Milled holes undersized - any ideas?

Using ESTLCAM, from a dxf file produced with Autodesk Fusion 360 I am trying to mill out a set of holes (circles).

The holes have a diameter of 10mm and i’m using a 6mm bit - which i’ve checked the size using calipers. When I measured the holes they consistently came out to 8.2mm.

With ESTLCAM I am telling it to mill out the inside of the hole with allowance set at 0mm. The spacings between the holes is perfect and they are (roughly) round. I am a bit confused as to why the 1.8mm undersize, any ideas?

If your z axis is really long it could be flexing, depending on your milling speed.

With further investigation today, I think it could be Z axis linked. It decided to drop down during a carve completing the rest of the cut much deeper. I might try to take the Z axis apart to investigate. Anything to look for?

Thanks for you help by the way as well!

Move your work piece up, make the z axis as absolutely as short as possible.

Ok, thanks. I think i might actually trim the feet next weekend to lower the machine even more. I’ll try the updated firmware and repetier settings for the z drop as well.

You can try it out by just raising up the work piece. It isn’t as good as making the machine shorter but pretty close. How long is your z axis now? You don’t need to change the firmware for size change.

My Z axis is the standard start size. It does flex a lot less lifted up. I took the Z axis off and cleaned/lubed - seems to be much smoother. Now just need to tweak the square a little. Thanks for your help, such a brilliant project and machine.

So I finally got time to use the machine again, and decided to make a test piece to make one piece of wood (pine) fit into another. The outline of the pine cut out ok, although the milled pocked is not square (as per ESTLCAM view). I also noticed one side of the cut out wood was a bit wonky, rather than a nice clean cut. I raised the work piece up, and it didn’t seem to be struggling to cut.

I was using a 6mm bit, 4m DOC, 12mm/s feed speed, 45%, 180 degree and 700w router - anything stupid about those settings? I’ve attached some photos to illustrate.

The ESTLCAM view (wouldn’t allow me to upload both)

Pretty big bit, typical is 1/8" (3.17mm), Machine size and Z axis length play a large Part as well.

Those settning are what I use for a 24" machine with a 4" z axis and an 1/8" bit. You might want to change it up a little, either slower cuts or shallower Depth of cut.

Ok, I will try with a 2mm bit. I think in inches my machine is 30 x 42ish. I still need to further drop height. In general a smaller bit will load up the axes less?