Month: May 2011

A group of 34 mm NEMA 17 steppers arrived from the usual eBay seller, I wired one up, and popped it in place under the original cork sheet. The bolts sit on steel washers riding atop compliant bushings from the batch of 43 mm NEMA 17 steppers that drive the extruder and Y axis.

[Update: Not that you’ll ever find another one, but here’s the straight dope directly from the motor label…

• Astrosyn P/N FH5-1043 02
• Minebea 17PM-J034-P2VS
• No. T6Z01-03

]

Putting a cork sheet under each motor was a nice idea, but  it didn’t work as intended: the bolts quite effectively couple the vibration to those resonant acrylic and plywood sheets. I had to cut the two front bushings in half to ease the bolt heads under the X axis stage at that end of its travel, but the motor is now isolated from the Y stage. I’m sure the bolts touch the slots, but even I am unwilling to fit compliant bushings around the bolts.

Lashing the cable to the side of the box should suffice for strain relief, as the jacket forces it to flex in a large upward loop with no sharp bends.

That picture’s inverted so the flash lights up the stuff sticking out of the bottom of the box. The thin silvery arch is a cable tie around the motor connector holding it securely against the motor frame, but the Y axis follower bearing just to its right actually sticks out slightly more.

This motor has six active terminals and could run in either unipolar or bipolar mode. The wiring harness has four leads and that’s why I bought it: the MBI driver board expects a bipolar motor.

The winding resistance is a mere 2 Ω, compared to the MBI motor’s 35 Ω. Of course, I don’t have any specs for this motor, but similar Minebea 17PM-J0xx motors run around an amp with 130-180 mN·m of pull-in torque, compared to the MBI motor’s 14-ish mN·m. I expect to run it around 500-750 mA at half the rated torque, where it’ll dissipate maybe a watt, tops, with no overheating.

… 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.

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…