Question about steps per unit/mm

I’m setting up the firmware for my skr v1.3 and had a question about the Steps Per Unit settings. I am using 5 X DRV8825 drivers 1/32 microstepping, with dual Z and Y steppers and Dual endstops enabled.

In Marlin 2.0 in the configuration.h the steps per unit are set to reflect x, y, z, and e1 (#define DEFAULT_AXIS_STEPS_PER_UNIT { 200, 200, 3200, 100 } . Do I need to have 5 values there since I’m using 5 stepper drivers or do I only need 3 values for X, Y, and Z axis’?


1 Like

x and y will be the same scale, so you would still use 4. Yours should be 200, 200, 800, whatever you want. That’s assuming a 4 start T8 leadscrew.

Thanks Jeff,
I’m using a single start M8 lead screw with 2mm pitch, that’s where the 3200 comes from.
So my values should be DEFAULT_AXIS_STEPS_PER_UNIT { 200, 200, 3200, 3200 } ?

1 Like

Yeah. Just beware that you’ll be spinning that motor shaft quick, so you’ll lose torque at higher speeds. I would also test your Z speed. A conservative value would be in the 2.5-4mm/s. It might be fine at higher speeds (every build is different).

But those values look like they should be right for 1/32 stepping.

ok, I appreciate the advice. I went with a single start over a 4 start because I thought it it would be better for torque since it’s only moving 2 mm per rotation as opposed to 8 mm per with a 4 start ( am I thinking about it backwards maybe?) When I built my prusa mk2 clone 3d printer I went with single start lead screws for better accuracy and seems to achieve it over my other printer with 4 start lead screws.

At the end, you do get more force (from the torque). But in between, you have a lot hanging on the screw and when it comes to friction, you get no advantage. FWIW, it comes up a bunch. A lot of people do that and many prefer it, especially if they have heavy routers like the 611.

Stepper motors are kind of goofy in that when you go faster, they resist the current more. There’s no problem when the steppers are moving slow enough, or with a high enough input voltage, but if you are moving fast, with 12V, the steppers will resist the current enough that the drivers will basically be saturated, sending all 12V to the steppers, and at that point, you will start to lose torque (at the motor shaft) and the friction can be enough to skip steps. It is fine at lower speeds.

The reason I say 2.5mm/s is because a conservative value with a 4 start is 10mm/s. Many people go faster than that, and the weight of the router will be smaller for your motor, so you can probably go a bit quicker. It is easy to test though. Just send some moves from the top to the bottom, and increase the speed until it skips. Then back it off enough you’ll be confident to not skip steps again. Ideally, put this new setting in the firmware as well as CAM so you won’t accidentally skip steps when a nice workpiece is in the machine.

TL;DR: Either slow down the Z or get a 24V PSU and you’ll get good torque.

Great advice, much appreciated.

1 Like