[originator]

I have a design I can't find a solution for. Imagine a pievce of 8020 extreude aluminum tube, the 2020 to be exact which is a 2" x 2" OD square tube. Inside there is a Delrin nut that runs through the center on a 1/2 acme screw. This tube currently mounts vertically on a stand, permanently mounted to some legs on the ground. This tube supports 3 other arms with steppers moving stuff around to do some work. I want to take the vertidcal tube, remount it on some type of apperatus so that it can spin around 180 degrees, controlled precisely and smoothly by another stepper. I have no idea on how to mount this other than some crude flange that is metal to metal. Has anyone ever seen something that I could borrow from that is designed to rotate a vertical tube driven by a stepper for the full 180?

Thanks

[vger]

I don't know how much wieght you must support, however, borrowing from an assembly that was a carosel sorting bin for a microfiche duplicating machine, here goes. The unit was about 30 inches diameter and 18 inches tall. 2 large bearings were mounted in aluminum plates, one at the top and one at the bottom connected together with several vertical pieces. It had a stationary 3" post up though the middle of the bearings. Rotation was accomplished with a stepper motor and timing belt monuted under the whole assembly. If you reversed this and mounted the plates to a shortened version of your stand and attached your 2 X 2 post to the center post, might work. For very smooth and precise rotation without metal contact, I've seen flat teflon washers and bushings used in leiu of metal bearings for support, and plastic (probably delirin) ring gear with worm gear drive from a motor. The teflon washer and bushing arrangement was a focus assembly on a large lens assembly (about 15lbs) in a 35 mm microfilm camera.

[originator]

Thanks vger for the good ideas. The height of the vertical boom is 6 feet. The vertical boom has a 4 foot boom attached to it that extends straight out, and contains a few more nema23 steppers, so the total weight of the whole thing is under 25 lbs, plus the torque of the horizontal boom with forces that want to tilt the thing over at all times. Ideally some type of precision base comprised of bearings and a means to rotate it. A screw and nut would be great, but I can't picture a way to get 180 of motion like that. Like you said, a stepper with a belt is workable. A stepper against a gear would work, although the deflection would likely require the steppers gear to be supported between two bearings to keep the load off the stepper.

[Runner4404spd]

how about a harmonic drive? add some bearings to take the load and build a base for it. that way you have all the precision you will ever need. or for something less precise how about a rotary table?

[originator]

I will google up a harmonic drive, never heard of it but I'll check it out. I just found a rather bulky concept that could be scaled, here is the idea that is quite simple. This idea could be mounted using 1/2 id bearings top and bottom, very precise, quiet. A stepper and screw could be added to get 180 out of it with the right design.

Aha the rotary table is the ticket!

[Runner4404spd]

ideally with a rotary table, you want to keep going in one direction. there is some inherent backlash in the gear, so many repetitive motions back and forth and you can lose position. the best option would be to couple the rotary table with an encoder feed back so you actually have position of the table.