Z axis drops - Super hot steppers

Hey All,

I’'ve got a dual-endstop Rambo MPCNC, bought the hardware+electronics kit directly from V1.
Most of the time things are good - I get really good dimensional accuracy etc. But I find that after running the machine for a while, maybe an hour or so, I occasionally get some unexpected drops of multiple millimeters in my Z axis, which as you might imagine, ruins everything.

I notice that my stepper motors, especially the Z axis motor, are extremely hot. I don’t have a reliable way to measure the temperature but I can’t hold my hand on it for even three seconds. I’m using the latest V1CNC_Rambo_Dual firmware. I’ve got a heatsink on each of the stepper drivers and a fan cooling the Rambo board. I’ve also got ferrite beads on all of my wiring in attempts to minimize electrical noise.

Any ideas of what might be causing my Z axis issue? And any thoughts as to whether it’s related to the motor temperature? Wondering if I should try to change the max current for the drivers in the firmware? Or maybe adjust the Vref? If that’s even possible with this board. I really don’t know if the temperature is causing these issues but since it only seems to occur after the machine has been working for a while, it seems like it could be a likely culprit.

Would really appreciate any ideas! Hate to keep ruining these nice pieces of walnut…

The current is controlled by the firmware. I’m working from memory, but I think it is M900.

It sounds like it is set too high. Not sure how that happened. The goal is less than 50C on the motors. I am not sure how long your hand can sit on 50C, but I think it is longer than 3 seconds.

Edit: M907:

Thanks Jeff!

Interesting I didn’t realize you could control current using Gcode commands. Figured I would have to re-flash.
Any ideas for what kind of reference values I should be aiming for to experiment with?

I’m quickly scanning the firmware and I see the MAX_CURRENT under
shows 1000 milliamps.

Then under
shows 800 milliamps.

So I guess I can just trying bringing the current down a bit less than 800?

(btw my whole workflow is based around V1Pi, thank you!)

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Those are for TMC. If you send just M907 it should respond with the values it has set. You can see a bunch of these settings by sending M503.

IIRC, rambo is 0-255, and it has funny scaling. I think the default co fig sets them to about 135. I hope this function doesn’t do any odd scaling. I don’t have a rambo to see what it is after a clean flash.

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Thanks, I appreciate the help.

Weirdly when I send M907 I get no response beyond:

And M503 isn’t giving me any milliamps information.

Figured I would try M906 as well, but it’s invalid, which i guess makes sense if the drivers on the Rambo are not TMC.

Shoot. Looks like it isn’t configurable on the fly.

It is the DIGIPOT_MOTOR_CURRENT in Configuration_adv.h. But I can’t imagine the setting from Ryan wouldn’t be right. Do you know what version of the firmware you have? 425D?

Thanks! Yeah it’s good to have a reference point even if I don’t actually change the current in the firmware. And yes I’m on 425D.
I should probably look into some alternative cooling methods like motor fans or something before messing with the firmware. Although, to be honest, I’m still not even sure that high motor temperature is what causes these occasional Z drops.