Multiple steppers on one axis

[chj.sullivan]

So I just finished up the drawing for my latest CNC project. A 32x72x6 working area Gantry unit. I planned to use a single ball screw on the Y axis. But this plan will require very large Extrusions to supply the strenght needed across the very long table when all the numbers are added up. Not a big deal, but I started to think maybe there is another way.
I plan to use the Gecko 320V Stepper Motor drives. Have not planned the motor selection yet as I don't have an exact weight of the gantry unit.
I'm now thinking that if I planned for two ball screws to drive both sides of the Gantry I could use much small Extrusions, and supply better support for the table. And have a lighter Gantry unit.
I know you can run Steppers wired paralell to each other off one Drive unit. Or I have been told you can. And I've seen units built this way. But I'm not framilliar with any possible problems this may cause.
After a fair amount of "Searching" I still have many questions.
Do you need to use "matched" steppers?
My understanding is that as long as the two motors power requirments together do not surpass the limits of the Driver you OK. But I'm not sure what the reasults of running to motors together are. I mean as far as do you double the Amps/Phase of a single motor? How about the Impeadence?
Can someone please help guid me here.

[BobF]

Use 2 drives and synch in sofware.

[Crevice Reamer]

Most of the buffered breakout cards will have enough drive to run two Geckos. The 201's take about 15 to 20 ma of Step & Dir drive to work reliablly. If you find a board that will provide 30 to 40 ma it will work by just tying the Step & Dir for each drive together at the same terminals on the breakout.

If you use the new Gecko 203's you can use any buffered breakout since the drive level requirements are much lower.

Do not try to run two steppers off one Gecko. Steppers have distinct detents at the poles (200 per motor). The microstepping and more importantly the low speed anti-resonance you pay extra for in the Geckos will suffer and the results will be lost steps or other ugly things.

Running two steppers from two separate Gecko's mechanically linked on the same shaft will have some negative results as well. Trying to keep the motors in sync (poles at the same position on both motors) is impossible so you have the same cancellation of microstepping and possible loss of torque (motors fighting each other for a position). It will work, but honestly running a bigger motor is ususally more cost effective. You can get 1200 oz-in motors for $140.00 from Homeshopcnc.com

Running two motors on a gantry with one one each side (dual drive) that are independant to double the torque has some advantages. With MACH you slave one axis to another and the you can actually make the steps per unit on each axis slightly different to perfectly calibrate for varience in mechanical components. You can run separate homes and the gantry will automatically "square" itself any time you do an axis reference. If you make one of the Home switches adjustable then you can use it to perfectally square the gantry.

CR.

[chj.sullivan]

Well the break out board explanation was what I was looking for.
But I use Mach 3 and did not think of doing it that way. Which is really quite short sighted of me as I seem to have a link in "favorites" that leads to a post on ArtSoft Forum concerning this very topic.
Thanks for the help.