Here’s how you measure the height of the extruder head over the build platform to calibrate the Z-axis travel: a taper gauge.
Home the Z stage, zero the readout, move the stage downward by known increments until it’s less than 4 mm above the platform, then slide a taper gauge under the nozzle until it touches. Read off the actual nozzle height above the platform, add that to the distance you moved the nozzle from its home position, and you have the total Z axis travel.
For example, right now my TOM Z axis travel is 115.3 mm. Plug that into the homing routine in start.gcode and you’ve got perfect nozzle height control.
Here’s a Starrett No. 270 taper gauge showing the nozzle 1.65 mm above the platform. One might quibble with the last digit, given the bit of snot hanging from the nozzle, but it’s pretty close.
- You won’t accidentally ram the nozzle into the platform
- The gauge flattens out small belt waves
- You don’t squint at tiny vertical differences
- Assumes a flat platform, which really should be true anyway
- Seriously spendy (see below)
The gauge has inch divisions (0.001 in) on one side and metric (0.05 mm) on the other. I’ve put plenty of hours on the metric side in recent weeks.
Fortunately, I’ve had that gauge in my tool cabinet forever; I’d be reluctant to cough up the C-note required to buy one these days. That Enco page gives some other choices, none of them, alas, inexpensive. If the link has rotted out, search for Starret No 270 taper gauge and you’ll get close.
I think you could construct something similar by gluing or soldering layers of brass shim stock: 8-mil (call it 0.2 mm) shim stock would probably stack up in 0.25 mm increments under sufficient pressure. You could measure the resulting steps to get pretty good accuracy, even if they’re not regularly spaced. Perhaps a gauge that measured 1.00 to 3.00 mm in steps of 0.25 mm, stacking eight thin layers atop a sturdy 40-mil / 1 mm base strip?