Another plasma component...

perfect, thank you

paused motion isnt working in my case also sir
using linuxcnc-plasmac

Guys

I know this is hopelessly out of date given the rapid pace of events the last few days. Here are some thoughts on THCad and general arc voltage calibration.

1. None of our cutting equipment is laboratory grade or metrollogy grade. So don't agonize over getting multiple decimal point precision.
2.The calculated THCad card calibration using the Mesa values for your card are a good starting point. Then check this and against a battery and good DMM tweaking the scale as needed.
3. Do a little research and determine the the correct cutting current and cutting height for the consumables you are going to use the most.
4. Now it is time for live cutting and using your DMM. Pick a work piece that is in good shape on the surface and at least close to the thickness and material type you think you will use the most. Error on the thick side as this will be easier to level and use slower cutting speeds. The hard part is that first you need to get your machine adjusted so that your torch runs true and at a constant height above the work piece over the area you will be using for testing, this is probably the hardest part for an inexperienced new builder. You can mount a dial indicator to you torch mount if you are careful and move slowly. Write your self a small line cutting program for a line 150mm to 250mm long. Test this program to make sure it moves the torch in a straight line at the commanded height.
5. This is it! Connect your DMM across your work piece and electrode connections. Make a cut at the correct cut current, torch height and speed while recording the voltage displayed on your GUI and the DMM. Using Halscope can be a big help here. You will need to do this more than once (I have done 100's of line tests over the years).
6. Adjust your arc voltage scale to correct the error between your DMM reading and you GUI display value. Again you will be repeating this and step 5 a number of times to get it.
7. Notice I didn't say anything about a book value for arc voltage? If you don't have a big manufacture machine you don't have a cutting chart arc voltage value anyways. The arc voltage value on your display is now the correct arc voltage for your machine using the material, consumables ,cutting current, torch height and speed you just did your test at. You should do this for each new material and it's set of cutting conditions that you use. This way you build cutting charts for your equipment and connections.

My machine is setup so the GUI arc voltage value matches the values from the DMM when cutting. I have also checked it with a high voltage (168v) power supply. The arc voltage scale factor is not the same number as calculated using the Mesa factory calibration but is quite close. This is because at the factory they weren't using your power supply, torch, table connections and maybe voltage divider. My cutting arc voltage differs from the published cut charts for the consumables I use by nearly 10v when cutting at the correct current, height and speed. I have checked this many times with feeler gauges so I know it is true.

John.

Is there a way to turn THC off in the gcode. I have tiny holes to cut 1/4" and cant seam to get corner locking to work. Its always on if I select it. I looked through the read me and did not see any gcode commands to turn thc on and off. Had corner locking problems with small triangles as well. Right at the tip when it reverses direction the torch dives and hits the sheet.

Awesome detail! Thank you John.

Maybe it's a silly question but I'm still left with one:

Connect your DMM across your work piece and electrode connections

Can you define or illustrate where best to probe the electrode connection? Do I need to open up the machine while it's running?

My plan before was just to put the probes across the input pins of the thcad.

But, if per #7 you're not going by book values, isn't it all moot?

AgentWD40

The best place to connect your DMM for measurements is at the electrode and work piece terminals in your power supply. This will expose you to potentially lethal voltages so don't do it if you don't have the knowledge or skills to do it safely. Do not operate your machine with the cover off! Connect extension wires to the measurement points so you can run with the covers on and carefully insulate the meter connections. If you have a CPC port use those connections. Since you have already checked the THCad calibration with a battery connecting the meter at the THCad card input will give you little benefit without knowing the arc voltage by another means.

The book values still have great value as they spell out the correct cutting conditions and consumables for each material listed. The published arc voltage values can be an aid in diagnosing poor cutting as well. Remember it is the correct cut height that matters not the arc voltage which is only close when everything else is correct under lab conditions.

Not everyone here has a larger HT machine and torch combination that came with pages of cut charts for most likely combinations of materials and consumables that are within the power range they can work at. That is why I presented this the way I did. You can adjust your GUI voltage to match the book value if you want but there is no guaranty it will stay in calibration when change to a different consumable type. Or if you are like me and have a TD power supply and an HT torch the arc voltage in the book is a rough guide at best. However the cutting parameters for each type of consumable and material happen to be right on if you are controlling for cutting height and let the arc voltage be what it will be each one.

John

islander261 wrote:

The best place to connect your DMM for measurements is at the electrode and work piece terminals in your power supply.

Got it! Thank you.

This will expose you to potentially lethal voltages so don't do it if you don't have the knowledge or skills to do it safely. Do not operate your machine with the cover off! Connect extension wires to the measurement points so you can run with the covers on and carefully insulate the meter connections.

Well that's why I asked! It didn't seem like a very bright idea, lol

Since you do this procedure regularly would you permanently install these extension wires? Or I guess you'd just be installing a cpc port and running these leads there?

If you have a CPC port use those connections.

As far as I can tell it will only provide the internally divided voltage and not the raw voltage.

Since you have already checked the THCad calibration with a battery connecting the meter at the THCad card input will give you little benefit without knowing the arc voltage by another means.

Yeah, that thought occurred to me after I posted.