Tim Hatch

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Mantis Mill 15 Aug, 2012

I just realized I forgot to write anything about last year when I put together the Mantis bot 9.1. Cutting took a little planning, but I made it out of a single 2 × 4 sheet with almost no waste and all the remaining pieces are useful for building a second copy. While I didn’t make it for <$100, I did learn quite a bit about what matters for a small CNC machine like this and probably could get the incremental cost to $200 if I built more than one.

The single most expensive parts are a 3/8-12 ACME tap ($80ish) used for making the leadscrew nuts, and the electronics (the Mantis site assumes you can manufacture them yourself, which requires… a bot, or toner transfer (which I haven’t had good luck with)). I was able to use some older reprap boards, which connected with test clips to my parallel port. To date I haven’t had a short, although the clips do tend to fall off.

As you can see from the photo up top, I made it out of plywood with a thinner outer layer than spec’d. This is because MDO is comparatively difficult to find, but you can find 12mm cabinet plywood at a place like Lowe’s or Home Depot. I’ve since found a local lumberyard that stocks MDO in precut 2 × 4 pieces, which I plan to go with for my next project.

What’s good about the Mantis design?

  • The method of aligning the bushings by epoxying after drilling the stacked rails works great.
  • Tapping your own leadscrew nuts isn’t that bad. I haven’t figured out how to make an anti-backlash version yet, and since they get epoxied, are difficult to replace… I haven’t put a whole lot of stress on it yet.
  • Leveling the bed is fun.
  • It basically solves the chicken-and-egg problem, by being able to fabricate everything with standard tools (save the ACME tap). I used a circular saw, jigsaw, vise, drill press, a few drill bits, and a screwdriver set.

What weak points does it have?

Although I understand one of the primary goals was to make a really inexpensive machine, there are several points that I feel could be improved for an additional couple percent in cost.

  • Spindle is weak, noisy, and underpowered. The bearing design is great, but the motor driving it is a toy. Even mounting a dremel works better than the lost cost spindle (and I’m working on an RC brushless design at the moment).
  • Leadscrew-to-motor mounting is difficult. Without access to a lathe, I did my best to center drill the leadscrews, but it wasn’t enough. A redesign of this would end up with a regular skate bearing on the end away from the motor, and a flex coupling on the end with the motor. This would require turning down the 3/8 leadscrew, but with a lathe this wouldn’t be a big deal. It would also mean your motor bearings would last a longer time, and allow for an upgrade with thrust washers too.

What are problems it didn’t solve?

  • G-Code generation. Skeinforge has a mode that can generate subtractive commands, and you can use cad.py… but that’s about it for open source solutions.

So far I’ve done basic engraving with it using a V bit in cardboard and plywood, some drilling with PCB bits, and attempted to make a couple of circuit boards using a 1/64 endmill. My biggest lesson learned so far is that if you crash the bit into your work piece without it spinning, the tip will break.

When I pick it up again next, I plan to construct something more stable for electronics (if I want to keep using EMC2, a parallel breakout board with Pololu drivers; if separate, something based on the reprap Gen7 boards). I’d also like to build a larger one, the ~6 × 4” area is pretty limiting.