Small Hole Diameters

Little help please. I have calibrated my machine and the manual moves work perfectly. But when I cut 20mm holes with either Easel or Fusion360 the holes are only 19.5 to 19.7. The bit is a 6mm end mill (and is 6.02mm)
What can I check for?

I have calibrated x$100 and y$101 and z$102

The 96mm between holes is correct, but the holes should be 20mm, not 19.5mm. The 6mm end mill is cutting the correct width.

Most likely system deflection, as in the machine is not rigid enough to resist the sideway force of cutting completely.

I assume you have disabled “Rest machining” under the tool setup.

I would try a separate / finish pass with "2D Bore and adjsut offset to take up the difference, the Bore path will basically give you vertical cutting force instead of sideway force during 2D Pocket/Adaptive passes.

There is also the possibility that you bit is not exactly 6mm. End-mils are rarely exactly the size advertised.
I would suggest measuring the bit with calipers and inputting that number into Easel/fusion as your selected bit size, ru a test and see if that fixes or improves the quality of cut.

I’ve seen this issue with my X-Carve as well, holes are positioned properly, but the size is always just a little off. I spoke to a friend of mine who has been a machinist/tool and die maker for nearly 35 years regarding this issue, he thinks the machine may not be rigid enough to hold that tolerance. He said that there are 3 ways to fix it:

  1. Adjust your tool paths with regards to your end mill’s cutting behavior, be well aware that you may have issues running future jobs as you will have to test cut and adjust paths each time.
  2. Get an end mill/cutter/high speed drill that’s the correct diameter and drill it.
  3. Use a small end mill to create a guide hole, then use the proper sized drills in a drill press to drill thru.

I generally use option 3, as the price of a high speed carbide drill set is a bit out of my price range compared to regular drill bit sets.