Has anyone else had problems with the bolt circle dimension for mounting a new chuck or chuck adapter plate? The CAD model clearly indicates 85mm bolt circle. I’ve 3D printed 85, 85.5, and 86 mm gauge plates, and none of those line up with all 5 holes on the microarc’s face. I became suspicious when building a plate to mount the add-on collet chuck vertically in my 1100 MX - the 85mm bolt circle was not the same as the bolt circle on the collet chuck.
Been a long time since I’ve looked at it or made any faceplates for mine, but I do remember running into exactly that problem.
Going back to my various faceplate designs, the 3-bolt pattern is a really bizarre 3.307”/83.947mm circle.
It’s probably to prevent any expansion of the chuck mounting plate from effecting the drive interface geometry. Some kind of physics magic. I’d be surprised if the mounting holes don’t have weird geometry in the threads too.
I think i resolved this. The bolt circle is at nominal 84 mm, with a 5 bolt pattern - 3 evenly spaced + 2 at 180 degree spacing; all appear to be M6. The 2-bolt pattern solved my measurement problem, as i could use 2 carefully measured socket cap screws and the distance across the heads of the cap screws - was within 0.1mm of 84 mm and a 84mm 3D print matches perfectly with 6.2mm clearance holes.
Interestingly, the 2-bolt pattern does not re-use one of the 3-bolt pattern, so this places two pairs of holes 30 degrees apart on one half-circle of the mounting ring. I will use these to mount my chuck, as it appears I can orient the chuck (67x105mm base) clearing all 4 holes … possibly.
Finally, on my Microarc, the inner location bezel is 72 mm, outer plate 100 mm, with both undersized by about 0.2 mm. A machined faceplate at nominal would be a bit loose (0.007”), so I need to decide if i live with that or cut the outside ring , center on lathe, and then incrementally bore the inner bezel to fit.
not sure if this is what you are observing, but I recently tripped over this. There are 2 sets of three holes at different radii - one for the 3 jaw chuck and another for the collet chuck. I discovered this while setting up a CAD file for the microarc as a fixture with both the 3jaw and collet.
I recently swapped between the collet chuck and 3 jaw and realized that my physical MicroArc indeed has 5 mounting holes - three spaced for the chucks and two that are 180 apart as described above.
There was something goofy about the CAD model I found, but that is irrelevant.