Z axis zero problem

Hi
I have been making and modifying a Mendel at the school where I work and have managed to get it to print well, the problem that I have not been able to resolve involved zeroing the z axis. The z axis stepper motors work well when raising the print head, the problem is when you ask it to return to the z zero position, the stepper motors just buzz unless you press the limit switch then they bring the head down about 10mm and stop. Because of this is it the head stops in any position above the bead I have to keep pressing the z home button and flick the home limit switch to get it to move. I have checked the switch and it functions ok so I have just put up with it but I do wish I could solve the problem. Help
(enthusiastic amateur)

Send command M119 to the printer.
It will send back state of the end switches.
If it shows Z as triggered then you need to correct your end stop.
If you are using mechanical switches you can change pin on switch( No to NC or NC to NO).
Or you can change setting in config.h
const bool Z_MIN_ENDSTOP_INVERTING = false; // if false set to true , if true set to false to invert the logic of the endstop.

Resend M119 to check

Thanks I will give it a try today.

Had a go today and all the limit switches working. It has altered a little now, when you ask it to Z zero the motor just buzzes until you flick the limit switches and the head rises a little then stops and buzzes for about 2 seconds then moves down until it hits the limit switch.
?

So you have 2 Z steppers? are they wired in series or parallel to the driver and what vref is the driver set for?

Ben

It's normal behavior when homing for the motors to stop when they hit the limit, back off a bit, and then come back down at a slower pace. This tells me that your Z motors seem to be working at slower speeds. Speed can be a factor when motors don't seem to work. Many people report that they can move manually, but motors don't move while printing.

I'd do one of two things - drop your homing speed a bit - I believe this is settable in the firmware. The other thing is to verify the potentiometer settings on your stepper driver board - perhaps you need to bump it up a bit.