It seems as if you are narrowing down to the problem.
I wanted to post the article I read showing the different speeds vs stepper torque.
/quote
By Tony Jeffree:
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 can be
surprisingly bad on the standard mill - my worst axis had about 5 thou
backlash), and secondly to convert from Taig's 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 Taig's "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.
The differences here are basically down to differences in motor
inductance;
the higher the inductance, the worse the high speed performance will be
for
a given supply voltage. So, given unlimited driver voltage, these
differences would not be a problem. However, in the real world, where
drivers suitable for a machine like the Taig (e.g., the Xylotex board)
are
often limited to a supply of around 30-40 volts, choosing a motor that
has
lower torque and low inductance is often a better deal than choosing
one
that has high torque but higher inductance.
Needless to say, I will be using the 140 oz-in 3A/phase 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.
/quote
James  |