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Mantis notes

November 7, 2011

Very long boring post with no pics, summarizing lessons learned about cutting and drilling the Mantis panels, and about the construction process thus far.

For cutting panels and drilling:

  1. Peck holes for later 3/8” match-drilled holes should be done for all sets of holes.
  2. The Z axis has 2 identical bottom plates and 1 top plate. All 3 need to be match drilled at the same time but the top plate is hard to align with the bottoms because it only touches them for about ¾” width and has no landmarks for alignment. We should design an extra throwaway piece to butt against the top piece that will put it in the proper position for match drilling. This extra piece should incorporate the spindle bearing hole that is present in the 2 bottom pieces so that we can use that to hold it in the proper position.
  3. When drilling, double-check the 90 degree alignment of the bit vs. the drill press bed.
  4. When match drilling the 3/8” holes, have a piece of 3/8” rod, just under an inch in length, available. Even a hacksawed piece of the unthreaded portion of a 3/8” bolt should work. Then after one matched hole of each pair is drilled, push this rod into those matched holes to help rigidly hold the two pieces together while the second hole of the set is drilled.
  5. A decent V-block for the drill press vise would have been hugely helpful when drilling the delrin rod for the leadscrew nuts. I improvised one by cutting Vs into blocks of wood but a more accurate one should be printed in ABS with a high infill.
  6. I should do a video of details of panels and the drilling process. FabLabCarolinas should do that when we produce another Mantis.

Questions about my substitutions/ changes:

I did not bore the 5mm holes in the ends of the acme rods to accept the stepper motor shafts. Instead I used 2-part spider couplers. Because they do not have any stability in the axial direction, I glued the coupler halves together with 2-part urethane adhesive. They have a bit of flexibility laterally- I hope that this is actually an asset as the carriages move back and forth- the carriages are constrained by their smooth rods so I am hoping the leadscrews will not cause any extraneous movement.

The question is how they will hold up over time and if there is any rotational slop that will cause a loss of accuracy. If so we can print ABS 5mm-3/8” couplers and try those. Boring the acme rods on a lathe or using ABS couplers might be a better choice for making up Mantis kits. Also, these spider couplers are a bit of a pain since you have to source the 2 halves from 2 different vendors and they add about $10 to the cost.

When I started the process there were no standard electronics and some of the options looked pretty ugly- thus I used the GRBLshield for Arduino, designed for the MTM Snap mill. I don’t see any problem with that; I can recalculate the steps/mm for all three axes and change the firmware, but the toolchain for creating the Gcode might be a big pain. We’ll have to see.

I had to make my own GRBLshield (with toner resist/ PCB chemical etching) but they are now being produced commercially, with much better stepper motor connectors than the ones I hacked together.

BTW: using an Arduino in Ubuntu requires a little massaging; I’ll have to backtrack what we did to make it work and post about that as well.

I have a question about whether my power supply (scavenged laptop 18.5v brick) is going to be sufficient to power this, esp if I want to add a fan to blow over the stepper drivers, but I won’t know this until I actually start running it and see. They may not get too hot at all. Fix #1 is to stick sinks on all 3 chips, then add a fan. The 18.5v might fry a fan; don’t know. This is minor stuff; an old ATX should work fine if I have to go that route.

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