[gotfdirtball]

I'm in the early planning stage of a very simple CNC plasma cutter. Initially I was planning on just a simple Z axis going to a pre-determined Z dimension for torch height but I see a lot of discussion on this forum in regards to torch height control. Is THC absolutely necessary for a simple 'backyard' quality machine or will my first idea work?

Also, in looking at some video on youtube and some project pictures from here, I see water trays under alot of the machines. Same question, what do they do other than contain the smoke and sparks and how necessary do you think they are for just a 'backyard project' level machine?

Thanks in advace for any help.

[plain ol Bill]

THC - well it depends on what you want to do I think. A once in awhile used table, with back yard quality has been done for years really without a THC. But if you intend to cut frequently and make multiple copies of a product out of thinner materials without beating your head against the wall I think one is necessary.
Going to use the machine out in the back yard? Then you don't need a water table or ventilation. Got neighbors close? They won't approve very much of metal dust wafting through the neighborhood air. If you plan to use the machine indoors you are going to wish you had ventilation or a way to catch the dust in water.

[jimcolt]

Torch height control for plasma....is it necessary?

The most important parameters to ensure accurate cut quality with a major brand plasma torch are: Good quality consumables in good condition, excellent motion control with excellent velocity control, accurate torch to work distance for piercing as well as for steady state cutting.

Can you cut with damaged, worn out consumables? Yes, with varying degrees of edge angularity...and slower than manufacturers recomended cut speed.

Can you cut with rough motion and at varying cut speed (velocity)? Yes....rough motion (even the slightest vibration) will show up in your cut face....varying cut speed will affect edge angularity, dross, hole roundness and warpage.

Can you plasma cut without an automatic height control? Yes....however one pierce too close to the plate will damage your torch nozzle (causing all of the issues above)....and varying from the manufacturers cut height after the pierce will affect edge angulartity, dross, warpage, hole roundness.

Many machine operators think that they can control torch height with a handwheel....or with a motorized up / down switch on the z axis.....and they can.....however I challenge anyone to maintain the accuracy needed with a plasma torch to provide consistent height while piercing and cutting. The need of an operator having to constantly watch the arc and adjust torch to work distance also kind of takes the whole idea of a cnc machine and makes it a manual machine!

I have also been told many times that " my plate is perfectly flat.....so I don't need torch height control for my plasma".....read on, here is more about the accuracy required!

There are two major categories of plasma cutting.....there are conventional plasma systems and there are high definition class plasma systems. Conventional include the air plasma systems that most users have on their entry level, low cost, or homebuilt cnc plasma tables. High Definition class plasmas (also read, expensive industrial grade plasmas!) are typically reserved for highly productive industrial plasma machines that are expected to produce high volumes of high accuracy parts 3 shifts a day in an industrial environment.

For conventional plasma systems....the accuracy required in order to produce consistent cut quality results from a high quality major brand plasma torch (all bets are off with low cost imports...as there can be many variations in quality withing the plasma system itself)...the torch height control needs to be able to sense the surface of the plate before each cut...then retract to the manufacturers suggested pierce height for the particular material being cut...before firing. If the manufacturer says the pierce height for 3/8" steel should be .150" off the plate, and the pierce delay time should be .8 seconds....then to avoid damage to the nozzle from either pierce splatter blowback (too close to the plate or too short of a pierce delay time)...the distance and time need to be accurate within about 10% of the specs. Once the pierce is complete and succesfull.....the torch should index rapidly to the manufacturers recomended cut height while the part program is still on the lead in (scrap) part of the cut path....and must get to the cut height.....if the plasma torch manufacturer says cut height for 3/8" is .080"...then it must be at plus or minus .010" of this height to produce the best cut quality. Too high produces a positive bevel and dross, too low will promote collision with the plate and produce poor cut quality due to reduce kilowatt output from the plasma (low voltage). Can a human with a finger on a toggle switch control torch to work distance accurate to plus or minus .010" from 4 to 8 feet away from the torch? Is your plate level to within this tolerance?

High definition plasma systems require height control accuracy that is in the plus or minus .004" range in order to produce accuracies that they are advertised for....and to provide consumable life that is acceptable.

So can you cut with plasma without a torch height control? Yes. Just understand that you are leaving some of the the most important variables....torch to work distance and pierce height....wide open.....and the effect will be poor consumable life and poor cut quality as a result of consumable damage...and also as a result of cutting at the incorrect height.

The cost of height control is always an issue....in fact, since I work for Hypertherm I have (on more than one occasion) been accused of just selling height control systems! Hypertherm does build height control systems....but our primary products are plasma systems. We have (over the years) been forced to design and produce torch height controls with advanced features, advanced accuracy and high reliability...just to keep up with advances in the plasma cutting torches. Our best HPR400xd high definition class plasma is no good without good motion and good torch to work distance! Hypertherm's height controls are generally out of the entry level or homebuilt plasma cutting market due to their complexity, size and pricing....so no, I'm not on this site promoting our height controls. Rather, I would expect see our plasma torches used to the best of their ability.......a good performing height control is absolutely necessary in order to get the best performance and long term low operating cost (consumable life).

Best regards, sorry for the long THC response!

Jim Colt

[jgramlich]

I did something similar last week with one of our handheld 80A jobs. We've got this thing we call a "tractor torch", about twice the size of a shoebox, runs on a track and can do about 40 inches per minute at full speed. When we got our plasma table it stopped being used except for beveling plates occasionally (it can hold the torch head at different angles).

So the other day I see over on Controlled Automation that they've got a very similar deal for splitting beams but it uses a plasma head instead of oxy-fuel. So I decided to swap out our oxy-torch for the hand-held plasma torch. Took some experimenting but I did get it to work pretty well.

There is definitely no torch height control on this setup. The manual plasma comes with a drag-shield which would work for 90 degree cuts. I had to remove it to do a bevel, as it put the tip of the torch just a bit too far away when angled.

Your plasma's user manual should have a section with machine settings for various types of cuts that also includes some THC parameters. For my particular application, if I recall correctly, I had to hold the torch head something like 3/16-5/16 from the plate. So I just eyeballed the torch height, ran the tractor down the cut (by hand before actually cutting) to see if the torch head collided with the material. When I got my height where I wanted it, then I started the cut. It worked okay and didn't mess up my torch tip too much. I would definitely rig up some sort of spring-loaded wheel for cheap-and-dirty THC if I were going to use this in any sort of volume.

The end result was pretty good though. The cuts are pretty clean if you take your time to set the thing up properly and I can now cut/bevel a sheet about 2-3 faster than if we were still using the oxy torch with the tractor setup.

[jimcolt]

Here are a couple of pics of a "Bug O" brand track burner cutting and beveling with a Hypertherm Powermax45 plasma torch.

Jim

[jgramlich]

Well now I'm going to have to put some pictures up... :-) I'll try to get that later today.

[jgramlich]

Here's a few photos of the device, the plasma unit and some cuts that I was able to make with it.

Well...I'll have to make another post with the rest of the images...above is the original oxy-fuel head, our manual plasma unit, a couple of images of the torch head with the drag-tip on it. Also on the torch head you can see my kludgey shim of electrical tape and the aluminimum trigger lock. The top view of the tractor shows the speedometer and the clutch engage and the last two there are of the tractor in its track.

[jgramlich]

Here's some pictures of cuts I've done. The first is a piece of 1/2" A36 steel plate cut at 45 degrees. The other two pictures are of bevels of 3/8" plate. On the one you can see towards the end where the cut is a bit messy...that was part of the testing.

I tried a couple of things, originally trying to hold onto the plasma torch while the tractor was moving to steady it...didntworksomuch. Any motion at all makes changes to your cut quality.

The bevels this way turned out pretty great. Previously we'd been using the oxy-torch for this and the cut was very wavy, plus all the melted dross/slag/whathaveyou from the oxy-torch. The plasma makes a significantly better cut, and about three times faster. Much less grinding, much more gooder.

[jgramlich]

The point being of the posts I made below with the pictures: This setup uses no torch height control. I set it manually about 1/8" to 3/16" off the surface of the plate, made sure the tip was not going to collide before cutting and then started the cut.

[jimcolt]

And....I contend that without height control.....you will have shorter consumable life and as a result poorer cut quality.....as well as a lower level of automation....even on a track burner.

Jim Colt

[gotfdirtball]

Man this website is superb! Thanks for all the info fellas.

I think in the end, my setup will be much closer to the tractor setup than a high speed low drag production unit. I'm essentially a motorcycle enthusiast looking for a better way to make brackets, levers, gussets, etc. After using info from this sight to build an XY plate for my bridgeport, I was just thinking I could use the same type motion control in conjuntion with a mounted hand held cutter (such as pictured in the tractor pictures) to build a quick and dirty CNC motion plasma cutter.

To be honest, after reading all this, I think my first step needs to be learning a little more about plasma cutting in general. I mean I had no idea that you "pierce" at one height and move to another to cut LOL.

Thanks again for all the feed back.

[WSS]

A case study here. I recently started using a CNC cutter with a THC. Previous to this I was using a magnetic tracer similar to a track burner. I am using a Hypertherm 1650 with a T100M torch. On the tracer (as hard as I tried to get it right ) I could not get through a job without changing nozzles and the occasional electrode. I would clean the parts every 10 or so cuts. When I Was able to use our CNC/THC, I never even took the torch apart. I made through the entire project and never had a bad pierce (.69"). The only cleaning I did was to grab the shield with my glove and give it a twist as the gantry was starting to rapid move. Just a one time comparison, but I was happy with the THC's ability to track and retract for piercing. I believe It could do another project before I need to change.

WSS

PS: The job took about half the time and I was actually able to sit on a stool to watch the show! The hard part was hitting the enter key with my gloves on to start another cycle. I need a keyboard with a bigger enter key.

[jgramlich]

If I'm not mistaken, a lot of these kits you see are conversions of handheld plasma units to CNC. Another note on that track burner, it's so simple that we're always doing edge starts on it, so no pierce height issues there.

Also on Jim's comments from earlier, he's right about consumable life for sure. If you are doing x & y cnc instead of just x I can't imagine that it would be that great without torch height control. There are probably options out there that do work without it, but probably not well.


The thing on pierce height (and I'm sure I'm not telling the whole story, but...), especially with the thicker plate we cut, think of it like the lightsaber in the newer star wars movies when Liam Neeson takes down that blast door. When he initially "pierces" the door, the handle of the lightsaber is 2 feet away, but as he begins cutting, he moves the handle to only a few inches from the door.

Nevermind the physics issues and having your bare hands just inches from dripping steel, but the plasma arc is a bit like that lightsaber. On a 1 1/2" plate you can really see it well, as the tip of the arc starts just at the surface of the plate when it begins the pierce. About 80psi of gas pressure pushes the arc out of the torch head and then melts its way through the plate as the torch head moves down into it. Once the tip of the arc is through the plate, then you can begin your cutting motion.

It's quite a show too. :-) As its piercing, 1 1/2" steel sprays everywhere as its instantly melted by 10000 degrees of hot buttered love.

[jimcolt]

jgramlich,

I can tell you would love to see one of our PAC500.......1000 Amp plasma torches cutting some 5" thick stainless....the arc is about 3/4" in diameter, the noise is similar to standing behind a 747 on takeoff, and the whole area around the cut glows this erie pink color from the argon/hydrogen based 1000 Amp arc. When you cut off the edge of the plate the arc stretches to about 6 inches long before it extingushes with a loud snap. Thats what got me hooked on plasma cutting back in 1978....when this system was Hypertherm's only product!

Jim Colt

[jgramlich]

1000 amps?!?!? Drool....