How to calculate required stepper torque for indexer project

[3IMOH]

Howdy All-

I am working on a home built indexer that is similar to a rotary table. I have some various sized disks that need holes machined at various points radially on the edge. My plan is to mount a fixture on the end of my CNC table that will rotate these disks under my spindle. This fixture would have a stepper motor mounted at the center of the disk, with the shaft oriented horizontally.

The stepper would need to be able to spin the disk to various points to allow the spindle to make the cuts. This indexing motor would not be moving during the cut, so it does not need to counteract any cutting forces. My plan was to actually have a clamp that locked the disk to the fixture during cutting, to prevent any issues.

The disks range in size, but the maximum will be around 44 inches in diameter and weigh around 20 pounds. the back of the disk and the fixture will have UHMW plastic to minimize friction.

My question (finally) is this. How do I calculate the torque needed to spin this disk given the diameter and mass? I do not need to spin it very fast (1 to 2 revs per minute would be fine). The only purpose of the motor will be to accurately index the part.

I am hoping I will be able to direct mount to the stepper. I am considering various belt drive reducers, or even purchasing a super low geared stepper, but I have a 7.2:1 geared motor that I am hoping I will be able to use. It is rated at 79 inch pounds of torque.

Any thoughts would be appreciated!

thanks,
Howie

[jalessi]

Howie,

Most rotary or indexers have between 72:1 up to 100:1 reduction.

The diameter of what you are working on is really big however Tormach makes a very inexpensive eight inch CNC rotary table. You could make a base plate that would raise the rotary up 25 inches and use a adapter fixture to mount your part to.

http://www.tormach.com/Product_RotaryTable.html

After you finish running your job you could sell the rotary for a slightly discounted price, everyone here is always looking for a deal!

Good luck with your project.

Jeff Alessi
jalessi@aol.com

[jeffs555]

My question (finally) is this. How do I calculate the torque needed to spin this disk given the diameter and mass?

The torque needs to overcome two different forces, friction and inertia and you need more info to calculate either.

Torque required to overcome friction can be calculated, but it is much easier just to measure it.

Torque required to overcome inertia depends on how fast you need to accelerate the part. If you are not concerned with how long it takes to rotate, then this torque is not important. If you are, then you need to look up formula for rotary inertia.

[jalessi]

Check this board out I think you will find what you are asking for. They have a very good engineering forum.

http://www.physicsforums.com