Re: Motor Size estimate?

At 04:22 PM 10/14/99 -0400, you wrote:

>Could anyone guess what kind of motor torque I would need to operate
>the crossfeed of a 13" south bend lathe through either the existing 8
>tpi acme screw or a 5tpi ballscrew

Just for ref, I ran plain screws on my lathe for a while during the
conversion phase ... I used various combos of manual and CNC control
throughout this while making the parts to hang the screws. I was able to
run the long feed with 8 TPI and direct drive, using stepper rated at 450
oz-in at I think 3.5A; I am actually only giving them 2.8A. I'm sure your
carriage is a lot heavier though and a 13" lathe is going to take more
force at the cutting edge. You can probably get by fine with 600 oz-in and
ball screw on long feed, esp if you drive with a belt reduction. In that
regard, I pushed the HS out of line on my lathe the other day using the 450
motor, 5:1 gearing, a 5 mm pitch b-screw, and a wrong command. The HS sits
on flat ways and is clamped down by two 10mm bolts. It's never moved under
cutting forces and I've jammed it down to stalled before plenty of times.
This didn't seem to faze the screw, nut, stepper or drive, so I'd say 450
is fine for cross feed.

I'm using Techno-ISEL screws and they're kind enough to give a force formula:

M=F*p/2000*pi*E

M is torque, N/m, F is force, Newtons, p is pitch, mm, E is efficiency.
They give an example with 10mm pitch screw and E=90%: 50 oz-in direct-drive
produces 45 lbs thrust. That would be 90 lbs at same torque and 5mm pitch,
450 lbs at 5:1 and slightly over two short tons at 450 oz inches and 5:1.
No wonder it pushed over! That be enough for you?

>I'm considering open loop stepper, closed loop stepper, and servo as
>possibilities, probably with tooth-belt reduction.

Berg makes a real nice belt drive and pulley line in various widths. I
picked units rated at I think about 100 lbs working. I'm using open loop
for simplicity and it's really very good about following. I don't know how
you could lose steps unless you do crash it pretty hard.

>(For my application, a sort of software tracer attachment

You doing a digitizing thing to go with that?

Best wishes,

Hoyt McKagen


Belfab CNC - http://www.freeyellow.com/members/belfab/belfab.html
Best MC Repair - http://www.freeyellow.com/members/batwings/best.html
Camping/Caving - http://www.freeyellow.com/members/batwings/caving.html
If assholes flew, this place could be an airport !!