Re: [CAD_CAM_EDM_DRO] which steppers to buy?

Are you the same John doing the bridgeport conversion? If so, the
concept of 150 oz in motors is well, 'interesting'. You may need a
lot more than 3-1 reduction to even get the table to move at all.

Here's a test. Take out the backlash in the X axis and leave the
handwheel rotated so that crank pin is at the 3 or 9 o'clock position.
Measure the radius of the pin from the wheel axis. Tie a rope to the
pin and hang an empty paint bucket. Start putting junk in the bucket
until it moves. Weigh the bucket. Now put a big endmill in the
spindle, set up for a heavy cut, and repeate (make sure to feed
against the spindle rotation)

Multiply weight by radius and you have the minimum torque needed to
move the lightest axis... Now mulitply by (as a guess) about 4 if you
want any feedrate at all from a stepper. Scale by any reduction...

Another thing you can do is make up a collar to put a bridgeport
handwheel on the little stepper, and see how easy it is to make it
loose steps compared to the force torque needed to move the table. Or
go ahead and make up a mount to try it. Probably you will need to
make mounts for at least size 34 motors in the end, so if you current
ones are size 23 you might want to make the mounts oversize and drill
two sets of holes.

The low-performance CNC bridgeport at work has servo motors rated for
19 lb-in (that's 304 oz-in) and then some reduction. Steppers have to
be much larger than servos for the same application as their torque
falls off with speed. That's why according to a previous post the old
stepper bridgports used 1100 oz-in motors...

Chros

> Ok Group every one is saying to use at least a 200oz
> stepper .I called Smithy and asked then what they
> would suggest stepper s for the 17X20 3 in 1 they sell
> I was told that a 150 oz stepper and a 3:1 timing gear
> now I am confused is this rating they say too small I
> do have these stepper s now. I thought I should give
> them a try just for the heck of it . Am I looking for
> trouble using a small stepper I know that I can not
> get any real speed out of them , but going slow I
> think may be go till I learn more.
>
>
> John

--
Christopher C. Stratton, stratton@...
Instrument Maker, Horn Player & Engineer
22 Adrian Street, Somerville, MA 02143
http://www.mdc.net/~stratton
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