Z Axis Limit Switch

In order to determine when I've succeeded in reworking the Z Axis Limit Switch, I first run a calibration with the existing configuration and measure it.

First we make the bed conductive, by pressing a sheet of aluminum foil onto a dampened bed with a wet paintbrush. I suppose you could use a wet paper towel just as well.

Then we make the tip of the extruder conductive. I would have thought it was, being metal and all, but preliminary tests showed that some plastic had covered the tip from a prior build. So we wipe the extruder nozzle well with a paper towel with acetone on it. Test that the plastic is off by checking for continuity when a flat object like a screw driver blade is brought up to touch the very tip of the extruder nozzle. The metal shield around the heater wires is grounded, so you can clip an alligator clip to that.

Then we set up a digital multimeter as an audible continuity meter and fire up the machine panel of the ReplicatorG software.

By jogging the Z axis, measuring continuity and playing with the adjustment screws of the limit switch, you can get the switch to trip at the same time as the continuity meter indicates the extruder is touching the bed.

It turns out that the mechanical switch has about 0.4 mm of hysteresis. That is, the switch closes at position 0, but you need to back it off to 0.4mm before the switch opens again (see the video for an explanation).