Voltage Scale and Auto Volts

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12 Dec 2021 22:49 #228954 by CFE
Hi all,

We have a Hypertherm 125 for our plasma table, so we can cut up to 32mm plate. We are using a THCCAD-10 on a 50:1 voltage divider. and have set the voltage scale according to the sticker on the THCCAD-10 and the online calculator. We use MODE 1 with the ARC OK signal from the Hypertherm.

We are having trouble cutting thicker plate, 20mm and above. The machine cuts alright if we use IGNORE ARC OK, because it locks the THC. As soon as we unlock that the torch or goes up, or dives in the material. We use AUTO VOLTS. The Hypertherm manual gives a cut voltage of 169V for 25mm plate, but we see only approx 120V on the screen when it is cutting 25mm plate.

What does the AUTO VOLTS feature do ? Is it trying to get the plasma controller to cut at the voltage in the material table ? In which case we are way off the mark with our voltage. Or is the plasma controller sampling a voltage after ARC OK (when it presumably is stabilized), and tries to work around that voltage for the torch height ? In that case the voltage scale could be the culprit ?

We do not have full knowledge on how QtPlasmaC handles input voltage and works out the torch height, which makes it a bit difficult to troubleshoot for us. Any idea are welcome.

Cheers, Peter

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12 Dec 2021 23:36 #228961 by tommylight
During this, always have your finger over the ESC/F1 keys:
-check air pressure and adjust ONLY while the air is on (torch hissing)
-disable auto volts
-set THC to always enabled
-disable void lock and vad
-set the THC delay to 0
-set the pierce delay to what you think is OK for that material
-set the voltage to 150
-start a cut and check if torch goes up or down
--if it goes up = lover the cut voltage
--if it goes down = raise the cut voltage
Only after getting successful cuts with the above, start enabling and changing other stuff you might need.
One more thing not mentioned anywhere about cutting thick material, lower the acceleration of the machine, that is not negotiable. Make multiple configs of the same machine, and as a staring point i use anything above 750mm/s/s for under 5mm thick, 350-500mm/s/s for 5-10mm thick, and 350mm/s/s for anything above 10mm thick.

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13 Dec 2021 00:01 #228966 by phillc54
If AutoVolts is enabled then the THC target voltage is sampled from the actual cut voltage after the velocity has reached 99.9% of the g-code feed rate and then after the THC Delay time has expired.

If AutoVolts is disabled then the THC target voltage is read directly from the Cut Voltage in the selected material.
 

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13 Dec 2021 00:24 #228971 by CFE
Replied by CFE on topic Voltage Scale and Auto Volts
Hi Tommy and Phil,

Thank you for the info. @Tommy : out of curiosity, what is the reasoning behind lowering the acceleration ? I will give that a crack, just wondering why it affects the THC ? Do you also change the acceleration on the Z-axis ?

Cheers, Peter

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13 Dec 2021 01:09 #228973 by rodw
Replied by rodw on topic Voltage Scale and Auto Volts
I like auto volts. There is a natural voltage at a specific height above the material that stays constant.

But I would fire up linuxcnc's halscope and monitor torch voltage when cutting including a pierce and look at how long it takes to settle to a constant voltage with THC disabled and what the voltage is. That will be quicker than trial and error as you can do it with one cut.

I think you will find that you need to extend your auto sensing delay on thicker plate. The voltage is quite erratic on piercing so if its sampled a low level (possibly too early), as the THC is disabled , the torch will dive to bring the voltage down from the correct height to the incorrect sampled height.

Move it out to  2 seconds, 1 second, then 0.5 second (the minimum I would use)
2 seconds will be fine on that kind of material at slow speeds

 

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13 Dec 2021 01:44 #228981 by tommylight

@Tommy : out of curiosity, what is the reasoning behind lowering the acceleration ? I will give that a crack, just wondering why it affects the THC ? Do you also change the acceleration on the Z-axis ?

It will leave the start uncut due to torch reaching speed very fast, it will not give enough time for the arc to follow.
It does not affect the THC, THCAD coupled with LinuxCNC can do some very fast moves.
No need to change anything for the Z axis, just X and Y, do pay attention if you have tandem axis as both joint sections need to have the same values.
-
The procedure above is required to get the right voltages for your setup in regards to material thickness and plasma current, on all plasma sources voltage varies with current, some more some less.
After having that data sorted, enable the features like autovolts, check if the cutting voltage is close to what you got without it, then adjust the THC delay so autovolts gets consistently in that range (+-3 to 4V).
When cutting different thicknesses, a lot of parameters change, so having some starting values saves a lot of headache.

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13 Dec 2021 04:23 #228989 by CFE
Replied by CFE on topic Voltage Scale and Auto Volts
Hi all,

Thanks again for all the info. I have tried different things as per all suggestions from you all, but the main problem is still there and seems to be this : the machine starts cutting and after a undetermined time it just rapidly lifts up the torch and of course then it loses the arc. Sometimes it goes the other way around, the torch just dives into the material. On 25mm plate the feedrate is not very high, so the THC should be able to maintain height at its leisure. But it just goes awry in the blink of an eye.

The problem is that this happens very erratic, sometimes we cut half a meter without problems and then all of a sudden the torch decides to go whichever way it wants to go, buggering up a part. And sometimes this happens after say 50mm of cutting, and sometimes it doesn't happen at all. Something seems to trigger the torch to go up or down without a feedback loop to correct it.

Most of the time the THC works splendidly, it is very fast and is highly responsive. It's just this gremlin we are chasing and we can't put our finger on what it is.

Cheers, Peter

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13 Dec 2021 09:23 #229004 by tommylight
Try to catch what is going on with the voltage when that happens.
Seems like interference on the THCAD side, so first thing to change is the 5V going to it.

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13 Dec 2021 11:14 #229009 by snowgoer540

Try to catch what is going on with the voltage when that happens.
Seems like interference on the THCAD side, so first thing to change is the 5V going to it.

On this note, it may be worth brining up halscope and watching the voltage at idle while not cutting.  I found that I had my shield connected to table ground, and it was actually making the signal noisy, even at idle.  I removed any connection to the shield, and the noise went away.  I had asked PCW about it, and the basic answer was that whatever makes it less noisy is the best option.  
The following user(s) said Thank You: tommylight

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13 Dec 2021 20:07 #229025 by PCW
Replied by PCW on topic Voltage Scale and Auto Volts
Yes, plotting the voltage with halscope when idle and when cutting
should help pinoint what's going on. Normal operation followed by
crazy behaviour at random times sounds like a hardware issue of
some kind.

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