Mini MPCNC 1/7th Scale Model Build

Inspired by these tiny 1mm ID x 3mm OD x 1mm bearings I found cheap on Amazon, and by the lack of space at my current apartment for a full-sized build, I have decided to make a roughly 1/7th scale miniature version of the primo.

Parts for it were actually not that expensive (mostly):

  • $17/100 681ZZ bearings
  • $10/10 4mm x 300mm stainless steel rods
  • $6/5 miniature stepper motors (meant for a camera, 18 degrees per step, 3-5 volts, includes a gear on the end and some sort of mounting plate)
  • $8.5/2 miniature steppers with a 30mm screw attached (for the Z axis)
  • An absurd $24 for 200 M1 nuts (brass, I think, as that’s all I could find of that size on Amazon in any useful quantity) and $18/100 M1 tap bolts. But they won’t arrive until sometime in February.
  • So I also ordered a set of self-tapping small screws. $7/1000 in 10 varieties. I’ll probably mainly use the 100 M1 x 5mm from it if I don’t want to wait for the nuts and bolts.
  • Arduino with CNC shield - already owned.

Things still left to be done:

  • Movement system. I’m not having luck finding suitably small timing belts. I might go for a rack and pinion design, or just stick a rubber band or something on it. Or maybe try printing a belt out of flexible filament (or maybe even hot glue with some string as the core). This is basically meant to be a toy, so I’m not too concerned about this at the moment. As far as I know the motors won’t even be strong enough to move it, in which case I’ll probably just attach strings to it and it can sketch patterns from the springs.
  • Wires. What to use? Possibly I’ll use the wire from some broken headphones (with a mic for the 4th wire).
  • CAD models of the Primo parts so I can scale it down for the 4mm rods and 3mm bearings and 1mm screws. Unfortunately it’s not a uniform 1/7 scale, but varies from roughly 1/6 to 1/8 depending on the part. If I’m really lazy I might try printing a few of the parts without the extra work and assume I’ll be able to deform them to get the 1mm adjustments that might be needed (possibly with the application of some heat).
  • Receive the parts and actually build it.
  • Further in the future: maybe add a laser if it can be moved electronically, as that can be done in a lightweight manner (a couple mirrors and a lens are all that have to be moved).
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I wanna see it! Please update when you’re done!

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For belts try turntable or casette player belts they are probably close in size to what you are after.

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What about using rubber o-rings? I believe projectors use something similar.

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  • Wires. What to use? Possibly I’ll use the wire from some broken headphones (with a mic for the 4th wire).

Beware of strand strand count and the sleeve material in wires that move, especially in cable chains. I fine headphone wire very fragile and get wire breaks easily. YMMV.

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