I need to cut a lot of perfect circles - 1/8" acrylic. I confirmed the file is perfect, but the cutting isn’t perfect - the shape is skewed. What do I need to do to make them perfect? I currently use the default settings for acrylic.
Check your belt tensions, eccentric nuts, and, v-wheels.
I would also check to see if the steps per inch are correct. You will need a few things to do this down to .001".
You will need a test dial indicator that measures to .0005"
A 123 block you can clamp to your table.
A second 123 block that is know to measure 123
Attach the TDI to you carrage and clamp one of the 123 blocks down to the table.
Now move the carrage in the X axis and just touch the 123 block zero the TDI.
Now tell your machine to move 2" away from the 123 block.
When it stops insert the other 123 block 2" side between the clamped 123 block and the TDI. Do this very carefully so you don’t bump the TDI and move it. The TDI should zero out if your steps per inch is correct.
Do this for both axis. If your TDI does not zero then you may need to make adjustments to you steps per inch in your software configuration.
If after all of this your steps per inch are correct then You are going to need to look for binding that may make your machine loose steps.
Other problems can be electric noise that is causing lost steps.
Hope this helps.
Dave
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I’ve not had much success cutting perfect circles either.
As an experiment, I cut a circle and an opening the exact size for it to fit in. When pushed in, there were inconsistent gaps around the circle at various points.
I would be interested to see if anyone has done the same experiment with success.
David
What material are you machining for your tests. One of the best material for test is machinable wax. It has no grain to mess with you and if you make a mistake in your Gcode it does not break end mills the way harder material does.
Dave
I’ve been doing some extensive acrylic milling over the last couple of days, and I’ve found that a high spindle RPM, medium-low feed speed, and a shallow depth of cut makes everything run like butter. Currently I’m milling down through a piece of 1/4" clear acrylic at 14,000 RPM, 35 inches per minute, and .010" DOC using a 2-flute spiral upcut 1/8" end mill. It’s making beautiful cuts and the bit’s not chattering, dragging or binding - that can easily lead to lost steps and miscuts as well.
Assuming your machine is square and the software settings are correct, try some cut settings like mine and see how things turn out. Your mileage may vary, etc. 
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having the same issue, haven’t been able to make a perfect circle. Kinda frustrating. Checked the belt tension, nuts and wheels. Even tweaked the pots that control the x & y steppers.
I’m thinking it might be the electronics, the x-carve’s wiring scheme methodology isn’t the best.
Scientific explanation. David the Acrylic Master. 
Alan
I don’t understand is that a question or are you just being sarcastic.
Dave
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Just compliment.
OK thanks. I sure hope I can get my current project done before the end of July so I can get started putting my machine together.
Here is a link to the engines running in the vessel.
Dave
http://www.rcgroups.com/forums/showthread.php?t=1497645&page=116
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David,
Over the last 1/2 of last week I read through about 1/2 of that thread. I think I’m at about 2 years ago. You had just gotten the tug around the pond. Very cool build, you are a true craftsman, and a very patient person.
The Preston belongs to Mike Jones (hookpilot) I just designed the engines, built them and now I’m getting them installed in the vessel. Mating the engines up to the wooden vessel has been a real challenge
Hope to have it on the test pond soon.
Dave
Thanks to everyone here for your help.
I got everything to work now, it was a matter of making sure EVERYTHING was tight, belts, eccentric nuts, other nuts, etc. We put some blue threadlock on the bolts and that seems to keep things perfect.
I’m routinely making production pieces now!! YAY!!!
Debbie
DeborahAjaffe,
What were the final settings and bit type did you find to work best for you?
Brad