Thing-O-Matic Stepper Extruder: First Steps

Over the past few weeks I’ve printed the gears and plate from TheRuttmeister’s Coloso-Gear MK5 extruder Thing and flatted the shaft on a moderately husky (but not hyperthyroid) NEMA 17 stepper motor. While tearing the Thing-O-Matic down to add thermal switches to the Extruder Head, I converted the MK5 Filament Drive into a stepper extruder. Much to my astonishment, when I plugged the cable in and fired up ReplicatorG … It Just Worked!

Even more amazing: the first pinout arrangement turned the motor in the correct direction!

Coloso-Gear Stepper Extruder
Coloso-Gear Stepper Extruder

Some nasty pincushion distortion makes the larger gear look misaligned, but it’s parallel to the mounting plate and correctly engaged with the drive gear.

The motors arrived with short stubs of thin yellow wire on the IDC motor connectors, which I soldered directly to a much longer cable. The Parts Heap disgorged a chubby 8-conductor signal cable; I used pairs of wires for each motor connection, although one conductor would have entirely enough copper. The two cable ties around the motor prevent flexing those delicate wires as the Z stage moves.

Two tweaks to the MK6 Stepstruder profile in thingomatic.xml produced the right answers:

  • Set motor_steps = 1456
  • Set stepspermm = 48.2

Running the motor at 2.0 rpm for 30 sec should produce exactly 1 revolution of the big gear. I marked and counted the teeth on the larger gear as it rotated, and came up with 56 teeth. It’s a 51 tooth gear, so reducing the default 1600 steps/rev by 51/56 produces 1457. A defunct MBI stepper driver board that now only does full steps provides power; I resoldered all the chip pins and the fault isn’t due to external causes like no-lead solder.

Then run it for 60 seconds at 2.0 rpm and it’s under by maybe 1/10 of the tooth-to-tooth spacing. Adjust 1457 x 101.9/102 = 1456. Run it for another minute and it’s spot on.

I measured 60.45 mm for two revolutions of the big gear, so it’s 30.23 for one rev, which requires the aforementioned 1456 steps. Averaging more revolutions would yield more digits, but given the rubbery nature of molten filament, three significant figures seems entirely sufficient. I suspect this depends greatly on how deeply the extruder drive embosses the filament, so it’ll require some fine tuning.

Back of the envelope for the DC extruder at 255 PWM: feed = 45 mm/s, 0.35 mm thickness, w/t = 1.7 = 0.56 mm width gives 6.9 mm3/s. The filament is about 2.9 mm dia = 6.6 mm3, so it passed through the extruder at a bit over 1 mm/sec. There’s some windage involved in all those numbers and the extruding rate obviously depends on the temperature.

The stepper (from the usual eBay seller) is a Minebea 17PM-K150, which doesn’t appear in their catalog listing, so it’s likely one of their many custom motors. The stack length resembles the 17PM-K3xx series, which means roughly 1 A rated current. Setting the driver current to 500 mA (VREF = 1 V) produces enough torque that I cannot pull the filament back hard enough to stop it.

The step rate at 2 rpm is:

48.6 step/s = (2 rev/min) x (51/7) x (1 min/60 s) x (200 step/rev)

At that lethargic pace, the K3xx motors have something like 0.250-0.300 N·m of torque at rated current. At half current, call it 0.100 N·m and multiply by 51/7 to get 0.700 N·m = 100 oz·in.

The effective drive diameter is 30.23/π = 9.6 mm, so the available force on the filament is 0.7 N·m / 0.01 m = 70 N ≈ 7 kgf = 15 lb. Yeah, but that little 7-tooth gear will snap right off …

The reversal plugin cranks the big gear backwards at 35 rpm, which works out to 850.5 step/s. That ought to work, particularly seeing as how it’s not actually pushing anything.