[planescott]

hello, How would i find out what size rack and pinion would work for my
machine? I have looked at mcmaster carr rack and pinions and they dont
give out to much information on them other than sizes.
And i am assuming that i would need to gear it because one revolution of the motor would be about 3" for the smallest pinion.

scott

[jcc3inc]

Scott,
What size machine do you have? What are you going to do with the machine? Run plasma or oxyfuel, do wood routing, or machine metal??? All these influence the choice of rack. You may need to use gear reduction; I made a machine with no gear reduction, but it is for light routing or engraving. Provide us with more details for a better answer.

Jack C.

[planescott]

It will most likely be around 3'x5' with a fixed table style, the gantry will
weigh around 25 lbs. I will be cutting mostly plywood and hardwoods and
want to use a 3hp router in it. I am thinking 680oz-in stepper motors with geckos will be about right.



scott

[jcc3inc]

Scott,

I just weighed my 32" wide carriage and it runs around 75# with my Craftsman plastic router AND the power supply/transformer/geckos. I think your 25# is a bit light!

I understand that some of the NEMA 34 steppers are better at higher step rates than the larger ones. I think your choice of 680 oz-in is OK. I used 23 size single stack Pacific Scientific having ~ 200 oz in torque on my test machine.

My chioce of gear rack is 24 dp, 20 degree pressure angle. We had used it extensively on the plasma/oxyfuel cutting machines we used to make (retired now). I would choose a drive pinion of more than 20 teeth, maybe around 30 teeth so that more teeth are in contact with the rack. For a 30T pinion, the distance per rev is 3.927 inches. With 10x microstepping and a 3:1 reduction to the pinion, you will have 6000 microsteps per pinion revolution, or .0006545 inches per step, not unreasonable. To operate at a rapid of 250 IPM, this would be 6.366 KHz max stepping speed. The PacSci catalog shows a 2-stack rotor will have a torque of around 900+ in-oz (reduced to .707x because of the microstepping) resulting in a drive force of 1909 oz-in at the pinion. This gives a drive force of 1909/.625 = 3054 oz = 191#, less friction etc at 250 IPM. This is near the peak torque so it will be substantially the same at lower speeds.

I took this data from PacSci NEMA 34 two stack K32 or N32 motors. Anyhow, that is a quick overview of some of the possibilities.

If you will Email me, I'll send you pix of my 32 x 36 three axis test machineand you can see an alternate approach to the round ways so often used here.

Regards,
Jack C.

[DDM]

Hey jcc3inc or anyone else, you wouldn't happen to have any pictures of your setup for preloading the pinion gear to the rack or anything pretaining to the rack and pinion set up? I'm in the design process and I'm kinda at a stall point. My gantry is about 5 feet long right now and I'm a little worried about the gantry skewing on me or with only having the drive on one side. So if anyone has any pictures or links they could post I'd appreciate it. As soon as I get the rough design done I'll be posting a new thread.

Carl

BTW, In solidworks the frame is about 500lbs so far and I'm guessing the full weight of the gantry is going to be about 200lbs.