d expect this to produce an oversized part…).

Just some thoughts!


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