Circles not quite circle


We received our LongMill a few months ago, and I’ve been trying to learn some of the more basic software in order to use it. I’m an absolute greenhorn when it comes to CNC. Decided to start with a simple project: cutting out a series of small wooden tealight holders from pine. I drew them up on Carbide Create, and ran the gCode through UGS. The problem is, when the job was complete, the pocket holes and outer cuts weren’t exactly round. Using a very basic, single fluted 1/4" end mill. Not really sure what it is that caused it to cut wobbly like this, or how to go about fixing the issue.

Any help would be greatly appreciated!

@SKoutpost Josh: How are you holding the piece? Is it out of round throughout its entire depth?

Check the tension in all your wheels. Test on a piece of anything that is thinner - say 3/4".

Check your X and Y axis anti backlash devices. One, or more, probably aren’t tight enough. I had the same issue till I tightened them up a bit so that there was no more slop.


It’s an 11 3/4" x 24" board of 3/4" pine, screwed down to my wasteboard at the corners. I set the toolpaths so that it carves out the inner pocket to a depth of 1/2", then when those are done, it comes back and cuts the outer circle down to just nearly all the way through the board.

@SKoutpost Sorry, Josh. I somehow got the idea that your stock was thicker than that.
I’m with Paul. With the unit turned off, ensure that you have no movement in X and Y when you try to move the gantry and the router mount. If you have movement, tighten the anti-backlash nuts a small turn at a time.
Also, make sure that you have no fore and aft movement in your router. If the wheels on the X gantry are not adjusted properly, you will be able to rock the router back and forth from front to back. That will give you ovals instead of circles.


Found the culprit. Damn.

@SKoutpost Holy sh*&^%$t, Josh. What on earth caused that? They look like they have been chewed up and spit out. :worried:

I’ve no idea! I was testing the tightness of all the eccentric nuts and backlash bolts, noticed quite a bit of wobble on the X/Z, and some wobble on the router mount. Then I discovered a shattered wheel.