Voltage Scale and Auto Volts

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

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.

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.
 

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

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

 

CFE wrote:

@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.