Machsupport Forum

Mach Discussion => General Mach Discussion => Topic started by: MadHattan on August 15, 2013, 11:11:40 AM

Title: Need help with pulse per inch math
Post by: MadHattan on August 15, 2013, 11:11:40 AM

I have scratch built my own desktop sized CNC.  I have Mach3 working with my three stepper motors and limit switches so now I am attempting to dial in all the MACH3 configuration settings.  Calculating the actual pulses per inch should be simple.  This is what I have:

• Ball Screw, 5/8" Screw Diameter => 13/64" Travel/Turn (http://www.mcmaster.com/#5966k26/=o2q45e (http://www.mcmaster.com/#5966k26/=o2q45e))
• WantMotor Stepper Motor Driver DQ542MA => 40 pulse per step?? (http://www.wantmotor.com/ProductsView.asp?id=257&pid=82 (http://www.wantmotor.com/ProductsView.asp?id=257&pid=82))
• WantMotor 57BYGH420 Motor => 200 step/ revolution (http://www.wantmotor.com/ProductsView.asp?id=160&pid=80 (http://www.wantmotor.com/ProductsView.asp?id=160&pid=80))

My problem seems to be knowing what to put into the equasion for the Stepper Motor Driver DQ542MA.  The driver documentation shows that with the DIP switches set the way I have them [on-on-off-off] I should expect 8000 pulses per revolution.  I presume that the 8000 pulses per revolution is assuming that I am using their 1.8 degree/pulse motors, which I am.  That would indicate that the driver is giving me 40 microsteps per step (I think!).

Can someone check my math (see attached equation)?  

Thanks!

Title: Re: Need help with pulse per inch math
Post by: ger21 on August 15, 2013, 11:36:49 AM

I'd set the switches to 1/8 step (off-off-on-on - 1600steps/rev) - 8000 is way to high and will slow you down.
The ballscrews I think are 5 turns/inch, so 1600x5 = 8000 steps/inch

Title: Re: Need help with pulse per inch math
Post by: MadHattan on August 15, 2013, 11:43:05 AM

Gerry!  Thanks for the response.

I don't understand how you came up with 5 turns/inch on the ballscrews. I don't doubt you. I just want to learn and understand.

Thanks!

Title: Re: Need help with pulse per inch math
Post by: HimyKabibble on August 15, 2013, 11:52:21 AM

Those aree weird screws....  I've never seen ballscrews with that odd pitch.  As they are 13/64" lead, they need to turn 64/13 turns to move one inch.  Gerry is absolutely correct that it is useless to use 40X micro-stepping.  Anything more than about 10X gains you nothing of value.  So, using 8X as Gerry suggests, you'd have:

    200 Steps/Turn * 8 uStep/Step * 64/13 Turns/Inch = 7876.923076923 Steps/Inch

Regards,
Ray L.

Title: Re: Need help with pulse per inch math
Post by: MadHattan on August 15, 2013, 12:19:25 PM

Thanks for the assist!