The last wiring, the first calibration

Z axis still has trouble moving down. Trying some suggestions:

• Set Z axis speed lower. Seemed to work a bit, but not a lot.
• Need to Home Z, or it won't go below 0. That did it! Thanks to jamesdanielv on RepRap.org for mentioning this.

Next issue: Left and right X mounts are offset. Relatively easy to fix on the left-hand nut. After alignment, they stand 36.6 mm off the base plate at home.

Width is 190mm (due to different extruder design), depth is 200mm, and height is... curious. Seems to move more than it's supposed to. What it thinks is 160 mm is actually 193. Measuring it with a ruler says 20.5 to the bed. Changing firmware to use STEPS_PER_UNIT = 200*13.25/1 for Z. Also changed Z_HOME_POS to 205, X_MIN_POS to -95, and X_MAX_POS to 95.

Note to self: Do not move the X axis 200mm out, then home Z. It sounds nasty when the extruder motor hits the Z top holder and prevents reaching the Z stop.

Second calibration shows a Z movement of 204 mm when doing 20 10mm steps. Yielding STEPS_PER_UNIT = 200*13.25/1. That's a nice number and a good match. True height is 204mm.

Wired up the heater and Mister Thermistor. "Room" temperature is found to be about 80ºC - talk about global warming! Need to calibrate. Tried using an infrared gun, but that doesn't work well since it measures the outside temperature. Need to calibrate by getting the entire thing to a known temperature and then measure the resistance. Or measure on one of the spares I bought. Easier to do that at home in the oven with an oven thermometer.

Following RepRap.org's nice calibration instructions. Mostly good, except - the bed isn't close to level, about 1mm difference between corners. And not in the linear fashion I would expect from differences in corner support height, but more difference towards the end, which might indicate bending.

A sheet of paper catches in the max-x/min-y corner and at the max-x/max-y corner (but not quite in between), at the min-x/max-y corner there is about a millimeter gap, and at home there's maybe half a millimeter. Adding two washers on the home corner helped there, but it's definitely sagging.

Also fixed the inverted X belt.

Questions of the audience:

How home my Z steps/mm was off? Are threaded rods really different between UK and Germany?

Should I oil the Z threaded rod?

Update: Testing an equivalent thermistor at home showed it working as expected. I hope I haven't broken/short-circuited something when glueing it in.