Torch Height Control Tips

jimcolt

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.

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.
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.
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)

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

jeepsr4ever

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. Good info!! :t:

jimcolt

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.)

Jim

sadisticiron

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?? thanks jim in advance

jimcolt

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. Jim Colt

jeepsr4ever

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.

sadisticiron

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? thanks guys

jimcolt

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. Jim

sadisticiron

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..... 111!!! 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

jeepsr4ever

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.

jimcolt

Actually....if you look at the specs in the Hypertherm manual under "machine torch" and optimum settings you will see real world cutting specs for cnc machines. If you set your setting exactly the way the manual suggests, you will get a good cut. The specs listed by virtually all manufacturers for hand torches are the absolute fastest speed, with a new set of consumables. They can be achieved, with a new set of consumables, but not with the best cut quality. Why do manufacturers list their hand torches with these barely achievable specs? Because the hand plasma cutting business is very competitive.....many buyers make their purchase decisions based on price and cut thickness and cut speed. If you don't list the maximum speed, then you don't get the orders! Jim Colt

jeepsr4ever

:t: Thanks for the explanation! We have tried very hard with TD to get close but havent gotten close enough. We are setting up another powermax 45 unit, good to know we are closer with Hypertherm than with TD!

sadisticiron

ya mines an automated 101. not sure if that matters but seems to be pretty good so far. thanks for the knowledge

sadisticiron

i got it. i added the .250 to the pierce and the cut height as stated before due to the z axis limit switch, turned my cut speed to 60, set my amps to 60 and BAM... :? :? :? :t: :t: . the cut was so clean with almst no dross on a worn nozzle. thanks guys for the help

Creepy

jimcolt wrote
Actually....if you look at the specs in the Hypertherm manual under "machine torch" and optimum settings you will see real world cutting specs for cnc machines. If you set your setting exactly the way the manual suggests, you will get a good cut.

Jim Colt

I've been able to produce finished parts on the first cut in different thickness steel, aluminum and stainless with the basic PM45 machine torch cut variables entered straight from the chart.

Made a grill for a guy, never had aluminum OR stainless on the table before.....came out near perfect.

I say they work!

sadisticiron

are you adding that .250 or so to your cut and pierce cuts.

sadisticiron

bump for the last post im having a problem cutting holes no matter what size or thickness of steel being cut. the holes in the 3000thc come out as "U's" and in the 1000thc they come out oblong almost like a "d". not sure if its the post or the gantry's motor turn or speed. any help would be nice.

jimcolt

Good holes with plasma requires that everything is set up pretty close to right...and the torch height control plays a major part in hole quality...

Rules of thumb for good hole quality with plasma, and these apply pretty much to all holes under 1.25" diameter.

Cut speed for holes should be set at 60% of the optimum speed you are using for larger contours on the same materials. This slower speed allows the plasma arc to be more straight, yet will produce a little low speed dross on the bottom of holes.
Lead in for the hole should be as close to the center as possible, for a few good reasons. a. Piercing often produces a puddle of resolidified metal on the top of the plate...if this material is in the cut path of the hole, the torch cuts over it and it will cause some instability in the arc which will change the shape of the bottom of the hole.b. Starting near the center allows for more time for the torch height control to index from pierce height to cut height before getting to the hole contour. c. Starting near the center allows for more time for the plasma gas (air) to reach proper pressure and flow in the torch, which is critical for a good cut.
Accceleration on your machine (x and y axis) should be set at the highest rate that still allows the machine to stay on path. Plasma cutting of holes is always best at higher acceleration settings.
If your cutting machine is tight (good accel, no backlash) then a straight lead in for holes works best, and will leave a minimal ding/divot on the contour of the hole. If your machine is sluggish in the acceleration department, or may have some slop in the drives, a radial (curved) lead in may work best. Plasma likes to cross its own kerf at the end of the cut at 90 degrees, there is less arc instability when the straight lead in is used, however if the machine is sluggish a straight lead in will cause a divot in holes where the meachine has to decell to turn on to the hole contour.
Make sure the software programs your holes to cut in the counter clockwise direction. Plasma is directional. If cutting a ring, the ID cuts ccw, the od cuts cw.
At the end of the cut profile for a hole....never use a lead out that goes back into the center scrap material...this is bad for hole quality and bad for consumable life. Lead outs are normally used for oxy-fuel cutting. If possible program an overburn of about .200" where the arc off signal shuts off the plasma at the 360 degree point of the circle, but the motion stays on the hole radius and travels .200" past the 360 degree point. If the previous cannot be easily done with your software just cut the hole with no lead out...let the torch extinguish on the radius at 360 degrees. An air plasma is exothermic, meaning it does not instantly extinguish when the off signal is issued, so if motion stops at the same time the plasma stop id issued...there will be a divot in the cut edge.
Pierce height must be at the plasma torch manufacturers spec. If your torch manyufacturer does not supply this info...then pierce at 2x the recomended cut height.
Pierce delay time must be set at manufacturers specs. If your torch manufacturer does not provide pierce delay times...then just be sure the delay is long enough so there is no x, y or z movement until the plasma arc has fully penetrated the plate.
AVC (arc voltage control) should not be active on hole diameters smaller than 1.25". The THC should find the surface of the plate, set and accurate pierce height, once the pierce is complete index down to cut height...and remain at the cut height until the hole is complete.
Last, but certainly not least, worn or damaged consumables will screw up every hole shape. Nozzle orifice must be sharp and perfectly round, same goes for the shield orifice. If the above specs are followed carefully...you will get good holes as well as good consumable life.

The above assumes that the machine motion can maintain constant speed, and stays on pathe for hole cutting. A simple test with a pen in the torch holder, and a piece of paper on top of the plate can show your machines ability to cut a circle.

The best plasma torch in the world will not make a round hole if all of the machine motion is not optimized to work together. Incorrect torch height is the single biggest cause of ugly plasma cut holes.

Jim Colt Hypertherm

sadisticiron wrote
bump for the last post

im having a problem cutting holes no matter what size or thickness of steel being cut. the holes in the 3000thc come out as "U's" and in the 1000thc they come out oblong almost like a "d". not sure if its the post or the gantry's motor turn or speed. any help would be nice.

sadisticiron

gotcha. i think i got it all figured out. is it best to run your holes in your part as a seperate g code or just manually edit the gcode to cut holes at different speeds and cut heights. i manually edit but when u have 300+ lines of code for one part its kinda a pain. i got my hole cutting thing under control to with the new post from the guys at candcnc. i had a problem with only the lead ins and outs being cut. also the gantries steppers gears were coming loose from the set screws. oblong circles from that aswell from that slight miss step. i also changed my kernel speed to 25000 and turn my velocity down to 600. that helped alot with cutting. i just now have to figure out how to have over burn on my holes thanks jim

jeepsr4ever

You should not need to interpolate a hole twice.