Thing-O-Matic / MK5 Extruder: Resistor Connections

Having collected useful thermal numbers at low power levels, it’s time to fire that mother up and see what happens at temperatures around 200 °C. That, however, requires powering both resistors, rather than attacking one with clip leads as I’ve been doing. Given that I expect to change the resistors several times in the course of this adventure, soldering to the lugs seemed like a lot of effort.

I mooched some solderless lugs suited for 2-56 screw terminals from Eks, pulled off the plastic insulating sleeves, lightly crimped them on 14 AWG solid copper wire, and silver-soldered the joints. The crimp handles most of the current, while the solder keeps the interior from accumulating oxidation products at high temperatures: a gas-tight joint is a happy joint.

Crimped and soldered lug
Crimped and soldered lug

The resistor leads have holes just slightly too small for 2-56 screws, but a pass with a #41 drill does the deed; I think it’s an accumulation of solder rather than an under-sized hole.

The leads are stamped to shape and two of them didn’t have quite enough room for the lug. You don’t want the joint to look like this:

Misaligned lug
Misaligned lug

The briefest touch of a riffler file made them right, so as to look like this:

Properly aligned lug
Properly aligned lug

Then it was ready for insulation:

Extruder Head with lugs
Extruder Head with lugs

Note that the resistors are in series, not parallel (as per the Makerbot instructions), because I want a resistor failure to produce an unambiguous symptom: no heat. In addition, I expect to operate the heaters at much lower power, making higher resistances easier to drive from the +12 V.

In truth, those screw-and-nut connections aren’t the most durable or reliable joints, particularly without lockwashers under the nuts to soak up the differential thermal expansion. But they’re good enough for what’s coming next.