ACME w/NEMA23 Belt Drive

I wanted to do a test cut to verify calibration of my Z axis, so I used Easel to draw four lines with cuts to different depths; i.e., 1/16", 1/8", 3/16" & 1/4". Each cut was doubled. I suspected what might have happened, so closed Easel, opened UGS and read the grbl parameters. Sure enough, Easel had changed $102 to the default for an ACME lead screw. I updated my SO2 to the NEMA23 belt drive with ACME screw, so one revolution of the stepper makes two revolutions of the screw.

Will you be able to accommodate this change?


I do not under stand why you went with a 1 to 2 ratio and not a 1to 1. The nema 23 has the power to lift the Z axis at 1 to 1.


I didn’t “go with” the ratio. I bought the ACME and belt drive kits. The belt drive kit included pulleys that provide the 2/1 ratio. It made sense to me because the NEMA23 has the torque to handle the load and the system can move the spindle faster. The belt drive plate offsets the mounting so the spindle clears it.

That being the case the folks developing Easel will need to correct the problem.


It’s only a problem when one goes through the machine setup process in Easel. If I avoid that, I can use Easel.

Actually, the “normal” adjustment of $102 is half the value that Easel gives it. In order to have my Z axis dead on, I had to tweak that value a little.

I thought I should clarify the $102 value situation. Easel sets it to 188.947, the normal for an ACME lead screw. For those of us who updated to the NEMA23 belt drive, the value should be 94.4735. When I tested my Z axis, the movement was off a small amount which I corrected by adjusting $102=94.5.

Just wanted to update this, if you need to enter a custom value you can do that through Easel by using the Machine Inspector. Go to “Machine” -> “Advanced” -> “Machine Inspector” and you’ll get a readout of the settings and ability to send lines to change the settings.

Point of interest. Stepper motors can not do 94.5 steps. They can do 94 steps or 95 steps but not 94.5 It is just the way steppers are designed. You can do micro stepping if your driver supports micro stepping but it is still whole steps. It will do 1000 micro steps or 999 but not 999.5


I think that everyone is missing the mark on this. Yes, stepper motors can’t do fractional steps. But you have to look at the big picture, not just the details.

This value is used to determine the number of steps to move a distance. Calculations can take into account the fractional portion to make a move more accurate.

Say that you want to move 100mm.

If $102=94 then you would get 100mm * 94 steps/mm = 9400 steps.

If $102=94.5 then you would get 100mm * 94.5 steps/mm = 9450 steps.

So, 9450 +/- 1 step is more accurate than 9400 +/- 51 steps.

Not so?

Having issues with the z axis cutting too deep, almost double what should be the initial plunge depth. I had to reduce to 1/16 and it is pretty much cutting a full 1/8 on the first pass. Working on a 1000mm, NEMA 23 and an ACME screw kit.

If you’re letting Easel adjust your grbl settings, it will set $102 to the value needed for a direct drive or 1:1 pulley ratio. The NEMA23/Acme kit I got has a 2:1 pulley, so I had to cut the value of $102 in half.

Thank you. Simple as typing in $102=94.5?

The actual figure would be 94.4735. I changed mine to 94.5 after going through some calibration steps.


Cut the value in half and did a dry run (spindle off, along an already cut out shape). Looks promising. Crossing my fingers and will follow up if resolved. Again, thanks to you and the forum.