Hi,
I think I go with garyhlucas on this one. 425Oz/in works out to 2.2 ft lb or 3 Nm, with 5mm pitch 20mm dia screws to
give a stall thrust of 4000N or 400kg force, isn't that enuf?
The trick of course is to maintain that at speed which with steppers is all but impossible.
I use small Vexta 5 phase steppers of about 90Oz in thru 10:1 low backlash planetary gearboxes. The output torque, at low speed,
is 880Oz in or about 6Nm, tons of grunt but fairly slow. I run them at 2400 rpm with the gearbox output shaft at 240 rpm for rapids
of 1200mm/min with 5mm pitch screws. Its only a small machine so I don't usually fall asleep when doing rapids.
What impresses me is that 5 phase steppers are considerably smoother running than two phase steppers and the loss of torque at speed
is less pronounced.
I have recently bought another stepper for an A axis, a Vexta 5phase 34 size, 804Oz in direct. It supposed to run at 2.8A but I have it running
on a 1.4A drive at 3000rpm no sweat!
I'm using Vexta drivers and they produce about 150V peak, so its probably no wonder that they speed along OK....
If you want speed consider 5 phase steppers, they're rare and expensive likewise good high voltage drivers. Probably little to pick and
choose between them and genuine servos but they are QUICK.
Craig