I’ve been waiting to reply to this topic as I wanted to get as much data as possible but I’ve had a couple of PM’s from folks offering additional assistance and wanted to ‘clear the air’ so to speak.
@mbellon has been a huge help and I can’t thank him enough! Ultimately the cause was a few things being combined and I guess at this level of precision every little thing adds up to be a considerable amount.
The biggest issue was the inaccuracy of the end mills I’m using. Both the Onsrud 63-701 and 63-710 (one 701 is a .0625" and 710 is a .125") were slightly under the listed amounts. @mbellon contacted Onsrud on my behalf to get some details and the tolerance made up the amount that I was off. It is difficult to get a precise measurement from it since it has an odd number of flutes but after several measurements I figured out it was .115 which I later went up to .12 to compensate for a small wiggle I had in my part once it was milled. I additionally compensated for a slight inaccuracy for LEGO and adjusted from their spec’d 4.8mm to 4.85mm. Working at these small amounts is nuts but after a LOT of hours into this I feel good about having something that is repeatable.
I will now make a sample hole from time to time and get a measurement to see how much it is off and compensate for that. The same would go for wear on the end mill I’m using or for when I get a new one that may have a slightly different measurement but still within Onsrud’s tolerance.
I’m running an entire sheet at the moment so we’ll see how it goes. At some point I may very well make a custom fixture but for now I’m pretty happy.
It’s amazing with that little Nomad can do!
FWIW, I found that very shallow depths and higher feed rate offer a nice finish on the vertical wall. I’m doing depths of ~0.01 at ~14 IPM with nice results. I get the same accuracy by going a little deeper with a slower feed rate but the tool marks are much more noticeable. I’m sure this was stated elsewhere but you can never have too much info from lots of testing!