Step/mm with sfu 1204

Technically you don't need to know anything about the threads in order to calibrate the system, but it does help as a starting point .

1. With power off you can just turn a leadscrew 1 turn with your fingers and measure how far the carriage moved.
2. Fill that in here GRBL settings calculator under 'Z threaded rod pitch (mm):'
3. The motor is most likely 200 steps per rev so leave that alone.
4. Select '8 microsteps per step', (the standard on a Blackbox.)
5. now in the result section at the bottom for Z you get a steps per mm value.
6. In OpenbuildsCONTROL you can fill that number in for all relevant axes in the 'GRBL Settings tab', save and reset.
7. Now use the 'wizards and tools|Calibrate' menu items to calibrate each axis to a final value.

I have proven to myself that one can in fact calibrate an unknown system by just 'starting somewhere'.
I took an old printer mechanism with an unknown motor and unknown diameter wire drive.
I set the steps per mm to '1' and went through calibration 4 times to get to a final value.
A bit tedious but it does work (-: (and one does have to be careful with test distances lest it run into an endstop)

One might start with 1600 as that will give 1 turn of the motor on a stock Blackbox setup, (200 step per rev motor at 8x = 1600 steps per rev).
Then giving G0 X1600 (assuming X) should turn the motor 1 turn. If it only turns 1/2 turn then either the motor is 400step per rev, or the microstepping is 16x.
Either way you can now measure the movement and use the formula to correct the steps per mm value, do it several times to refine the value.

 

1204 = 12mm diameter, 4mm pitch. its a one-start screw.
1204 and 1605 is about the most popular sizes for hobbyists and are both single start. Multistart ballscrews are rare, and expensive. Consider the added complexity just manufacturing a recirculating ballnut with 2 or 4 channels (;

 

I was giving an example of an unknown system, and assuming a 200 step motor and 8x microstepping, 200x8 gives 1600 steps for 1 revolution of the motor.
This gives us a starting point, we can drive the motor 1 revolution and measure the carriage movement, and calibrate from there.

 

Think of that as the "number of threads" that will engage with the screw

In normal bolt-and-nut terms: Thats the difference between:


(I can see about "2 channels")

Here I can see maybe "5-6 channels"
and


And this one would have a tonne of "channels" as they call it

But its just one Helix/Start


- more contact area = larger load handling. No impact on the "pitch"



This would have "5 channels"

This is probably closer to the 1204 nut:



4 "channels" but its one loop

Send a 1mm move first (use Incremental Jogging to test), even if you are out by a factor of 500 it shouldn't crash (;

Yes, our 8mm pitch screws are +-200, so 400 should be good for 4mm pitch. The +/- comes in because screws are rolled, not ground (unless $$$) so there is some tolerance, theoretical is really close, but do check in Calibration