Guys - Please forgive me as I know NOTHING about this stuff. i am simply a Computer Geek trying to help a friend set up his CNC machine.

He has 3 Gecko Drives hooked up to REX motors. He is using MACH3 SW

We are trying to tune the motors and the whole Steps per unit are beyond me.

He has a 21 Motor Pulley, a 42 other pulley, 8 thread pitch and X, Y & Z Axes.

The documentation elaborates more on the Z but I dont have it on me.

We are trying to tune and the motors seem to work well sometimes, then grab and shake, then work. using the up and down arrows dont change direction all the time. I am totally lost.

Can someone tell me what my Steps per unit should be and what Acc and Velocity i should be setting this up for? Any help would be appreciated

200 steps per rev of motors (could be 400 but they are uncommon)
10 microsteps of the Geckos
2x motor revolutions for  gearing to turn screw one revolution
8 turns of screw to move one inch

So its 200 x 10 x 2 x 8 = 32,000 steps per inch

As for Velocity and Accel you will just have to do it by trial and error as all machines are different due to all sorts of factors such as motor size, axis weight power supply voltage etc etc. Start putting both low then increase one until you miss steps then back off a bit then increase the other until again you miss steps and back off. You can then fine tune by tweaking them again to get the best compromise of speed and acceleration.
Hood

This has been covered many times in this forum.

On the motor tuning page, you must enter the number of steps per unit. It depends on whether your friend requires his standard to be in inches or millimeters. It does not matter to the system, and Mach can deal with inches or millimeters, no matter what units it has been initially set up in.

It all seems to rest on the ptich of the leadscrew - whether this is in inches, or millimeters. I initially had a 10 turns per inch leadscrew, therefore I set my lathe up in inches. I have just changed my leadscrew to a ballscrew (more accurate) and this is 5mm pitch - so I have now set my machine up in millimeters.

Once the machine is set, all the standard measurements to set up tool tables and offsets must be entered in inches or millimteres. It then makes no difference to the operation of the machine since a code of G20 will make the machine operate in inches, and a code of G21 will make it operate in millimeters, and all values entered in the tables will be changed correspondingly.
When you say you have an 8 thread pitch - what do you mean - this must be accurate as you will see, and be careful that someone has not sold you a metric screw as an imperial. If he is using  a fixed machine, he will know.

To calculate the number of pulses - and note the word calculate - it is an follows - and all number multiply the previous answer.

The vast majority of motors now work on 200 pulses per revolution - check that these motors do - or alter the number accordingly.
Gecko 201 drives give a 10 micro pulse output - 200 x 10 = 2000
Your gear down appeasr to be 2 to 1 - 2000 x 2 = 4000
Finally we come to the pitch of the leadscrew  - on my original leadscrew it would have been 10 turns to the inch - 4000 x 10 = 40,000 (units in inches)
on my new leadscrews it would have been 4000 / 5 = 800 (units in millimeters)

That is how you do it . you will have to calculate the final  line yourself.

After you have done that enter the result in your x and y (and z) axis, and test it. Move right, stop and zero your DROs (by pressing the zero x, zero y), set up your measuring instrument ( I use a digital caliper). Then, using the MDI line enter G0 X1 (inches) of G0 X25(mm's). This should move right again (to avoid backlash) and measure the distance the table has moved.

Do not expect to be absolutly accurate (obviously the more accurate the better) because it is your measuring that is at fault, not the machine. If you are within 2 thou or even 0.01 (mm) then that is pretty good and shows that you calculations are accurate.

Have a go - come back if you get in bother.