Erratic homing behavior on large delta

First thanks for reading my post. My setup its this. Scratch built delta using ramps 1.4, with the latest marlin. My problem is this, when I use pronterface or repetier host to home the z axis only 1 or 2 of the carriages go all the way up to trigger the endstops. When this happens on the 1 or 2 that carriages that stop short I can hear the steppers making a humming noise. And then after about 5 seconds all of the carriages move down as if the procedure was done. When this happens the carriages are not at equal position after homing. Crazy thing though is that if I manually put the carriages relatively equal distance things are fine, at least for the most part. Sometimes I still get this behavior even when I manually preposition the carriages. This is fairly large build, do need to account for that in marlin for any reason.

Too much friction and/or insufficient motor current perhaps?

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Dc42, I was thinking the same for a little bit but when I check it manually I don't feel all that much friction. I know a person can up the voltage using the pots on the drivers, does that increase amperage too when adjusted? Forgive me, dealing with electricity is not my strong side.

Modern 3D printer electronics boards provide the facility to set the motor currents to known values with a firmware command. Unfortunately, budget electronics (as you have) has been stuck in a rut for several years, and it is not easy to know what motor currents you have set. So you will have to do the best you can. Here are two approaches:

1. Find out what the relation is between the voltage on the potentiometers of your driver boards and the motor currents. If you are lucky, the supplier of your driver modules will provide this information. More often, you will need to establish what stepper driver chips they use and what current sense resistors they use. Then you can measure the voltage on the potentiometer wiper with a multimeter and adjust it to get the current appropriate to your stepper motor.

2. The more empirical approach. If the motor and the driver are both cool, turn the potentiometer up. If they are both hot, turn it down. If the motor is cool but the driver is hot, and you think you need more torque from the motor, add a fan to cool the drivers.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].