plasma THC voltage

[duraflap]

Who can tell me:
What is the relationship between arc voltage and cutting height on the THC?
Thank you
Dennis

[Torchhead]

There is a simple electronic equation E = I X R (voltage equals Current times Resistance). Resistance can also be the "Load Resistance". If you hold any one variable constant and change one of the others, the third will change to maintain the balance.

Plasma is a constant current cutting process. When you set the current on the dial , the plasma unit will try to maintain that current (I). It's a balance relationship. Since the cut gap (arc length) represents the "load" (resistance), change of that distance (R) changes the other leg of the equation (E) in direct proportion.

In simpler terms the gap voltage is proportional to the gap distance if the current is constant. It only takes about .010 inch of difference to make about 1 volt of difference in the gap voltage. It's like measuring the change in air pressure to determine altitude.

Each plasma manufacturer has determined their specific voltage at a given cut gap. For some it's higher or lower than others but it will remain relatively the same for that given machine. The charts in the owners manual are based on a given torch, nozzle and cut current. If you use a value outside the chart (lower/greater current on a given tip) you find the voltage at a given gap is different.



Plasma cutting air has the highest voltage. A tip sitting on the metal has the lowest. measure the gap voltage, eliminate the noise, and be able to move over the change of just a few volts (out of hundreds) and you can "servo" the Z to move the torch tip small amounts to make maintain cut gaps that have to stay within + - .020 while moving in X & Y at 200 IPM.

[jimcolt]

Torchead is right on track...although I will add a couple of comments:

Think of the plasma arc as a resistor in an electronic circuit. The arc is made from the gas you are using (often air....but there are many industrial plasma's that use oxygen, nitrogen, argon/hydrogen mixes as well as many other gas combinations). When you change the amount of air (by increasing the air pressure) you change the resistance. When moisture (water from your compressor) gets in the air, the resistance changes. When the torch gets closer to the plate...the resistance changes. When the electrode and nozzle wear, the resistance changes. When you change cutting speeds, the resistance changes.

The arc voltage height control actually measures the the voltage drop between the electrode and the plate....one end of the plasma arc is attached to the electrode, the other to the plate. This measured voltage is compared to a voltage set by the machine operator....the height control compares the measured voltage to the preset...then adjusts the z axis (up or down) to make the measured voltage the same as the preset voltage.
So, at a known power level in Amps, and a known cut speed, with a known gas (air), and a new set of consumables....most plasma manufacturers publish a recommended arc voltage that will allow a certain torch to work distance to be maintained. The following will make that published voltage not work properly:

1. Lower cut speed. If you cut slower than manufacturers specs...the THC will move the torch closer to the plate...too slow and it will crash into the plate. If you want to cut slower than the manufacturers specs....increase the arc voltage until the torch is at the physical height that provides the best cut quality.
2. Faster cut speed. Faster speeds means the torch will rise too high off the plate. To cut faster, reduce the arc voltage until the height is best for your desired cut quality. Too high often puts some top slag on the plate, as well as increasing the angle on the cut edge.
3. Water in air will change the height....although the severity of the water...and other contaminates such as oil (from the compressor) can create different effects. Usually water will cause the torch to dive.
4. Electrode wear. Since the electrode naturally develops a pit at the arc attachment point...as it wears the arc gets longer....so the THC moves the torch closer to the plate to correct the voltage. It is common to have to increase arc voltage settings over the life of an electrode to maintain the correct torch to work distance. .010" pit depth will equal the need for approximately 2.5 to 3 arc volts increase.
4. As the nozzle orifice wears..the energy density of the arc decreases, causing the torch to get closer to the plate. Increase the arc voltage to compensate and maintain the correct distance.
5. Kerf crossing. At the end of the cut, when crossing the kerf into the lead-out....there will be a dive of the torch as the arc lengthens trying to find metal...and the THC tries to adjust. Many sophisticated industrial height controls have the ability to ignore kerf crossings.....and more sophisticated cutting software will add machine code automatically on kerf crossings that disables (freezes) the z axis momentarily to block the dive and possible collision. This also occurs when cutting holes...and the slug from the hole drops....the torch dives and tries to follow the slug.

There's a lot more, but I'll stop now. Hopefully this gives a better understanding of the arc voltage / torch to work relationship.....and may help explain some issues that commonly occur!

Jim Colt

[tinman13]

Thanks Jim, I always look forward to your replies!
By the way..... I realize that it is a competitors machine but would you have any ideas on my previous cut quality post? It happens more frequently on heaver plate, Have not seen it on 1/4" and below.
Hope I'm not hijacking a thread here!
Thanks, Steve

[duraflap]

Thank you both for the explanation of the relationship of arc voltage and THC.
So in a nutshell, I'll need to experiment more to fine tune my machine to determine where I need to set the voltage differently for say, cutting circles or short inner cutouts verses longer cuts or profiles.
Can I assume that the amperage and the THC setting can remain the same in the Sheetcam setting for the entire part and use the voltage level to adjust for actual torch heigth and feed speed change?
Being new to the cnc plasma I'm finding it considerably more complicated to master than my cnc router.
Thank you for your help.
Dennis

[jimcolt]

Dennis,

Always set the voltage last...as that will determine the height. The height controls cut edge angularity and dross levels.

Plasma has far more parameters to adjust as compared to most other processes. Interestingly....the High Definition systems on many industrial grade cutting machines are far easier to set-up. On most...the operator just chooses the type of material and the thickness...everything thing else is automated. Those systems are expensive, far more complicated (as many as 6 different gas settings)...yet produce better quality parts faster and less expensive than with an air plasma.

Jim

[Torchhead]

The MP1000_THC tip volts setting can be adjusted on the fly (as it cuts) from the:
UP / DOWN Buttons on the front panel
UP DOWN Buttons in MACH
UP/DWN buttons on the optional Hand Controller.

There is also a customizable table where you can preset material type/thickness and matching tip volts and bring it up and select from the list (before the job runs) . You can modify the settings in the list with the edit button. It keeps your notes for you.

It would not be difficult for MACH to send new tip volts based on speeds and feeds to the MP1000 BUT since it varies on other factors to be perfect it would almost have ot have a "teach" feature! If you can furnish me a chart of what they should be for a given feedrate and material then I can have it send the value as it cuts from MACH. Honestly with conventional plasma you have a job on your hands to get all of the numbers!

It's been my experience that just slowing down the Feedrate on smaller holes and changing the lead-in to perpendicular helps. You just group the inside holes together (layer in SheetCAM) and define the feedrate different. If you do need to use a different tip volts then you may need to burn the small holes as a different job and set up the target tip volts prior to cutting OR just manually adjust when it goes to cut the holes in the separate layer. There is a limit to what conventional plasma can do. It's only accurate to a point and if you try to cut holes smaller than about 3/4" dia on material thicker than 16 ga you start to get holes that are not perfectly round. Where I need perfect holes I "peck" the center of the hole with just a pierce point and drill the holes with twist drills. that still has an XY positional error of about .010 or greater.

The needed parameters for a specific tip and material (like pierce height, initial cut height, default feedrate, dwell at the end of cut, etc are all in the tool definitions in SheetCAM. You can setup as many tools (and or POSTS) as you want. I have different tools defined for different tip sizes. You can also override certain parameters when you define the cut process in SheetCAM.

I have multiple posts that modify the REFDISTANCE (inside the post file) so the auto touch-off frequency can be changed. In the default post the touch-off is only done every 20 inches of XY travel so where you are doing close pierces, it does not have to waste the time to ref before every pierce. On thin material where it tends to warp easy I set that to "0" so using that Post it touches off every pierce. Since MACH knows the position of Z as long as the material stays flat and level it will pierce at the right height.

You are correct. Cutting with plasma is a lot more complex than a router. Even doing the artwork is different. We do a lot of both.

You can spend a lot of time messing with settings to try and get the perfect cut. There are so many variables that it will shift on you. You can set everything from tip volts to plunge rate either in MACH or SheetCAM. You just need to find the best average and use that to cut with.

There are THC Corrections settings that prevent the Z from going too low or too high; there are THC Rate settings that change the dynamics of the THC feedback loop (how quick/fast the Z responds). Change those parameters are your own risk and keep the old settings so you can get back!

TOM CAUDLE
www.CandCNC.com

[Mongkol]

I like this post . It increases my understanding.