Part mill size does not match design

[szavoda]

I have been having some issues where my cuts were oversized (for profile cuts) and undersized (for pocket). This is on an Avid 4896 PRO with a 8.5 hp spindle. Material is 3/4" ply. Bit is a 1/4" compression. Other than the size, the cuts looks perfect.

investigated calibrating the axis motors and made some minor adjustments on step units and backlash, but nothing major. After I had both X and Y dialed in perfectly, I ran tests. I cut two 3x6 in. squares from the same 3.4" ply with the same compression 1/4" endmill I was using before. The pocket test was almost dead on, but the profile result was 3.0325" x 6.0325.

The only thing I can imagine is that I am doing something wrong in the software. I have tried cuts with both V-carve and Fusion 360 with similar results.

Any ideas as to next steps to troubleshoot?

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[DonFrambach]

Yes!

I would first r/o a problem with stepper calibration. To do that, I would make objects of several sizes. You mentioned making a 3x6 inch test piece. I would make a 6x12 and even an 12x24 test pieces. If you errors remain .0325 inches, that would tell you that your steppers are properly adjusted.

to r/o backlash problems, I would approach a line from opposite directions and make cuts. If the lines are offset, then you have a problem with backlash. If the lines are even then you have ruled that out.

After ruling out above, I would look at software. One common error is to have offsets set incorrectly. Is it possible that you have a .0325 offset set in your profile cuts?

[szavoda]

After more reading, and testing, I think a lot of it might to do with machine tolerance / rigidity and my feed speeds. I was running at about 400 ipm for the tests that produced the results above. I dropped the IPM to 50 and saw a significant reduction. I am also going to look at the climb vs conventional cut to see what the deflection is.

If I find anything exciting, I will post here. Thanks!

[CNCRim]

I have the same problem but with Mach 3 and I trying to use cutter comp to bring it to size but that is where the machine act funny.