Re: [CAD_CAM_EDM_DRO] Motor sizing

Tony Jeffree wrote:

>I know we all use the old "rule of thumb"/trial and error approach to
>deciding how big a stepper motor to use (with variable success), but has
>anyone come across a well-documented, definitive approach to stepper motor
>sizing (taking account of mass being shifted, desired speeds, acceleration
>rates, screw type/pitch,...etc. etc.) - preferably in software form?
>
>

It is very difficult, because the slope of the declining torque as speed
increases
is determined by the motor, the driver and the power supply, at least.
So, unless you have a torque curve for the specific motor/driver/PS voltage
combination to begin with, you can't even begin. These curves have been
published by some motor and drive makers for combinations of their
own components. But, holding torque is not a useful number, except
in that it indicates very rougly the dynamic torque at some speed, within
some ridiculous range of about 50%. People generally end up using
VASTLY oversized motors to be sure that the stall point is WELL above
what they actually expect to run at. The frictional drag of sliding-type
screws and machine tables can vary over such wide ranges that it makes
calculation almost a waste of time. You can weigh your table and know
that the weight won't ever change, other than vise and workpiece weight.
So, that is quite well controlled. But, the friction will go down right
after you lube the table, and then slowly creep up as the lube is wiped
off the ways. With many of the smaller machines with smaller table
to way contact area, the friction can climb to amazing levels, and on
badly worn machines, they can actually bind up at times. Cutting forces
on the larger machines can get to fairly large values, too.

I used a linear force of 1000 Lbs (including friction, acceleration and
cutting
force) as the benchmark for my Bridgeport retrofit. Since I have
snapped off
a 3/8" end mill in the stationary spindle when I hit it with a clamp
bolt while
jogging around during setup, I guess my motors are sufficient. (This is
a servo
system, not stepper.)

Jon