Hi,

As I'm new here, I'll introduce myself first.
My name is Maarten, 31 years old, and I'm planning to build the MomusCNC machine as I need a machine for my hobbies (RC air planes and DIY simpit).
Since I'm living in Belgium, I'm unable - at least not in an easy/ecologic/economic way - to use the imperial dimensions, I started converting the entire build to metric dimensions using FreeCad.

As I know others already did this as well, I hope to get some feedback on the following...
Here I go.

The original drawings give about 1.5 +- 0.8mm clearance between the ball bearing (22mm) and the aluminium bar when using 0.75" (19,05mm) thick aluminium and a vertically drilled 0.375" hole.

Assuming the 0.75x0.75" (19,05mm) bar was replaced by its closest metric equivalent of 20x20mm:
  • What size drill was used to make the hole holding the hexagon screw for the ball bearing? The expected size was 0.375" (9.525mm)? 9.5mm or 10mm?
    - When using 9.5mm, we have in total 1.5mm clearance for the M8 bolt (so that's already a good start)
    - When using 10.0mm, we have in total 2.0mm clearance for the M8 bolt (so that's a bit more than needed)
  • Where would you position the centre of the hole vertically?
    - Exactly in the middle of the aluminium bar, as was done in the original drawing (so in my case at 10mm)
    - At 9mm (measured from the top, in case the set/grub screw was positioned at the bottom)?


So there are 4 different combinations, each giving different clearance:
  1. 9.5mm drill + centre position: 1.0 +- 0.75mm
  2. 9.5mm drill + moved upwards 9mm from the top: 2.0 +- 0.75mm
  3. 10.0mm drill + centre position: 1.0 +- 1.0mm
  4. 10.0mm drill + moved upwards 9mm from the top: 2.0 +- 1.0mm


e.g. 3rd option

Momus convert to metric dimensions-momuscnc-01-defaultmetric-jpg

So there are two things to take into account:
  • the 'center' clearance between the ball bearing
  • the amount of increase/decrease in the clearance


My first thought was to use the 3rd/4th option, but the increased clearance is also applicable horizontally... which may result in too much horizontal movement (instability). Or doesn't it really matter that much? Feel free to say so

Btw:
  • for those that wonder about it... working with 0.5mm measurements is not an option for me
  • it not a one timer operation, I need to do this about 28 times... so I need to have it right (or at least as good as intended by the original designer meant it to be)
  • I really enjoy using the momus cnc document! If it wouldn't have been for the imperial dimensions I would have constructed it already


Thanks for your feedback.

Maarten

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