Drive system question

[CanopyFx]

Ok I have a few more. I spent about 5 hours yesterday researching drive methods. I am building a 4 X 4 table and I can not find affordable 3/4" or 1" ballscrews. I did find several 5/8 ones that are under $100 for a 6' section. I checked several sites and they said I can spin a 5/8 screw at 800 rpm if it supported correctly on both sides. So my calculations tell me (if they are correct) that a .2 pitch screw will move 160ipm (800*.2). Is this fast enough to cut? I dont care about fast jog times, I just want to cut (plasma and a trim router)

I also researched rack and pinion. For this I think I would need to gearbox the stepper for better resolution. Now that whip is not a problem the jog speeds come up. But here is the issue here. Are there any affordable gearboxes with minimal backlash, and where are they. I found several at 20arc that were in the $400 range. Well for that price I can get 1" ball screws. If you can post sources to help me find these gearboxes I would rather go this way.

In conclusion here are my questions

1. is 160ipm fast enough for plasma cutting and thin MDF and wood routing?
2. Where are the affordable gear boxes?

On a side note:
The planning of this CNC machine is driving me nuts. I cant sleep at night because I can picture in my head it how to make this thing move with the least resistance and not come flying appart on it maiden voyage.

Thanks
Joe

[Al_The_Man]

There are usually always deals on ebay, I picked up 3 DC servo's with planetary gear reduction for $27.00 ea.
Look for Bayside and others, if used and separate from the motor, make sure the input pinion is included.
Al.

[Mariss Freimanis]

It may make a lot of sense to use a much coarser screw pitch and a toothed-belt reduction from the motor to the screw. Compare a 5 TPI direct-driven screw vs. a 1 TPI screw driven by a 5:1 reduction from the motor. The advantages would be

1) Screw RPM is 1/5th the 5 TPI screw RPM; you can go 5 times faster before you hit the screw's 800 RPM critical speed.

2) The reflected moment of inerta is 1/25th that of a 5 TPI screw. That means the motor "sees" an inertia equal to a 2" long screw instead of a 50" long one.

3) Motor speeds, screw thrust and position resolution are the same in both instances.

Mariss

[CNCAddict]

I think 160ipm is very fast. For jogging it might not be optimum, but for cutting that seems awfully fast. Move your finger along at 2.6in/s and you'll see that it's a fairly brisk pace. If you want even more speed, go with a belt drive. They are cheaper than ballscrews and can move very quick. Stiffness will be down some, but reliability will be very high, and alignment is less critical. Also, dirt doesn't phase a belt, anything on the surface will just press into the rubber as it moves around the pulley doing no damage. However, dirt on a ballscrew or rack makes me cringe.

[limbo]

Also for rack & pinion you can use a toothed belt reduction. Cheap and no backlash.
A strong stepper can even direct drive a small pinion on rack. In that case microstepping can be used to increase resolution. Personally I dislike acme screws because of their low efficiency. Ballscrews are much better but still limit max. speed due to whipping. I'd choose a courser pitch for plasma and light wood cutting. As CNCaddict said, 160 ipm is fast for cutting but not really fast for positioning.