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

Tony,

EXCELLENT POST!

Yep, there's more to this stuff than brute force.

My old 1A, 7.4V 116 oz.in. motors were tooling one of my Sherline
5400 mills around at 45IPM the other day using the new GeoM control
box with its 34VDC and 7062 uni-chopper drives...

Usually they've been running with my old 297/298 drives in modifed
halfstep. They'll go faster, but in that config we run 'em at 40IPM.

Ballendo

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Tony Jeffree <tony@...>
wrote:

>
> 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
>