Update on my parallelogram situation. Did the following:
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Took everything apart
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Added another board on top of my existing table to achieve a surface as flat as possible
- For me this was with 1 mm in each direction using a flat edge; side to side, front to back, and diagonals.
- I used used jack hand plane to flatten the board underneath, then glued a 5/8" melamine 4’x4’ piece on top with construction adhesive.
- Followed the Onefinity assembly video with these additions:
- Used a center punch (wrapped it in a few layers of hockey tape to get it just the right size for the mount hole) to mark each hole. This worked surprisingly well at getting dead center.
- Pre-drilled each mounting hole using a 8" long 1/8" drill bit per forum.
- Used #9x2" GRK screws (GRK Fasteners | R4 Multi-Purpose Screw) with #10 washers to mount. These allowed for a bit of play for adjustment, but easy to use and stable.
- Compulsively checked the diagonal distances and for square against the known straight bottom each of the mounting board. Moved the gantry back and forth several times while checking.
- Mounted front corners as per video with gantry in front, then all back mounts with gantry at the back (4 on each), then the rest of the front mounts.
- Check distances and square again (then once more).
- With everything seemingly accurate (and connectors etc. attached), used the game controller to mark each corner as per this: Square your X and Y axis - CNCnutz Episode 212 - YouTube
- Those measurements were as perfect as I was able to measure: 81.5 cm on edges and 115.4 cm diagonals.
Discoveries:
- One of my y-rails had a twist. I fixed it using this awesome video: X rail doesn't sit flush (was really easy)
- One of my combination squares isn’t square!
– Used this to check: Checking Your Combination Square - YouTube
Conclusions:
- Having a moonscape tabletop may not be sufficient for mounting the Onefinity.
- Check your rails for twist (can’t do this with a flat top for reference)
- Check your measuring tools for accuracy!
With thanks to those with constructive comments and kind offer to supply the mounting template @Aiph5u, @TheyCallMeJohn, @TMToronto