My calibration still isn’t as close as I know it can be so I created a file today for some testing and calibration. This is a busy piece but served its purpose. Here’s what I cut with a 1/4" two-flute downcut spiral bit running 18k rpm and 125 ipm. The depth of cut is 0.1875" and it is cut with one pass leaving 0.005" on the side walls for a final clean up cut to get the fuzz off. The depth needed to be just deep enough to allow my dial calipers in for good measurement.

Calibration test - 8-11-18.jpg

I started on the bottom left, ‘Before’, and made my first cut. You can see it was out +0.005” on Y and -0.010” on X. I changed the settings in Mach4 and then ran the top left. Ignore the diagonals, those are to ensure I didn’t get too close to the screws. The circled dimensions are exactly as they’re supposed to be. I then ran the top right and it’s exact on Y and out +0.005” on X. The middle right was next and you can see it is exact on Y but out -0.002” on X. Bottom right is exact on X and +0.005” on Y (I wrote it wrong on the piece). The circle is very close, as well.

However, all of the inside circles are 0.015” to 0.025” undersize and I don’t get that. The outside squares, rectangles, and circles are close enough to be acceptable. I think the variance is due to the material which is a sort of soft Purebond plywood from Home Depot. If I was cutting hardwood I think the tolerances would be much closer.

So how do I get the inside circles to cut accurately? This makes it very difficult when I’m doing inlays and inserts, fitting dowels or bolts in holes, etc. I can’t change settings now or the other measurements will be off and I don’t want that. Is it something not set correctly in Fusion 360? Something in Mach4? It's not mechanical; the machine is a Saturn 2x4 from Fine Line Automation and is very rigid and tight and if it was mechanical the squares, rectangles, and outside circles would be off, as well.

Thanks!

David