Re: [CAD_CAM_EDM_DRO] Servos vs Steppers costs

Robert Allen & Marsha Camp wrote:

> I am getting ready to retrofit the motors and drives on
> a 2 axis ball screw driven table on an old mill in almost
> unused condition as I have posted previously.
> I was looking at the Gekko G201 drives at first but
> after reading about the G320 and G340 I'm wondering if the
> steppers are the way to go.
> The table had 350 in.oz. 5 wire unipolar motors on it, w/1:1
> drive ratio, w/an ancient drive and would move at 10
> in./min. I was going to use the G201 drive and some 720 in.
> oz. bipolar motors and was expecting about 20-30 in./min.
> from this set up.

Unless you've already bought these motors, you might think
about servos. You will have to buy motors, encoders and amps.
If you can get a good deal on some servo motors (and there are
deals available, like taking them out of scrapped tape drives,
etc.) this may be a real good way to go. It WILL be more
effort, but you should have a much better machine in the end.
The bigger the machine, the more trouble steppers will be.
The really huge steppers needed to move at a reasonable
rapid feed rate will usually be HELL to find, and expensive,
even used, if you can find them. then, you have the problem
of finding a stepper driver that can drive them properly.

> I am curious if maybe servos are the way to go now that a
> drive is available for $100.
> I saw 420 in. oz. servo motors on Camtronics web site for
> $100 w/encoders and not knowing the difference between the
> ratings of steppers and servos or even stepper and servo
> drives is this even close to being a possibility?

Sure. Make sure the motors are optimized for very high torque at
VERY LOW speeds, as you are talking about direct drive, here.
(If you change to a belt reduction drive, you can get by with
smaller motors that spin faster.) 420 Oz-In may be OK if that
is the continuous duty rating, but you may need more for direct
drive if that is the peak rating. With a 5 TPI screw, direct drive,
120 IPM feed is only 600 RPM at the leadscrew. So, you want
the motor to have a low Kv, so that it develops reasonable voltage
at these low speeds. Otherwise, at 600 RPM it could be only
getting 5 or 10 Volts across the armature, and not developing
much power.

I can calculate some values based on the 420 Oz-In torque.
Since it is direct drive, you can treat the 5 TPI screw as if it
were a drum with string wrapped around it, with a circumference
of .2", the radius is .2/(2 Pi) = .03183". So, the pull on the
'string' would be 420 Oz-In / .03183 In = 13195 Oz, or
825 Lbs of linear force on the table. I have a small Bridgeport
mill, and I have a setup with about 1116 Lbs linear force at
the peak current of the servo amps. So, your arrangement with
those motors direct driving the screw may be a bit weak, if the
420 Oz-In rating is peak.

Jon