Hi,
ideally the steps per unit should be calculated but many people get confused and use a measurement approach.

It seems strange to me that even the most mathematically challenged person seems to grasp compound interest
on their loan......all without a calculator....and yet cannot get their head around 'steps per unit'.

What pitch are your ballscrews?
Do you have belt reduction from the stepper to the ballscrews? If so what is the reduction ratio?
What have you set as microstepping? If you haven't or don't understand I would recommend 8 microsteps per
fullstep or 1600 steps/rev.
What are your native units?

Tell me these numbers and I will outline how you calculate the 'steps per unit'.

Craig

Hi Craig,

I have belt drive for "X" and "Y" with 24 tooth pulley. Microstepping can be either 1/8 or 1/16.  Native units is mm.

"Z" axis is screw with 2mm pitch direct drive from stepper.

Please explain CPR and Mach3 steps per motor revolution.

Thanks,

Roger

Hi Roger,
CPR stands for counts per rev. It is usually applied to an encoder.

If an encoder has 2500 lines, it sometimes called pulses/rev or PPR then because of the quadrature placing of the opto-detectors
it will have 10,000 counts per rev or CPR.

If you set your microstepping to 1/8^th (recommended) then your steppers will rotate exactly one revolution when the drivers are presented with 1600 pulses or CPR.

Lets tackle the Z axis first. If the stepper rotates once with 1600 pulses and the z axis will advance 2mm, being the pitch of the ballscrew.
The 'steps per unit' value that you put on the Z axis tuning page will be:
StepsPerUnit=1600 / 2
=800

Easy!

Now the X and Y axes are not so simple. You say they are belts with a 24 tooth pulley? What pitch are the teeth on the belt?
For the purposes of demonstration I'm going to assume 3.15mm pitch (1/8 inch).
StepsPerUnit= 1600 (steps per motor rev) / (24 teeth x 3.15)
=21.16
You will have to round to 21 steps per unit.

Note this is very course resolution, every pulse will advance either the X or Y axes 1/21 (=0.05)mm. Is this what you expected or wanted?

Craig

Craig,

Lets see if I have this right. I was hung up on the encoder and trying to factor the 4 into the equation.  Apparently the items factored into the calculation are:  Motor Steps / rev (200), Microstepping (16), and distance moved per step or rev.

My calculation: 200 X 16 = 3200 Steps per revolution.  (Belt 3mm pitch, pulley 24 tooth) 

(3200) / (24 X 3) = 44.444 pulses per mm      Is this correct??

You mentioned previously that 8 is the preferred microstepping factor.  Is there a reason to not use 16?

Thank you much for your help, it is much appreciated,

Roger

Hi Roger,
CPR stands for counts per rev. It is usually applied to an encoder.

If an encoder has 2500 lines, it sometimes called pulses/rev or PPR then because of the quadrature placing of the opto-detectors
it will have 10,000 counts per rev or CPR.

If you set your microstepping to 1/8^th (recommended) then your steppers will rotate exactly one revolution when the drivers are presented with 1600 pulses or CPR.

Lets tackle the Z axis first. If the stepper rotates once with 1600 pulses and the z axis will advance 2mm, being the pitch of the ballscrew.
The 'steps per unit' value that you put on the Z axis tuning page will be:
StepsPerUnit=1600 / 2
=800

Easy!

Now the X and Y axes are not so simple. You say they are belts with a 24 tooth pulley? What pitch are the teeth on the belt?
For the purposes of demonstration I'm going to assume 3.15mm pitch (1/8 inch).
StepsPerUnit= 1600 (steps per motor rev) / (24 teeth x 3.15)
=21.16
You will have to round to 21 steps per unit.

Note this is very course resolution, every pulse will advance either the X or Y axes 1/21 (=0.05)mm. Is this what you expected or wanted?

Craig

No need for rounding in Mach3. Mach will be perfectly happy with fractional steps per.