Plasma cutter advice please!

So I need some advice here and there are some real brilliant folks here, so if I may be able to pick your brains a bit.

It seems that I have to be about the only person here who does not have a plasma table yet, but that is about to change.  I have a router that I am about to replace that is 1m x 1.5m with a 5'x9' beast.  Usually when i build a new manchine, i desing and cannibalize, but since i will be building on a much larger scale than my existing one, there are absolutely zero parts that will be usable on the new machine.  so the original plan was to make a dedicated 4axis to be used as a lathe of sorts, but my source of plasma cut parts will be moving soon, so i decided to make a plasma table.  the problem is that while i am no stranger to cnc, ive never messed around with plasma, so i am starting an inch above zero here.

so it is a few quesitions, so hopefully john, paul, andy, phil, or rod will chime in and share some ideas and experience.  my design isnt set in stone and i havent started any cutting, so i am still flexible.  the overall design is basically cribbed off my smaller router that i built a while ago with a few mods.  overall a super solid and modular design, 

first, the machine i have is 1000x x 1500y.  idealy i would like the x to be the long axis, but that entails purchasing a new gantry beam.  is there any advantage one way or the other that is a necessity for a plasma besides the usual mass and momentum that i am missing?  my thoughts are that having the longer axis open would make loading and unloading easier, but is it worth the cost of purchasing a new beam?  i am working on a bit of a budget here, so cost is definitely a factor....  on the other hand, having the short axis open would make it easier to do oversized stock, but i dont really think that would be a real necessity for me, but neither did having a plasma either

second, is a thc an absolute necessity?  if so, i was thinking a mesa thcad2.  i absolutely love mesa cards and the machine is running a 7i96 right now.  is that compatible with a thcad2 or do i need a different card?

third, i have a vulcan 45 plasma, but it doesnt have any control ports on it.  putting one in is easy enough, does anyone have any experience with on of these and if so, how did you wire it up?  and how big of a resistor would i need between the vout and the thc?  should i put a fuse in it as well or is it fine with just the resistor?

forth, how about clearance height?  the z axis i have has 6" of lift overall, but im pretty sure thats overkill.  but if i can reuse it, that would be nice.  it really isnt cut out for routing, has more flex than i can tolerat, but i figured it would be ok for a plasma since there is zero contact.  i can modify it if needed, but if not necessary, then why bother.

and finally, i am undecided on how to do the material height sensing.  i was thinking of a spring loaded carriage on the z with a switch or proximity sensor.  i want sure how a proximity sensor would handle that much noise that close and if it would spaz out.  same as an optical sensor unless i found how to shield it.  i really would like to keep it as simple as possible.  touch and done.  

also if there are any other things that i might be missing here feel free to chime in, all advice is welcome!  im going with ballscrews and pretty sure that my speeds wont be enought to worry about whip, so i should be fine there i think. 

thanks for time and input here, much appreciated!

1. Long axis OK. With Plasma, light and rigid is the go as velocity and accelleration is king. The gantry beam does not need to be expensive. I used 80mm x 40mm  x 4mm aluminium rectangular section.

2. Yes. Usa a THCAD2 with youe 7i96

3. I don't know anything about your plasma but you shoud use a blow back non high frequency start torch

4. 6" is fine

5. proximity sensor is fine. Normally you would have a sliding section the torch was attached to. This would slide up when the torch hits the material and trigger the prox sensor.

rodw wrote:

1. Long axis OK. With Plasma, light and rigid is the go as velocity and accelleration is king. The gantry beam does not need to be expensive. I used 80mm x 40mm  x 4mm aluminium rectangular section.
 
40x80mm is what i am currently using for the beam.  it is a belt drive with nema 23 425oz/in (2y 1x) and i am going with 1605 ballscrews which those motors should accelearate no problem.  same as my other router, but i am going to do some trimming on the the x carriage to make it lighter.  i was just torn between if i there was a layout that was more prefered than the other.  if i keep it as is, i dont have to buy another gantry beam which dont come cheap, although i get my aluminum extrusions through misumi and they are quick and reasonable.  but still if i dont have to then ill keep it as is.  problem is that what i was thinking was having the y axis as the short axis and the x longer.  that would mean more of the table open for loading and unloading.  and possibly easier to have against a wall.  as it is, it is in the middle of my shop and a bit of a pain really.  i was just curious on other peoples takes

3. I don't know anything about your plasma but you shoud use a blow back non high frequency start torch
so the plasma i have is this one:
vulcan 45 plasma cutter
i know it is used on plasma tables because it is a supported unit for the langmuir crossfire plasma table.  there just isnt jack out there for documentation on how anybody has actually wired this one up to work

great @tommylight knew you would have some great advice! so how big of a resistor should i use for the output lines? and i was going to wire in a couple of aviation connectors on the back, one 4p and a 2p for the thc. there is a nice flat spot above the cooling fan that would work, provided there is room once i crack it open. should i wire the thc inside of the unit itself? i was going to put it inside of my control box. my current one sits about 2ft away from the unit, so somewhere im thinking 2-6ft. is that too far to run?

ballscrews are a poor choice for plasma as they limit velocity and acceleration which are king with plasma 
Normally I would say small nema 34 on each Y, with 5:1 reduction, 30 mm pinion,
Nema 23 on x , 3:1  or 5:1 reduction on the X
Tommy uses belts a lot.

rodw wrote:

ballscrews are a poor choice for plasma as they limit velocity and acceleration
 

 

how fast aside from jogging would i really need? i was under the impression that realistically i wouldnt need that much of a top end. as for acceleration, i have one screw driven machine and for all intents and purposes, i have near instant acceleration. i was just looking into ballscrews for the accuracy.

currently i am running 15mm HTD-3M steel reienforced belts with i -believe- a 18T pulley, its been a while since ive had to do anything with the pulleys.  i am pretty sure that is about the smallest i can run on a 10mm shaft.  i had problems with the kevlar belts and stretching and they are not NEARLY as strong as the steel ones. i broke many a belt before i switched over.

rodw wrote:

Normally I would say small nema 34 on each Y, with 5:1 reduction, 30 mm pinion,Nema 23 on x , 3:1  or 5:1 reduction on the X 

[/quote]

for the size of the machine, i should be able to get away with the 23's that i have. 34 would probably be overkill and excess weight to throw around i think. these 23's that i have are pretty stout and about year ago i completely ripped the shaft off of one of my y motors jogging too fast. that was a bad day.

but if i can get away with using belts, that would save me a nice penny, seeing as that i already have all the hardware and spare belting. how do they do they hold up overall?

thats pretty sound advice. i was going to put a pair of outputs in the back of the machine and mount the thc in the controller box. i thought that would be a bit of voltage to be slinging around the shop.

the other thing i was thinking of would be using something like this:
cnc4pc.com/plasma-voltage-divider.html
mounted inside of the chassis and then running low voltage lines out to the thc.  would that be better or safer? or overkill?

also, rod says you use belts on yours. i've seen some of your retrofit/refurbs, but i never saw anything of yours that used belts.  how do they hold up for you?  any problems with them?