[cstmwrks]

So this is just a working thought right now. I have a Northern Hydraulics 375 model 40 amp plasma cutter and I would like to be able to do some small projects with CNC control. I suspect that my little 375 is not ideal, but as often as I do plasma cutting it is not worth it to buy a more expensive machine.

So rather than attempting a complete CNC controlled torch, my thought was to just make a mounting bracket with and old fashioned boden cable remote trigger control. My CNC table runs off MACH3 and I use V Carve for creating all my designs. A modified PP to add a dwell after the Z movement to get the arc started and then your off and cutting.

Is there any reason why this could not work?

[digger doug]

The smallest Torchmate (the 2' x 2' one) I watched it demo at the LWS, has a manual height adjustment.
Just the simple rack and pinion with knob like on a bug-o.

They simply used a 1/8" shim on top of the plate,
wound down the torch till it touched, then removed the shim, and hit "go".

Worked fine for what they were doing.

You'll have to make sure the plate is leveled up all around, and realize your
not going to get the utmost accuracy out of your machine, but give it a try.
(and post some pics here)

[SignTorch]

I'd put a relay in place of the remote trigger so mach3 could actuate the relay to fire the torch

or mount the trigger on the Z axis so it gets pressed when the torch is lowered to the pierce height - if that's what you meant then that's not a bad idea - might need a manual override method in case it stops in the down position

I don't see any reason it wouldn't work...

[Eddard_Stark]

After talking with people who have used high frequency start plasma under PC based control I definitely wouldn't walk away from the E stop button, nor would I be surprised if the G Code isn't executed perfectly.

Having said that I thought about implementing similar setup with a pilot arc plasma for occasional use. If I used it much I would probably make a simple floating head like the poster above alluded to.

[jimcolt]

Just to clarify the different starting methods and descriptions that plasma torches use. It is funny to me that even a few of the plasma system importers don't know the differences between high frequency start....and pilot arc start.....or if there even is a difference!

-First...High frequency start in a plasma torch is the common method of developing an approximately 15,000 volt @ about 2 megahertz (which is high frequency) discharge inside the plasma torch...usually between the negative electrode and the positive nozzle. The energy dissipated by this discharge heats up the plasma gas (air in an air plasma torch) to its ionization temperature (and the air then becomes more electrically conductive). With highly conductive air between the electrode and the nozzle.....the DC energy (again, negative electrode, positive nozzle) then finds a path between electrode and nozzle...and the force of the air flowing through the torch forces this DC arc out through the nozzle orifice. The effect....when fired in the air is called a pilot arc. It appears as an intense bluish arc coming from the nozzle orifice.

-Blowback start. More modern air plasma torches do not use a high frequency discharge to ionize the air. Rather....they activate the DC power and moments later start the air flow. Before the air arrives in the torch plenum (between the nozzle and electrode).....these two parts are directly shorted (in contact with each other)....which in effect is a dead short on the output of the plasma cutter. When the air arrives in the plenum....it forces the (spring loaded) electrode to slide back ("blow back") away from the nozzle.....and the release of the dead short creates a fairly intense (current is controlled however by the DC power supply so damage is not done to the electrode and nozzle) short circuit breaking spark....which ionizes the gas......this results in a pilot arc that is forced by airflow through the nozzle orifice.

So......both high frequency start torches and Blowback start torches can be pilot arc type torches...and when fired in the air the result is identical, an intense bluish arc exiting the nozzle orifice. Th function of the pilot arc is to (when in close proximity to conductive metal that is connected to the positive plasma work cable) quickly burn through any low conductivity surface conditions (corrosion, paint, coatings, etc.) on the metal and to get the pilot arc to transfer to the material and start the cutting process. A strong pilot arc also allows cnc machines to set a higher pierce (transfer) height.....which allows the torch to pierce the metal more efficiently and without damaging the nozzle orifice. Once current flow is sensed on the work cable....the power supply ramps up from pilot current to cut current.

The advantage of the Blowback start circuit is:

- No need to generate a high voltage, high frequency current....so there are less parts in the power supply, better reliability, lighter weight, lower cost.
- High frequency, by the nature of its voltage level and RF frequency can easily couple to low logic level voltage in computer cables, electronics used in cnc machine drives, and can even feed back into power lines. Often, especially on cnc machines that use office computers as the cnc machine control......this can and will disrupt operation. Blowback start technology does not have these issues.

It is strongly recommended that for PC or Laptop controlled cnc plasma machines...that you use a Blowback style torch on the plasma system.

There are a couple of other types of torch start technology that involve contact with clean metal........some of these develop a high frequency discharge between the electrode and the metal (workpiece).....meaning the torch must be very close to the plate, and others are a DC "scratch start"......meaning you need to scratch the nozzle on the workpiece in order to start the plasma via a DC discharge. Neither of these are recommended for cnc plasma applications as the torch nozzle will quickly be destroyed by starting in contact with the workpiece....which causes double arc damage to the nozzle orifice.

Hope this clarifies a bit! Watch the advertisements and brochures for many imports, be sure you are getting a blowback start torch. A "Pilot Arc" torch can be either blowback or high frequency, and often the sellers do not know the difference!

Hypertherm developed blowback start in the mid 1980's, all of its Powermax air plasma cutters use this technology. Since the blowback patents expired a number of years ago...most major plasma torch manufacturers use this technology currently, however......many low cost imports still use copies of older technology, high frequency start plasma torches.

Jim Colt Hypertherm

[Greolt]

It is strongly recommended that for PC or Laptop controlled cnc plasma machines...that you use a Blowback style torch on the plasma system.

This is good advise. If using CNC plasma to earn a living, then take heed.

However don't think that it's impossible to use a high frequency pilot arc cutter in a CNC table. (Jim is not saying it is impossible)

My table that I built uses HF pilot arc and works quite well for my purposes. Cheap Chinese plasma. Cheap Vietnamese THC.

Would I prefer a Hypertherm? Of course. But it would cost more than twice as much as I have in my entire setup, including cutter. (I am in Australia)

I am a hobbiest (is that a word?) and cut arty decorative stuff out of 1.5 and 2.5mm steel. My biggest problem is that I do not have a system to handle the smoke.

All this is just to say, do not abandon the idea just because you do not have the best stuff to work with.

A lot can be achieved for moderate expense, if you are careful with design and electrical layout.

Again, if you are doing production work then don't fluff about, just get the good stuff.

[msimpson99]

Can a blow back torch also have HF? Why do some models allow you to turn off the pilot arc?