CNC Plasma Table

[f350ca]

Got to start somewhere.
Rack and pinion drives for the x and y axis are popular and seams to make good sense. Thought about ball screws but don't need that kind of precision and I'm worried the dust from the torch would end up eating them. Acme threads with anti backlash nuts I think would be a possibility.
Any thoughts?
If I go rack and pinion how crazy would it be to manufacture my own. Lacking a tool grinder, I could build a simple jig to set the 20 deg angle on a HSS cutter and grind it on the bench grinder. Then cut the rack 10 1/2 inches at a time on the shaper, index the blank and start over. The same cutter would make a hob to cut the pinion.
I know dreams of having too much time on my hands, but I need to get a metal fix, been spending too much time around wood.

[PixMan]

Balls screws would be sealed pretty good, and of course you could shroud them with sheet metal. It's more than just accuracy, they offer high-speed positioning, fewer parts required and not much can go wrong.

How much travel are you planning on having for this table?

[f350ca]

Need just over 4 feet on the gantry, enough to get the end of a sheet in, as for length, somewhere between 2 and 4. 2 x 4 foot would probably cut 99% of the parts I need, but 4 x 4 might be more practical, as long as the end of the sheet can hang out.
High speed is nice but usually equates to higher cost as in bigger steppers and the supplies and driver to run them. If this thing will cut out my parts accurately the time saving will be there, moving between parts doesn't really matter.

[Mayhem]

A friend of mine is looking into some of the kit form ones and from what I have seen, the speed is important to the finish of the cut. The thinner the material the faster the speed of cut required.

[stevec]

(10-10-2012, 10:07 PM)f350ca Wrote: Need just over 4 feet on the gantry, enough to get the end of a sheet in, as for length, somewhere between 2 and 4. 2 x 4 foot would probably cut 99% of the parts I need, but 4 x 4 might be more practical, as long as the end of the sheet can hang out.

Greg, I was thinking 2 foot + sections could be added and removed when not required to save space, it's not the type of machine that you or I would use on a daily basis.
I also toyed with the idea of using lengths of timing belt in place of gear rack but the rack could be more precisely anchored to the "add on" sections I mentioned.
Darren has a point in "The thinner the material the faster the speed of cut required."

[EdK]

Speed may be important under certain circumstances but what is that speed? I think that needs to be answered before one can make an informed decision as to what method of positioning is best for the application.

Ed

[Mayhem]

I cannot recall what the speeds were but the manufactures of the systems I saw had charts that gave a range in inch/minute for a given thickness. Torchmate was one IIRC.

[EdK]

Looking at the Torchmate web site, they have models that use either "anti backlash screws" or rack and pinion. The rack and pinion version has a higher traverse rating than the "anti backlash screw" version. I don't know if "anti backlash screw" means acme screws or ball screws.

Ed

[f350ca]

Good point on the minimum speed, hadn't thought about that. Will track down the manual for the plasma cutter, it listed voltages and speeds to different materials and thicknesses. When cutting by hand speed seams to help but has to be consistent (which of coarse you'd achieve with the cnc drive), one small jerky motion and you loose the cut.
On one of the previous dreams of having one of these I'd thought of the timing belt method Steve. The belts are pretty strong but my concern was stretch when you changed direction. Kind of a bounce effect, maybe nothing to worry about though.
I like the added sections. That would be doable with a rack and pinion.

[4R8]

for the price and durability you can't pass a rack and pinion.
speeds are easily adjusted (to an extent) by changing the pinion gear used.
Ballscrews would be great but to accomplish the speeds needed you risk the screw reaching it's critical rpm which produces lead whip. multi-start screws with a coarser lead can be used to avoid this. As mentioned though, plasma dust is an issue as it's so fine, it gets everywhere! a carefully designed layout can minimse the exposure to the dust but replacement screws (if) affected will be pricey.

I'm currently most of the way through a build at the moment with the table pretty much built and working on the drive mechanisms at the moment.
I'm using racks and 'V' rails for motion with steppers and belt reduction for the drive. I'll start a build here shortly once i snap a few more pics.
Should i place it here or in projects? it''ll be a while until completion with my minimal shed (shop) time lately but it's finally looking like a machine