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I thought I'd post this as it is an often misunderstood part of the CNC plasma cutting process. There are a lot of things that go on in the background of THC (Torch Height Control) with an automated plasma system....and it is a very important part of the plasma cutting process. A cutting machine without automatic torch height control will exhibit poor plasma consumable life as well as poor cut quality. Unfortunately, even with a good THC, improper settings or lack of a basic understanding of how the system works can lead to the same results as having no THC! Here's a simplified explanation of what has to happen for every cut cycle with a proper THC:
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 mottion 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 noticably changes during the transition between cut height and avc, then you should adust 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 thunb 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!
Another thing to consider is the amount of movement it takes to trip the Z axis switch. The larger BTA tables are right around .250 so this must be added in Sheetcam to the Z axis pierce and cutting height.
Those that don't pay close attention to the accuracy of pierce height...buy more consumables, especially nozzles (tips) for their plasma torches!
There, as I mentioned above, are different methods used on different machines for finding the surface of the plate. This function of the height control system is often referred to as IHS (initial height sensing). The Bulltear machines use a limit switch and a floating head, essentially the torch indexes down, contacting the plate, which floats the movable potion of the z axis assembly, tripping the limit switch. This type of unit needs to be set-up in the software to account for the hysteresis (backlash) in the limit switch, ensuring that the torch will retract accurately to the pierce height.
Other IHS methods used on plasma cutting machines for initial height sensing:
Floating Head...mentioned above.
Ohmic Contact. This method uses electrical contact sensing between the shield (front most component on a shielded torch) and the plate. This method works quite well on clean surfaces, and is probably the most accurate method of finding the surface of the plate, however does not work on primed, painted, masked or rusted surfaces, and also does not work in submerged (underwater cutting) applications.
Stall Force Sense This method uses adjustable stall force sensing by measuring z axis motor stall or current (depending on the design of the circuitry) to sense contact with the plate. This process works well although on thin plate it tends to deflect the plate, rendering the pierce height inaccurately. Works very well on thicknesses over about 12 ga, and on all surface conditions.
Inductive sensing This method uses inductive probes that sense the plate when close. Typically only used on very high current (400 to 1000 Amp) high end industrial systems where robustness is most important and accuracy is least important!
Optical or sonar sensing These methods are occasionally used...but have only had limited reliability due to surface conditions ans well as shop conditions (smoke, fumes, etc.)
ok.....now that you said that. my recommended torch height for my TH 101 on lets say 1/4" steel is, .188 at 80 amps running 102 ipms with arc volts at 107. you are saying i should set my torch height at .250 plus the .188 recommended for a total of .438, does that sound right? that seems like a lot.
im asking because my 101 will not cut 1/4" with the recommended setting. the only way i can get a clean straight cut is running at .200 at 120 amps with arc volts being at 120 and at 30ipm. i get a lot of dross that chips right off but id rather not have that much. im i setting it up wrong??
No....you don't add the pierce height and cut height together....the torch would likely not even transfer to the plate from that height!
Most plasma system manuals provide a mechanized cut chart. (at least the Hypertherm manuals have this info). On this cut chart should be recomendations for pierce height and pierce delay times for each thickness of material.
Pierce at the pierce height, the as soon as the pierce delay time is over, the torch should index down to the cut height. If you cannot find the recomendations for pierce height in the operators manual for your plasma, then the "rule of thumb" for pierce height is 1.5 to 2 times the recommended cut height. If you cannot find the suggested pierce delay time...then adjust it by trial and error...the torch should remain at pierce height with no x, y or z movement until the plasma arc is blowing metal sparks out the bottom of the material.
With your 101.....if you want good quality on 1/4" you should be using either a 40 amp or a 60 amp nozzle. 120 Amps will provide good cut speed but a lot of edge angularity on 1/4" plate.
Physically the Z axis will slide to trip the limit switch. Depending on where you put your Z axis limit switch you will need to add this travel to your pierce and cut height to get the proper height.
i wasnt talking about adding the pierce and the cut height. it was stated to add .250 to each of those on the big BT machines. my limit switch is in the same place as shown in the setup for my 4x8.
im trying to get good quality cuts with the right height and not blow through tips. i had a problem when first starting to where the pierce height was to low and not letting the torch fire because it was touching the plate. im just trying to get everything setup correctly
on 1/4 inch plate is it recommended to start on an egde as to not blow crap back at the torch during its cut. also is it recommended to use cold rolled and P.N.O plate rather then hot rolled?
Sorry, I guess I misunderstood. I am not totally familiar with the software for setting the offset for the limit switch on the Bulltear machine.
Your plasma goes up to 120 Amps? You do not need to edge start until you get to about 3/4", as long as pierce height and pierce delay are set correctly.
You can use whatever plate you want, however hot rolled will produce a bit more dross on the bottom edge.
ok sounds good. maybe thats one of my problems is that im using 1/4 hot rolled and getting a high dross. im thinking i have it set up wrong.
its an 80 amp machine but i get a better cut if i set the cut profile to 120. dont know why can i have something wired wrong. or is it just me..... could be both. ive had alot of long nights with the table and the learning curve of autocad, sheetcam, and mach.....
the specs to cut 1/4" from TD are not letting me cut 1/4". but if my pierce and cut height are wrong that could be problem. i see alot of people online that cut with similar tables and smaller plasmas and getting better cut quality then i am. your help is much appreciated
It is my experience that the mfgs dont really get very close to real world cutting speeds and feeds on the smaller plasmas. Drop your feed way down and do some test cutting. The speeds will be the learning part of cnc cutting. It does take practice.
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could be both. ive had alot of long nights with the table and the learning curve of autocad, sheetcam, and mach.....
