THC Problems - Torch drags after 20-30 pierces

[Welder Kenny]

Hey guys is anyone else have a proto THC that works great with new consumables then after about 24 or 30 pierces it starts to not respond like it should?? mine starts to drag on the sheet and when I try to make adjustment on the front of the THC nothing happens?? I'm confused.

[acourtjester]

You did not state what Plasma unit you have, if this is all on one cut routine (G-code) or many small cuts with different G-codes.
What software you are running. Did you mean Proma or proto, if Proma which model.

[weldguy]

Like AC said we don't know what plasma cutter your using but it could just be normal consumable wear. As you cut your electrode essentially will get shorter, you will notice a divot in the end of a used electrode. Its not much but it is shorter and as a result your THC will lower the torch to maintain the same arc length. This will of course put your nozzle or shield closer to the plate so as you cut you may need to increase your THC setpoint voltage a volt or 2 as you go. Give it try and it may solve your issue. Also be sure to clean dry air!!!! Can't emphasis this enough and a Home Depot filter isn't going to cut it.

[Welder Kenny]

Sorry guys I know better , I'm running a Hypertherm 65 powermax with sheet cam and Mach 3 my THC is a proma I put a new electrode in and cleaned the shield, plus adjusted my THC up to 90 volts, I'm cutting 1/4 A.R. 500 pieces are around 12 x 8 inches square. Also had a few UC 100 errors.
Thanks for all the help I know just enough to be dangerous.

[weldguy]

Thanks for the extra info. Try my previous reply and see if that helps. When you see the torch getting too close simply adjust your THC from 90v to 92v or whatever works.

[Welder Kenny]

Thanks guys, That's going to be the plan adjust voltage up on 1/4 plate, Dumb question do I need to set it lower when I cut 14 ga. or just set it and forget it? I cut 3/8 plate in the morning and by lunch I maybe running fine cut electrodes and 45 amps on 14 ga. should I be lowering my thc voltage setting?

[acourtjester]

If you have not done the setup to find the arc voltage for each metal thickness you should, no need to guess what it is.
this is a procedure to do that will work with the Proma too.
https://youtu.be/EyZJVMtzKzY

[Welder Kenny]

Thanks guys and Gals as always you have helped me out of a jam, keep up the good work. Happy Plasmaing

[weldguy]

Thanks guys, That's going to be the plan adjust voltage up on 1/4 plate, Dumb question do I need to set it lower when I cut 14 ga. or just set it and forget it? I cut 3/8 plate in the morning and by lunch I maybe running fine cut electrodes and 45 amps on 14 ga. should I be lowering my thc voltage setting?

Whenever you change material thickness, amperage, cut speed, etc. you will need to change the THC voltage. I double check my cutting height with each new job I do and adjust the THC voltage accordingly to achieve the appropriate cutting height of .060" for steel and aluminum and .020" for stainless. You have a Hypertherm 65 so refer to these cut charts https://www.plasmaspider.com/viewforum.php?f=164 for a starting point and do a few straight line test cuts in the material you will be cutting to confirm the chart settings are getting you good cut quality and proper cut height. Adjust if needed. DON"'T GET HUNG UP ON THC VOLTAGE NUMBERS. The number doesn't really matter, its the cut height that matters and you adjust the voltage number to achieve the proper cut height.

[Welder Kenny]

Thank you weld guy, I do have the charts in a book right next to my plasma I will set my machine up to cut close to the charts, I've been running my cut speeds a little slower just to make sure all cuts well.( hard to go back and recut the steel) Are the voltage setting referring to the THC? Also I need to double check my torch height. Thanks a lot its nice to know you all will share what you know works..

[jimcolt]

A few things about height control:
1. Arc voltage and cut height are proportional. Increase the voltage preset on an arc voltage based height control and expect the torch to rise. Decrease the preset voltage and the torch will run closer to the material.
2. Yes.....voltage must be set differently on every different thickness in order to maintain the recommended cut heightlisted on the cut charts in the Hypertherm manual. Why? Thicker material requires a longer arc, a longer arc has a higher arc voltage.
3. If you are cutting along and the height is correct (usually .06" between the torch and the material) and you decide to cut slower.....expect the torch to move closer to the material. Cutting slower makes a wider kerf (cut width), the plasma arc has to reach out to find the material, this makes the arc longer (higher voltage).....so the height control moves the torch closer to the material. For best cut quality you need that physical cut height of .06", so you must increase the preset voltage on your height control when cutting slower for best results and less chance of collisions with the material.
4. As the electrode and nozzle wear the arc voltage will get higher (the pit that wears in the end of the electrode makes the arc (voltage) longer (higher). The height control then moves the torch closer to the material in order to maintain the preset voltage. The best cut quality height is .06" for most materials (again, cut height is listed in all of the Hypertherm cut charts. As the electrode wears the operator must increase the preset voltage in order to maintain the best cut height to compensate for electrode wear.
5. The arc voltage listed in the cut charts is correct using high end industrial test equipment in a lab. When you set the arc voltage to the suggestion in the operators manua, you have a new set of consumables in the torch, the cut speed is as suggested in the manual....then you will get the suggested physical cut height with the suggested preset voltage.....assuming your machines height control is calibrated as well as the ones in Hypertherm development labs (it probably is not!) So if you set everything correct including the speed and arc voltage, but your torch is just about dragging on the material.....what do you do? Increase the arc voltage until the torch is cutting at the suggested physical cut height. The voltage is just a number for feedback purposes, the physical cut height is what is important. Always tweak the voltage at the specs you are running in order to maintain the physical cut height.
6. Adjust the voltage (higher) as the consumables wear. Adjust the voltage (higher) as you cut thicker material. Adjust the voltage (higher) if you decide to cut slower.
7. There are a dozen or so low cost standalone height controls used on a number of different low cost (any cnc plasma under about $18k is considered low cost by those that have been in this business for a decade or four) cnc machines. They work better than no THC, but need a lot more attention than the high end (higher cost) integrated THC's that you will find on higher cost Light Industrial and high cost Heavy Industrial machines. The better engineered THC's will not ever require a voltage setting.....you simply set a physical cut height and the machine auto calibrates the height on every cut cycle. Because of auto calibration, you never have to compensate the arc voltage for consumable parts wear or for speed changes. If you set .06" in the height control settings, the torch will maintain at .06". Expect on the lower cost machines that the operator needs to understand the importance of height (cut quality, consumable life) and the relationship with arc voltage. Those that understand height control will get better cut quality and lower operating cost than those that don't! Or you can spend more up front for a Light Industrial machine with an integrated height control. Hope this helps a bit on height and voltage issues! Jim Colt

[Welder Kenny]

Thanks Jim, Makes perfect since, I'm used to my manual hand operated plasma that I drag by hand. getting into this cnc stuff is a learning curve.

[Rodw]

As always a gold mine of information from Jim.

The linear relationship between torch voltage and height is well understood. When I sampled 16,000 torch voltage readings while cutting, the regression formula tracked the actual voltage with 99.4% confidence. Frankly I was amazed as I expected a change in slope based on samples containing just a few readings I did earlier.

The better engineered THC's will not ever require a voltage setting.....you simply set a physical cut height and the machine auto calibrates the height on every cut cycle. Because of auto calibration, you never have to compensate the arc voltage for consumable parts wear or for speed changes. If you set .06" in the height control settings, the torch will maintain at .06". Jim Colt

This is exactly how we are now doing it in LinuxCNC these days. I've never really bothered to look at the torch voltage while cutting for months. I have faith in the machine's ability to probe accurately, sample the voltage after the spike from piercing subsides and track the material surface at the programmed cut height exactly regardless of how much the material warps. Thats because the voltage is being measured and the height adjusted 1000 times a second to keep the voltage (and therefore the torch height) at the desired level.

[Welder Kenny]

I Just went and cut some 3/8 A.R.500 and THC worked perfectly kept torch at .06 must of the way around it did jump to .122 for about 30 sec then went right back to.06 I'm happy and will watch the zed axis and make adjustment from there, I want to thank all that helped me to understand arc voltage and THC you guys are awesome.

Happy cutting
Kenny