[CPierce18]

I'm in the process of designing a 5'x7' plasme burn table. Im trying to keep the cost down. Anybody ever use Nylon 101 for the gear rack? It will be shielded, so I'm not so worried about the dust and sparks. Any suggestions/comments?

[rustamd]

why not just use steel?

Here is very affordable place:

http://www.stdsteel.com/gr_stock.htm

[ewest]

I don't see any problem using a nylon rack. I wouldn't worry about accuracy either. There's no cutting force anyway. Just use something with a larger tooth so you don't risk shearing off the teeth or grinding them up when the machine does a hard acceleration/deceleration in corners and whatnot. Maybe the money you can save by not using metal can use re-directed to using a helical gear rack instead of a spur. That way you can have smoother motion with larger teeth.

As far as I can tell, the most abuse that the racks in MY cnc plasma takes is from the horrible, evil, gritty dust that plasmas are so good at making. It's about two years old, and the racks look the same as when they were new.
I will do a little preaching now - hopefully you already know this, BUT////
When you're building this thing, remember, (unlike PracticalCNC) that the little quarter inch shaft (hopefully larger!) that the spur gear is mounted to IS IS IS IS the part of the machine that has all the force of moving the gantry applied to it. Keep your bearings close to the gear, and if you can, support it from both sides. Just my two cents. Shield the hell out of your wires, especially if you're using servos, and do everything you can to keep the plasma dust out of delicate areas, like your bearings and encoders. Spending the extra few bucks on sealed components is well worth it. And again, same rant as every other guy in here gives, USE GOOD LIMIT SWITCHES! They'll protect you and your investment. At least use a micro switch with a roller on it, like from All Electronics.

[CPierce18]

Any suggestions on the rack and gear size? (for both Nylon and Steel) It’s been about 7 years since I’ve worked with gears. Also I’m having trouble deciding what size steppers to purchase, so any comments on that would be greatly appreciated. Currently I have 269ozin on my wood cnc router, but those will probable be getting sold w/ the table. I figure it will be (2) motors driving the X-axis (rack on each side), (1) motor for the Y-axis (rack), and (1) smaller motor for the screw driven Z-axis.

[ewest]

I like your idea of running a motor on each side of your X (Actually, I think you mean your Y - the one that runs towards and away from the operator and front of the machine) It's too easy to put the motor on one side, with the spur gear on that side having 4 inches of shaft between it and the motor, and the other side having 5 feet of shaft. Makes for some funny torquing under load/acceleration.

Anyway, for the plasma, I would imagine you could probably get away with a 16 pitch gear set. Kind of big, but for plasma, it's fine. As far as motor size - you're on your own. There are a ton of ways to calculate the power required from your motor. Anyway, you're not concerned about cutting force, because there is none with a plasma -all you care about is the acceleration, and remember, with the plasma, you want to maintain a relatively constant cutting speed, which means that when you come to a corner, you want one axis to stop almost instantly while the other one starts almost instantly. The thinner the material, the faster you go, and the better acceleration/deceleration you need. When we cut 16 ga, we feed at 160 IPM.

For gears, go ahead and look at http://www.qtcgears.com/ they'll send you a catalog whether you want one or not, and it's got an absolute TON of information about the physics and geometry and movement of every type of gear you'll ever encounter.

Good luck.