This one I mentioned in another post is built on a box-like frame. Four 2x4s are the sides and a sheet of MDF with a hole is the base. The corners are these custom pillars that also include the bottom corner and replace the legs.
I had a heck of a time deciding how to get the thing to sit square. In the end I chose an arrangement where the screws hold the ends flush with the side of the pillar but there are slots so the board/pillar joint is not constrained from the board twisting or shifting horizontally. I can lay it on a flat surface with the screws loose and then tighten the screws and it stays flat and doesnt try to twist out being planar.
The sub-floor is the working surface and I will be able to swap it out depending on what I’m doing. For now its another sheet of MDF and the machine is just resting on it, but my hope is to experiment with:
5-axis (will need some Z room)
unconventional 3d printing
wood joinery on ends of boards
steel with flood coolant, needs a bucket or tray which is hugely annoying with low Z clearance
I wanted these to be contained inside the box instead of outboard, and yet not consume any of the working space.
I have a plan for repeatable and precise squaring while keeping the motors wired in series (necessary for printing and 4+ axes with the standard 5 drivers). Basically home the machine with ordinary endstops along two rails and then manually cog the other stepper motors on the other two rails to alignment markers.
I’m using the old tool changer Z axis but it’s not functional at the moment. With a small working space it might not make sense to consume working space with tool parking slots, but there is always the possibility of some kind of turret in the sub-floor maybe. Thats a long way off if I ever go that way.
You’ve said before that the flex in the corners when milling was negligible. So I am assuming the motivation was just to make it strong enough to be portable? Now it can keep it’s square even while not attached to the bottom most surface? Is that right?
Good thinking accounting for wood movement on the 2x4s.
Yes exactly. The legs/corners are plenty strong in the normal configuration but if I just had a thin deck with a giant hole, I would be concerned about the deck flexing.
Right now the 2x4s are only secured through the corners and are not affixed to the deck along their length, but if I secure them along their length it should be quite rigid in staying flat. I would first want to check that the x/y movement is parallel to the sub-floor and shim the corners if necessary. No surfacing a spoil board should be necessary.
Finished up my repeatable squaring/positioning with series-wired motors:
I used some Avery labels to print 8" long adhesive rulers and little short segments with a Vernier-style gradations (similar to belt stretch approach before) so in theory I can eyeball position to less than a mm.
I also used my mirror “infinite hallway” approach to ensure that the zero position is parallel to the side rails to within one click of the motors.
I think this would be very slightly better than holding against endstops when energizing because there is no guarantee that the motor quantization will not be off by one cog between the two motors, depending what phase (in the cycle of four full steps) they happen to be in when energized. And for the same reason, position non-repeatability could be in the range of 0.4 mm.
From a more practical standpoint, holding against stops is inconvenient (just slightly) because I have to hold the center assembly with one hand and then select and run the ‘wake-up’ gcode file with the other (basically G92 X0 Y0 followed by G1 X0.1 Y0.1). So I find doing a proper G28 X Y and then a few seconds of forcing it square is nicer, nevermind the accuracy.
