Hi
I am new to cnc having just built my own.  I am using fusion 360 for cad and cam and Mach3. My machine is running great and cuts squares about as perfect as I can measure.   My problem is that when cutting a circle it’s cutting about 2/100 small. I am not leaving any “stock to leave”or any of that.  Any ideas on what could be happening?  Thanks in advance.
John

What is the smoothing and tolerances settings and are you use R (radius arcs in the pre post dialog) or I and J

Thanks for the help -

Smoothing .0004
Tol .004
Radius Arc is not checked and I do not see an I or J.

I am attaching the G Code file.
It cuts a 4x4 box and puts a 1" hole in the middle.
Totally appreciate the help.  I am going to recut it now.

John

Try doing it with the feed and speed reduced for the circle have it at 15 in/min 

Same problem.  Tomorrow I am going to try to run a 1, 2 and 3 inch hole and see the errors.  I'll let you know.

cut the circles realy slow like 5 in/min if it's still out run a Vcarve toolpath of 5 letters if this is out you have big problems or backlash

I made an adjustment to what i am guessing is a step correction?  I moved a fixed distance on the DRO and put in the difference.  It was not supper precise but I then ran a test of 4 circles, 1, 1.5, 2 and 3 inch all came to within 4 thousands.  I think if I work on the setting a little more i can get that even better.  I did run it at 5 in/min.  wow too slow.  I need a new bit, I plunged it in on another test way to far and fast and toasted it!  Nothing like learning.
Thanks for the help!