Could be some backlash in your X and Y axes. Since you said home built we don't know if your axes are belt driven or screw driven or gear driven the kind used would make a big difference.
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That would be handled in Fusion 360, not in Mach3, but the same principle applies there.unfortunately i do not use mach3 to tell you where that is, but i am sure there is a tool page where the tool diameter can be entered. have your tried the same box test with a different tool? is there an allowance or offset in the program?
TimPa You nailed it. I cut two grooves and measured .216” with a 1/4” spiral down cut bit. I had measured the bit and a drill hole previously at .25” Changed my tool diameter in F360 and got accurate results. Thank you for solving this mystery!besides all of the great responses above... the best way to calculate and adjust for actual tooling dimensions is to perform a straight line cut, preferrably in the same material you are using, and the same feeds and speeds. measure the width with calipers as accurately as possible. then enter that value for that specific tool cutting diameter.
unfortunately i do not use mach3 to tell you where that is, but i am sure there is a tool page where the tool diameter can be entered. have your tried the same box test with a different tool? is there an allowance or offset in the program?
im glad it worked out for you. i just repeated something that i have read tho, no credit here...TimPa You nailed it. I cut two grooves and measured .216” with a 1/4” spiral down cut bit. I had measured the bit and a drill hole previously at .25” Changed my tool diameter in F360 and got accurate results. Thank you for solving this mystery!
Thanks everyone for the good comments. I have been doing additional troubleshooting. I declared victory too quickly. I changed my spiral downcut bit to straight flute and measured the cut. It is 1/4". Ran additional tests and came up with similar issues in both axis. Both bits are Whitehead solid carbide. I would like to still rule out that the issue is not with F360. Could someone be so kind as to share a G code file that they created with a simple square recess(say 6x6"x1/2" deep grooves in a 12X12"x 3/4" stock). That way I can confirm it's the machine and not the G code. Thank you. SteveI'm a bit late to this and it sounds like you solved your problem but there is another possibility. I would check my F360 CAM setup - there is a "stock to leave" option that is on by default (at least, it was a year ago). It leaves a little stock to allow for a light finishing pass. Also, it is hard to accurately measure a groove for width and 34 thou is waaay off. Even bits made of Chinesium aren't usually that bad. I think (based on what you said) your original test was an inside cut. Have you tested an outside cut?
I think you're missing the point, Doc. I'm not having an issue with my machine; Steve is having an issue with his. He's using Fusion 360 and asking questions about accuracy.that could be a source of your problem.