# GRBL + 28BYJ-48 - wrong resolution, moves in steps instead of mm

GRBL Shield v3
Stepper:
Sepper turn = 64 steps
Screw == M8 == 1.25 mm / turn
Stepper turn = 1.25mm
Hey guys
So… I made a CNC with a GRBL Shield Plus 3 28BYJ-48 steppers (cutted the + to make bipolar)
It works fine… A little slow but ok.
My problem:
Using Universal G-Code Sender (tested others) I have to order to move 64 mm so it will do a turn (move 1,25mm due to M8), meaning it’s moving steps instead of mm
Where can I change the option so the resolution is changed to 1:64?
Thanks

So steps per mm is 64/1.25. The \$100 (x), \$101 (y) and \$102 (z) commands are used to set the steps per mm for each axis. See configuring grbl.

A handy 3D printer calculator page.

@g F
You got before me. Setup is mandatory.

Thanks, but my problem is that I don't know how to setup, and which values I should change.
\$100 = 250.000 (X-axis travel resolution, step/mm)
\$101 = 250.000 (Y-axis travel resolution, step/mm)
\$102 = 250.000 (Z-axis travel resolution, step/mm)
But the calculations give me 16.000, and steppers just won't move while trying to.

I looked into this motor as I was not familiar with it. In full step mode the motor itself is 32 steps per revolution and that is geared down by approximately 64:1 which means that the output shaft will take 2038 steps to turn 1 revolution. When the lead screw turns 1 revolution the axis moves 1.25mm so the steps per mm is 2038/1.25 or 1630.4 steps / mm. See this page for more information.

Eh. A minor opinion… 32 * 64 == 2048 giving a final result of 1638.4 steps per mm, as RJPF says.
What about the lack of movement? No stepping still?

geared down by approximately 64:1

Yes, I was too lazy to type 63.68395:1 which is the actual ratio. Thus 2038 steps per rev. See the page that I linked in reply #5.

Ok, after hours of research I think I found the right parameters.
I needed to change the F (steps per mm) to compensate to the resolution (that's why the steppers would get stuck with the default speed of 500)
First, I changed the A4988 power rating to 600ma (the power the stepper needs), I would give a link but the tutorial is in Portuguese, still, there is more out there (this is necessary to not burn the steppers, still did lots of tests before finding info about this on my research)
Then, I did several calculations and hardware tests until it seemed right
I'll show only the calculations that seem right:

Steps in One Revolution: (360°/5.625°) = 64 Steps

Total steps = Steps in One Revolution x Gear Ratio = 64 x 64 =4096 steps

M8 = 1,25mm pitch

Steps per mm = 3276,8
This should be the number of steps per mm, but I had to multiply per the number of phases, so
3277 x 4 = 13100

So, I had to change this values on config:

\$100 = 13100.000 (X-axis travel resolution, step/mm)
\$101 = 13100.000 (Y-axis travel resolution, step/mm)
\$102 = 13100.000 (Z-axis travel resolution, step/mm)
\$110 = 10.000 (X-axis maximum rate, mm/min)
\$111 = 10.000 (Y-axis maximum rate, mm/min)
\$112 = 10.000 (Z-axis maximum rate, mm/min)

\$100s are the resolution per mm
\$110s are the Speed, I'm giving it 10 mm/m (It can go up to 15, 16) to compensate for the extra effort.
programming the G-code for the final test now.

So the A4988 is set for half steps?

but I had to multiply per the number of phases,

Why?

@gF How to create such a transmission,63.abcdefg....? I could't dream of that.

@RJPF Good work, well done! If You need an increased accurazy use half step, quarter step, 1/8 step, 1/16 step. It will serve You but the available torque will be less.
look at the \$S20.... \$S30 if they will serve You.

@groundFungus I don't know, but that what worked...
Might not be perfect at 0.1mm, but seems to be what gives me the right mms...

I already have Nemas coming from China, but I'll be using them on my second CNC, with more robust and calibrated materials.
I'm hoping I can use this CNC, plus the 3D printer I'm getting for my Birthday ( was yesterday but takes a few days to arrive) to make the pieces I need for the next printer.
Anyhow, It's finally working.
IT's slow... really slow... But:
Its power consumption is almost 0
Connected to a laptop (instead of my desktop) I also decrease power consumption.
My CNC isn't perfect, I'll not even get a precision of 1mm, but it's a good start as I learned the mistakes I needed to learn before I invest in better materials. (I round the cost of this machine at €30, €40)