Need help with leveling bed with two Z axis motors

mperdue

I'm running firmware 1.19.2 and trying to set up the bed leveling feature using two Z axis motors. It goes through all the motions but when it's done and I measure things it is low on one (the left) side.

I have the motor for the left side connected to P2 and the right side connected to P3

in the config.g file I have the following

M584 X0 Y1 Z2:3 E5:6:7 P3 ; Driver 0 For X, 1 for Y, Z=2:3 Extruder 5-9
M671 X-58:452 Y150:150 S1.0

My bed.g file contains

G28 ; home
M401 ; deploy Z probe
G30 P0 X20 Y150 Z-99999 ; probe near left leadscrew
G30 P2 X380 Y150 Z-99999 S2 ; probe near right leadscrew and calibrate 2 motors
M402 ; retract probe

When I enter G32 the printer homes, moves to the left center of the bed and probes, moves to the right center of the bed and probes again. This is just what I would expect to see. However, when I measure the distance from the rail the extruder and probe are suspended from to the bed the left side is about 1.5mm lower than the right side. Mapping a mesh of the bed confirms this. If I manually adjust the bed to level it and run G32 a couple of time the left side is low again.

What might I be doing wrong?

dc42

Does the reported correction applied by G32 increase each time you run it? If so, that probably means that you have specified your leadscrew coordinates in M671 in the opposite order to the motor driver numbers. You have told the firmware that the motor attached to driver 2 (the Z motor connector) drives the leadscrew at X-58.

mperdue

No, I originally did have them reversed but I swapped them. The motor driving the screws on the -X end are connected to the Z driver. The other one is connected to the remapped extruder connector. The difference seems to be remaining constant and is actually about 2.8 mm measured manually.

dc42

What type of Z probe are you using? I suggest you measure its trigger height at both probe points.

mperdue

I’m using a BLTouch. I’ll take a look at trigger height at both locations. What is the best way to verify trigger height?

dc42

See https://www.duet3d.com/wiki/Using_mesh_bed_compensation#Checking_the_trigger_height.