I am considering doing a CNC conversion on an older Bridgeport. I am
wondering what the torque reqm't would be for stepper drives. I have
some 300 oz steppers, but I don't think they would be big enough.
Maybe ok for the X/Y, but definetly not for the Z(if I move the knee).

Anyone out there have a good number to stick to these? For now, I
plan to use the stock ACME screws, but would like to upgrade to
ballscrews at some point. I know that the ACME screws are going to
have a higher torque reqm't.

JW

Jeridiah writes:

Anyone out there have a good number to stick to these?

I have one of the ones Bridgeport used, which is a big Slo-Syn item with
something like 300 to 600 oz-in stall torque. This was gear-belted 1:1
onto the ballscrew.

For now, I
plan to use the stock ACME screws, but would like to upgrade to
ballscrews at some point. I know that the ACME screws are going to
have a higher torque reqm't.

Roughly, mechanical efficiences are 50 percent for acme versus 90
percent for ballscrews. So you'll need to double the capacity of your
motors (and power supply) for acme, which will probably cost you more
than the ballscrews.

Stepper motors are not a good solution where speed counts. They are
hideously poor performers (slow, loss of torque at any decent speed)
unless you have a very expensive controller and power supply. Servos
are a much better and cheaper solution, especially on the scale of
making a big, heavy milling machine table move into resistance.

See my pages:

http://www.truetex.com/servomod.htm
http://www.truetex.com/ymount.htm

(Jeridiah) wrote in message . com...
I am considering doing a CNC conversion on an older Bridgeport. I am
wondering what the torque reqm't would be for stepper drives. I have
some 300 oz steppers, but I don't think they would be big enough.
Maybe ok for the X/Y, but definetly not for the Z(if I move the knee).

Anyone out there have a good number to stick to these? For now, I
plan to use the stock ACME screws, but would like to upgrade to
ballscrews at some point. I know that the ACME screws are going to
have a higher torque reqm't.

JW

You could use them with a 2:1 reduction.
WHEN(not if) you want more speed, go for it.
In the meantime, you stated that you already have the
smaller motors. You will learn alot just from the
experience of getting them working.

Pete

I did a retrofit on a Chinese mill/drill. I used 425 in-oz motors with 0.1"
pitch lead screws on the X and Y. I would have preferred even more torque
but the motors were free. The machine worked OK for most small jobs and I
didn't have problems with loosing step except at fast speeds. Step motor
torque goes down fast as the speed is increased. On a Bridgeport I think
the lead screw pitch is larger like 0.2" or 0.25" so I would definitely
recommend a 2:1 reduction on 300 in-oz motors. Most retrofits and even
Bridgeport CNCs have the z axis motor on the quill not the knee. I think
that makes more sense considering things like drilling cycles.

John


Jeridiah wrote in message ...
I am considering doing a CNC conversion on an older Bridgeport. I am
wondering what the torque reqm't would be for stepper drives. I have
some 300 oz steppers, but I don't think they would be big enough.
Maybe ok for the X/Y, but definetly not for the Z(if I move the knee).

Anyone out there have a good number to stick to these? For now, I
plan to use the stock ACME screws, but would like to upgrade to
ballscrews at some point. I know that the ACME screws are going to
have a higher torque reqm't.

JW

I would like more too, but the motors and drives I have were free.
Just didn't want to go through the work of doing all this to find out
the motors are not strong enough. The drives can handle more than the
steppers need, so that is one option.

I think I will go for a 2:1 reduction. I am well aware there are
going to be a lot of limitations to the system, but when I get it
working I can then determine the value(to me) of upgrading to
eliminate those problems.(the ultimate would be a full servo system w/
enough power to direct drive, but that get's expensive).

Has anyone every done a blended system,some stepper and some servo?

JW

"zephyr" wrote in message ...
I did a retrofit on a Chinese mill/drill. I used 425 in-oz motors with 0.1"
pitch lead screws on the X and Y. I would have preferred even more torque
but the motors were free. The machine worked OK for most small jobs and I
didn't have problems with loosing step except at fast speeds. Step motor
torque goes down fast as the speed is increased. On a Bridgeport I think
the lead screw pitch is larger like 0.2" or 0.25" so I would definitely
recommend a 2:1 reduction on 300 in-oz motors. Most retrofits and even
Bridgeport CNCs have the z axis motor on the quill not the knee. I think
that makes more sense considering things like drilling cycles.

John


Jeridiah wrote in message ...
I am considering doing a CNC conversion on an older Bridgeport. I am
wondering what the torque reqm't would be for stepper drives. I have
some 300 oz steppers, but I don't think they would be big enough.
Maybe ok for the X/Y, but definetly not for the Z(if I move the knee).

Anyone out there have a good number to stick to these? For now, I
plan to use the stock ACME screws, but would like to upgrade to
ballscrews at some point. I know that the ACME screws are going to
have a higher torque reqm't.

JW