How much torque is enough? (Was: COMPLETE 3 axis CNC...)

At 00:52 03/05/2006, you wrote:

>P.S. Still amazes me how quickly what once was acceptable becomes
>insufficient; how "testosterone" seems to have invaded DIY-CNC...
>I have machines with 150 oz.in motors going 240 IPM, carrying 3HP
>routers. Sherline's with "only" 116 oz. in. have been making money
>for me for over a decade. Yet many folks now seem to think that if
>you don't have the biggest, fastest...

Ballendo -

The problem is that people tend to focus on the motor torque without
understanding the effect that other factors (motor inductance, current
rating, supply voltage, leadscrew pitch, desired acceleration rates, etc.
etc....) will have on the system performance.

I did a very interesting test the other day on my Taig mill.

I bought one of the original Taigs with 140 oz-in motors, rated at 1A/phase
(something like 7 volts to drive that through the 7 Ohm coil resistance) -
it was the first of their mills to be imported to the UK a few years back.
A couple of years later I upgraded to their current 200 oz-in motors -
still 1A/phase, so even higher coil resistance - ~9 Ohm IIRC. I recently
decided to give the mill a makeover, firstly to improve on the drive
coupling and bearing end-float adjustment arrangements (which are
surprisingly bad on the standard mill - my worst axis had about 5 thou
backlash), and secondly to convert from the old "bi-level chopper" drive to
a modern microstepping drive. Having re-vamped the mechanical side to the
point where I can no longer measure any backlash, I decided to see what
maximum step rate I could achieve with three alternative motors, using a
driver capable of unipolar half-stepping at up to 2A/phase and a 24V supply
(actually one of my Divisionmaster units).

The first motor was one of the "standard" 200 oz-in 1A/phase motors. The
max half-step rate with this motor before losing steps was 1250
half-steps/sec, equating to around 9.4 IPM (20 TPI leadscrews).

The second was a 254 oz-in, 2.5A/phase, so with the reduced (2A) drive,
effectively a 211 oz-in motor, so very comparable to the first in holding
torque. This managed 4000 half-steps/sec or 30 IPM.

The third was a 140 oz-in, 3A/phase motor; again, with the reduced drive,
effectively down-rated to 93 oz-in. This managed 7000 half-steps/sec, or
52.5 IPM.

Needless to say, I will be using the 140 oz-in motors for the final
configuration, but will be driving them at their rated 3A/phase and will be
using a ~40V supply and a microstepping drive. If you do the math on what a
motor like that can deliver through a 20 TPI leadscrew, you reach the
conclusion that this combo can quite happily generate a force of around
100-140 Kilos at the tool tip (factoring in leadscrew efficiency and the
dynamic torque the motor can generate). I feel that should be more than
sufficient for a mill the size of the Taig, and certainly, the ability to
do rapids at >50IPM is way more than I will ever need (or will feel is
safe!) on this machine.

Regards,
Tony