Explain Please - Pierce Height, Cut Height

[solomanb]

Hello all
Could someone explain ignition height, peirce height,and cut height and how they are related to consumable life span.I have a 5x10 Koike table with ProMotion and a Hypertherm 85,(If I had three phase going by it would be larger). The peirce height is adjusted with a timer. No where in the manual does it tell you this much time gives you this height .It also has ATC which works fine. I need to add an air dryer to my system which I understand will add to the consumable life span .
This site is by far the best dxf patterns , ideas and of course info on software to hardware . I drop by this site everynight after work and weekends as well .
Thank you in advance

Solomanb
There are no big jobs some just take longer.

[jimcolt]

Every different brand of torch height control uses some different terms for the critical functions of height control. What is important to consumable parts life is the Pierce Height and the Pierce Delay time....the proper settings for both of these is listed in the mechanized cut charts in your Powermax85 operators manual. With your height control there is another function called "ignition height"....which is for some plasma systems that cannot start the arc at the pierce height...as it is too far from the plate. With your Powermax85 you should set ignition height and pierce height the same.

Cut height occurs after the torch starts moving....it will index down closer to the cut height....which again is listed in the cut charts in your Powermax85 operators manual. On most materials it will be .06" with thye 85. Arc voltage control takes over a short distance into the cut....you can set the arc voltage to the voltage setting in the Powermax85 cut chart as a starting point, then, if you notice the torch is running higher or lower than the cut height (.06")...adjust the voltage setting to a higher voltage for a higher height...or lower voltage for a lower height.

The following is an explanation of how a height control system works.


There are 3 cut height settings on a good quality THC. Pierce height, cut height, and arc voltage control.

1. Pierce height is preset by the operator to the plasma torch manufacturers recomended pierce height for a given material and thickness. The THC indexes down to the plate before the torch fires, locates the plate (there are a few different methods for sensing the top surface of the plate), then retracts to the pierce height. The torch then fires.....and should stay in place, no x y or z movement, until the pierce delay (recommended by the torch manufacturer for each thickness) times out.

2. Cut height is preset by the operator according to torch manufacturers recommended cut height. As soon as the pierce delay times out the torch rapidly indexes down to the cut height. At the same time x and y motion begin driving the torch through the part program.

3. Arc voltage control uses feedback from the plasma system to measure the voltage between the electrode (negative) and the plate (positive). (some call this "tip volts", which is incorrect as the torch nozzle is known as the "tip", yet it is the electrode and plate that the voltage is referenced from) At a given cut speed and fixed torch to work distance this voltage remains constant. If the plate is warped so that it moves away from the torch during steady state cutting...the arc between the electrode and the plate gets longer, a longer arc means the voltage gets higher. The torch height control see's an increase in arc voltage and signals the z axis drive to move the torch closer to the plate. Arc voltage monitoring takes over torch height functions as soon as the machine is within a certain percentage of the programmed cut speed.....so as soon as acceleration gets the machine to a certain speed...the arc voltage control takes over. Consequently when the x and y motion decelerates below this threshold percentage...the avc is disabled so the the torch does not dive during corner or small feature slowdown events .

If your torch height noticeably changes during the transition between cut height and avc, then you should adjust your arc voltage setting so that there is no change during this transition. So, if the torch indexes to cut height, starts moving, then moves further away from the plate....reduce the arc voltage setting. A rule of thumb is that 5 volts will equal approximately .020" torch movement......so reducing arc voltage by 5 volts will move the torch closer to the plate by roughly .020", increasing voltage will do the opposite.

Things that affect torch height when operating in avc (arc voltage control)

1. Gas pressure fluctuation. Changing gas pressure changes the resistance of the plasma arc, which changes the arc voltage.

2. Speed change. Slower cut speed makes the kerf wider, which means the arc is longer, which means the voltage is higher...so the THC moves the torch closer to the plate.

3. Worn electrode. Plasma electrodes wear by forming a pit in the hafnium emitter. If the pit is .020" deep, the torch will run closer to the plate by .020"....as the arc is .020" longer and the height control is trying to maintain a constant voltage.

Bottom line....the proper physical height is more important than setting the exact arc voltage that is listed in the plasma torch manufacturers manual. Adjust the voltage so the physical height is correct...don't worry about the voltage reading!


Jim Colt Hypertherm

[solomanb]

Jim Colt
Thank you for the explaination of the why's and what's . That also explains why a steady supply of clean/dry compressed air is essential to maintain voltage.
I will print this out and add it to my plasma cutting bible . Some day Jim, when someone asks me a question about this information I will be able to give them the correct and orderly answer as you provided to me . Sometimes knowing the why is sometimes better than doing the actual job !
Again Jim thank you.
Solomanb

[elwoood]

Thank you Jim for the clear and concise explanations in plain english . I bought a used system, the previous owner knew none of this info. Now I understand what is going on. You are an excellent instructor.
Thanks again,
Dave

[JeanB]

Amazing explaination,
They just install a new torch on our second plasma wicth is very hold (MG system 80). Since one year I'm running our new plasma hd 130 and I love it so much that I recommended to stay with Hypertherm to upgrade a bit the old one. Well I'm not disappointed. Like there's no hazard some time the mechanic guy and me had to try the new stuff. I'm coming to your explaination....
Our challenge here is very special. There's a reason why I try to understand very good what you say about pierce heigh, delay , cut heigh and avc. We have to cut hole in some kind of big dishes. So we have x,y and z. The angle of climbing can be from 30 degrees to may be 45 degrees (it's an approximative). So the torch have to start the hole (about 5 inches) at the lower side of the dish and rapidly have to adjust to climb the hill within unlock the torch of the magnet. We did tests yesterday (on a real dish who is sending to the production line....I've got to learn fast , not a big range for experience here) and my first whole wasn't perfect because the torch didn't react at all to the z axes. The speed was probabely too slow and the avc was disable. The second hole was better but the speed have been change. I don't know if you are gone understand because I have hard time to explain my situation , English is not my first language.
So I read carefully your explaination and try to really understand the difference about each one. By then I'll be able to do the proper setting to do the best quality cut . I will take your suggest too.
I'll start here, you said that after the pierce heigh and pierce delay done the torch will set down to the cut heigh and start to cut . The thing is my torch is already very closed of my materiel and after the pierce done the torch have to start cutting the hole and climb the hill fast. From now I put my voltage 165 and the speed 70. On my new machine like you said I adjust as I see physicly the cutting go. Piece of cake to adjust cut heigh . But the old one The box they install is different and as I want to change cut heigh during the program the torch is already crash in the materiel. Oh forget to tell you that they keep the old torch and attach it to the magnet support. The diameter of the torch is smaller than the new plasma. Anyway I see more info , I try the stuff last evening for one hour. I have to go back at noon today and teach them how to cut with it and go back to production.
Thanks a lot for your support and knowledge. I'll give you feedback of my experience and keep reading you.