Advanced Search

Yes, that was one of the original reasons the ohmic test was included. It was intended to check if there was a short like yours before starting a job. It does not happen very often. I only ever experienced it once and I found a bit of rubbish inside the tip,  much like you did. I have thought of setting it up to use one thcad. I'm sure it could be done fairly easily.

Oh ok so you’re not waiting for the voltage to go to zero to trigger the contact. Your looking at any voltage drop and once it does it’s triggering contact. That make perfect sense to use a thcad and watch the frequency output. Then you could set the trip frequency on the encoder input on your mesa board. I wonder if there is a way to double duty a single thcad-10.

You know I stumbled upon something the other day by accident that could work. I was using my fine cut consumables. And I was testing my retraction to make sure my actual retraction was what the display was stating it was. I held the ohmic button down and moved z down till it stopped from the ohmic trip. But I noticed when it touched I seen a +10v on the cut voltage. I did it agian and it was still there. So I started looking around and found a tiny slag bead stuck up in the nozzle bridging the nozzle to the shield. This was back feeding the ohmic voltage into the nozzle and it was showing up on the cut voltage.

Would there be a way to use this to sense ohmic. I don't know the ramifications to running the nozzle bridged to shield but it had been like that for awhile as I had just been cutting.

Stahlwerk
www.stahlwerk-schweissgeraete.de/Plasmaschneider-CUT
Good quality and cheaper than china, vertical machine torch is 40E in Germany, with 5 meter cable is 90E, bought both.
Mind the pilot arc, there are some without it so do not get those, it is easy just look for a red knob on the bottom front and usually it is written in spec info.

rodw and tommylight thanks for your answers. Although I have to admit I am stil confused.

I bought a cheap pilot arc plasma to play arround. It has an explicit connection between the tip and the workpiece. Ohmic sensing can't be done. Right?

Anyhow, I build a CNC Plasma and I am looking for a suitable frontend (plasma and torch) my wishlist includes:

1 plasma cutter (hopefully < $1000) with a european distributor
2 Ohmic or hypersensing (willing and able to build homemade electronics if I know what)
3 THC with a THCAD-x (best case with an avaibale THCAD-300)

Can someone propose a setup ?

roland206 wrote:

All of the pilot arc torch heads I know have an electrode connected to the plasma voltage (negative) and a shield connected to the workpeace.

Oh no!!!!!
Nozzle is not shield.
Nozzle can not be connected to workpiece, although in some "high voltage pilot arc" plasma's it may seems so when measuring, transformers n stuff...
Nozzle is connected to high voltage/high frequency when pilot is on
Shield is an extra layer not connected to anything internally, usually there is a retaining ring and a wire connected to it leading to ohmic sensing parts of the electronics, or not used at all electrically, just for protecting the air stream on multiple gas systems.
Pilot arc is between electrode and shield.
Cutting is between electrode (negative as you already stated) and material/table/clamp that is positive.
During cutting nozzle must be "off or electrically detached" otherwise it would burn to a crisp in 1-2 seconds.

except you are confused between the torch tip and the shield. Some torches (Hypertherm, and Thermal Dynamcs for example) Have a seperate shield that is not shorted to the workpiece.

So in general if you have an exposed tip you can't do ohmic sensing because its shorted to the material.

Some machines (ie Swiftcut) lower a seperate probe and ohmic probe off that. This can be done fairly easily in QTplasmac with a pneumatic cylinder as all the required signals are there.

I have a fundamental problem to follow the discusions on ohmic and/or hypersensing.
It would be great if someone can point to the mistake in my thinking.
All of the pilot arc torch heads I know have an electrode connected to the plasma voltage (negative) and a shield connected to the workpeace.
With a HF pulse on the plasma voltage (relative to the shield / workpeace) the arc is started.
This means that the torch head tip (shield) which is electrically connected to the workpeace can't be used for ohmic sensing because it's alrerady connected to the workpeace.
What's wrong ?