What Would Barbie Pack …

Nerf pistol on build plate
Nerf pistol on build plate

… if Barbie would pack heat?

It’s actually a snub-nosed version of that, cut down by 15 mm to fit the TOM’s vertical space; the nozzle homed 3 mm above the last of the 345 layers.

I wanted to discover three things:

  • Are there any axis skips in a 4 hour print?
  • Can ABS film + aluminum plate anchor a tall object?
  • Can I use up all the pink filament?

Answers: no, just fine, not quite.

I did not re-check the platform alignment after installing the new Y axis motor and fiddling a bit with the Y axis rods. Quite to my dismay, the platform was about 0.5 mm too high (crunch!), so I gave the Z axis leadscrew a mighty twist and salvaged the first layer during the Outline extrusion. Despite that, the first layer seemed to be flat within the usual 0.2 mm (eyeballometrically measuring the first infill, as the Outline was trashed) and adhesion was fine.

The grip delaminated a bit and the butt pulled the film up, which isn’t entirely unexpected for huge objects.

Nerf pistol grip - lifting and delamination
Nerf pistol grip - lifting and delamination
Nerf pistol - grip detail
Nerf pistol - grip detail

A better view of the grip showing the cracks:

I enclosed the build chamber before starting this print, but the temperature still isn’t all that high in the Basement Laboratory and the plastic was barely warm when I took it out. I’m not convinced any reasonable chamber temperature will solve the problem; it may work out better to assemble large objects from thinner parts.

This was the first full-up test of the X Rod Follower and the new Y axis stepper motor. Prior to printing this thing, I did a quick torture test (about which, more later) and dialed the motor currents back:

  • X REF = 0.63 V → 315 mA
  • Y REF = 1.76 V → 880 mA (in a 2 Ω winding)
  • Z REF = 0.54 V → 270 mA
  • A REF = 0.99 V → 450 mA (in a 2 Ω winding)

After four hours the Y, Z, and A steppers were barely warm to the touch and a thermocouple stuck into one of the X stepper’s bolt holes reported it was 38 °C, just above barely warm. I’m adducing evidence that the MBI steppers aren’t appropriate for the TOM’s requirements and that the default current settings are much too high.

Now, for some Nerf darts…

7 thoughts on “What Would Barbie Pack …

    1. I’m not sure what you mean. It’s printed with 0.33 mm layers and the overall height of smaller objects is spot on, so that part’s working fine.

      Most likely, successive layers aren’t fusing together firmly enough to withstand the shrinkage forces, which suggests the 210 C extruder temperature is too low. This being the tallest and chunkiest object I’ve ever printed means it experienced a lot more stress, but we did see a few small cracks in the larger companion cubes.

      I won’t futz with the extruder temperature just yet, seeing as how I’ve got enough variables in the air already…

  1. Which steppers are you running? The pololu’s seem popular but if anyone is intending to upgrade (myself included) a hint on ones which work well for others, namely you, wouldn’t go astray. I’ve always thought the default REF voltages on the TOM steppers caused too much heat but without someone like you able to prove it to me i was always just stabbing in the dark. Cheers.

    1. A quick summary; I’m still sorting this stuff out…

      The motor must have a relatively low winding resistance, down around a few ohms, and a rated current up around an amp, which will give a pull-in torque around 200 to 300 mNm. I’ve been using eBay as my parts bin, so I don’t have good specs on the motors, as they sport Minebea custom numbers, but anything in that range should work fine.

      The much-touted Holding Torque is irrelevant; you want the Pull-in Torque graph. If you can find that for the motor in question, it should be around 200-ish mNm at full current on the left (low steps/sec) side of the graph. Any torque higher than that is fine.

      So the Pololu 1200 would work for a geared extruder and the Y axis, but it’s too big to be an X axis motor.

      In order to make chopper current control and microstepping operate correctly, the driver chip must have many volts of headroom between the winding voltage and the power supply. That says you don’t want a “10 V” or “12 V” motor; that number is (winding current) x (winding resistance). Anything around 2 to 5 V should work.

      As nearly as I can tell, any stepper with those general specs will be an improvement.

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