Re: Driving gantry with two paired stepper motors

In a message dated 01/13/2000 7:31:59 PM Hawaiian Standard Time,
gpotter@... writes:

<< The two motors appear to be staying in

> phase with each other. I realize that you can't get
> the same torque from each motor (as you would if
> driven individually) but if you have a mechanical
> transmission, you also split the torque.
>
>
> My question: are there any potential problems with
> this technique, electrical or otherwise?
>
> -Richard- >>

Richard,
It seems to me, if you do lose steps, the homing procedure will be
interesting. You would need to mechanically stop one motor against a home
stop and make the other home. Of course your gantry will have a slight twist.
You would need two home switches in series to signal home.

Unless you are building something huge, I think coupling the two leadscrews
together with cog belts would be the easiest and most reliable. In X or 8mm,
belts can be had in fairly long pitch lengths. Most people make the mistake
of using sprockets that have too few teeth. This causes belt jumping when the
belt stretches under load. Use the HPT style belts whenever possible as they
are more jump resistant. I would overspecify the belt / sprocket specs so
they would be stiff under load. They would be more wear resistant. To allow
adjustment of the gantry to be square to the bed, one sprocket would be split
clamped on to one of the lead screws, the other would be keyed on to reduce
possibility of loosening and causing gantry twist. There is some frictional
loss in the cog belt drives but it is not very large. Get the largest motor
you can as nothin beats cubic inches to power through your cuts.


Peter
THRD, Inc.