Ball screw

[Ed Williams]

I'm a newbie. Thought I should get that out of the way first. I'm wanting to use this on my Y axis. I have a five turn per inch on my X. What would be the turn of the Kuroda ball per inch below. And I'm wanting to get rid of my steppers and go servo. The router table is 14"x36". What size servo's should I use and what gear ratio should I use. Keep it simple. I am.
Thanks
Screw Make: Kuroda
Screw PN: 25-25 C5S
Screw Overall Length: 700 mm (27.5")
Screw Threaded Length: 510 mm (20")
Ball Nut Travel: ~430 (17")
Screw Diameter: 25 mm (1")
Screw Lead: 25 mm (1")
Screw Drive End Diameter: 0.50"
Accuracy Class: JIS-C5

[dkowalcz]

"Screw Lead: 25 mm (1")" - sounds like 1" per turn to me.

Motor size - well, a lot depends on how fast you want to go, how accurate you want to be, or how deep of a pass you want to make, and how heavy the Z column is. I'll shoot from the hip and say 3 or 400 oz-in at the business end of the motor should be fine to start with, more is better, the gearing doesn't matter as much as long as it gets you there.

[Jason Ross]

Your moment of inertia for your ball screw is about 2.1 kg*cm^2 you need a servo gearhead moment of inertia of at least .4 kg*cm^2 for a 60mm motor a 3:1 gearhead would be adequate for a 90 mm motor you could get by without a gear box. This does not take into account your table's inertia. My guess is for the 60 mm you would need at least a 5:1 and probably a 10:1 ratio and for the 90mm at least a 3:1 and probably 5:1 gear reducer. Inertia mismatch is one of the determining factors in controllability of the servo system. Servos have good high speed torque so you do not lose much there. Precision gearheads are expensive but they also multiply your resolution of your feedback so you have very good controlability. With a 100mm or larger motor you probably would not need a gearhead.

[Ed Williams]

I know motors my oz/inch. What would be the 100mm in oz-in. And I was going to use Geck 320 drivers would this Encoder US Digital - E2-250-25-G have the wright resolution for my table.

Thanks

[Jason Ross]

About 350 -400 oz in depending on the winding selected. The encoder probably should have a higher resolution. I believe yours has 250 pulses per revolution through quadrature it would be 1000 counts per rev. Since your lead is 1" the best theoretical resolution you could have would be .001" without a gearbox. In practice you would have a resolution of about .003-.005" depending on your ability to tune the motor (something you would not have to do with steppers) It should be cheap enough to get a 1000 or 2048 ppr.

[Ed Williams]

Jason thanks for your help. Sounds like you know alot about this. I hope you don't care if I ask one more question. What is the specs on this motor I don't understand it.

Peak/Continuous Torque Gearing - 2.45/0.29 N-m

Torque Constant - 0.10 N-m/A

Gearing - 0.0118 m/rev

[Jason Ross]

Torque constant = 0.1*141*oz.in/amp (.1 nm/amp). I would assume that is on the motor before the gear box.
Your gears can do limited excursions up to the peak torque but it seems a little light to drive a 1" lead ball screw and you should limit your accelerations.
It seems like you have a large gear reduction so that inertia matching should not be a problem I have not seen gearing defined by the last number but it looks like it is around an 85:1 reduction. If so your inertia matching should be fine. Your top speed will be pretty low. If it is an 85:1 reduction your 250 ppr encoder will work well.

[Ed Williams]

I found this about the motor they said there is no gear head and the motor was 36 VDC No load Rpm at 36vdc =3,130 Maximum Continuous Current = 3A .
Thanks