Slag builup on nozzle - Opinions please

[Shane Warnick]

Hypertherm Powermax 1250, cutting 1/4" mild steel, 40 amps shielded, with 40 amp nozzle. 45 ipm, cut height 0.060, pierce height 0.12, pierce delay 0.4 seconds, auto zero'ed arc voltage on first cut, THC control on. So, I was cutting some 18" square plates for this job today, 15 of them I believe, anyways I keep getting this junk on my nozzle. It's not on the outer shield, but actually on the tip of the nozzle itself. When it builds up, cut quality suffers slightly, and the nozzle seems to push a pile of molten metal along right in front of the nozzle. I can stop the cut, clean the nozzle, and it cuts great for awhile. When it gets build up like this, the THC seems to really be working it's butt off as well, jumping up and down, but not always. I can see how that would be if there is a little pile making contact with the shield occasionally, twitching out the resistance and arc voltage. I stop the cut and measure, and best I can tell the tip is 0.060 or real damn close off the surface of the sheet. I have lead in set at 0.5 inches, and the thc is at cut height before the end of the lead in no problem. Here is a picture of the nozzle, on a T80M Machine torch.

So, I ask you oh all knowing plasma spider members, which direction should I run to chase this down? I see this on cutting thinner stuff with finecut but rarely this much buildup, it flakes off easy enough just seems to cause some angularity and the THC is working hard. I feel like it is blowback happening during the pierce, but should I raise the pierce height, or should I be using a moving pierce travel?? Is this truly the cause (in your opinion) or is it something else? Cutting too high? Too low? Too slow? I can cut faster (60 ipm still cuts fine) just has more angularity.

I just thought surely someone has had this issue, and I would inquire before I spent an hour or so making little lines on plate that I could be using to turn a profit with, trying to chase down something and perhaps running in the wrong direction. I am open to any and all suggestions, and would be real interested to hear from you if you have been down this road and have the map. I am leaning towards blowback of molten steel during piercing, and changing to a 0.5 inch moving pierce travel, but I have been wrong before. Once..... I lovingly refer to her as the mulligan now . Anyways, lest I digress, thanks in advance for your time on this.
Shane

[jimcolt]

Pierce height and / or pierce delay time are too low / too short. There is nothing else that I am aware of that will do this. Regardless of your settings.....you should physically check pierce height with feeler gauges and if it is too close, then add some height to the book specs to overcome the inaccuracy in your machines surface sensing. Often, plate deflects a bit as well when sensing the surface....then follows the torch back up making the pierce height too close.

One pierce too close will do this to a nozzle. Delay time as well.....if the torch is indexing down from pierce to cut height before the pierce is full through the material...this will happen. If the x or y motion starts before the pierce is fully through, this will happen.

I often will cut 300 to 600 pierces on my cnc with a Powermax85. My cnc uses ohmic sensing and I run the pierce height and pierce delay at exactly book specs. My nozzle and shield stay clean.

Jim Colt Hypertherm

[beefy]

After what Jim has said do a test pierce only, no cutting and see if it happens again. Even try five test pierces in a row, that should be a good indicator.

Interesting how it's only on the nozzle, it couldn't possibly be an air contamination problem ??

Keith.

[rikduk]

i would say too low/ pierce too short
i would do more than just pierces, more like small pieces, you need movement.
take your welding helmet, start the job.
check the arc to see if the torch moves down before the pierce is complete.
i would bump up the pierce height 0.010-0.020,
but i would bet on the pierce delay being too short.

[Shane Warnick]

Ok,

Having read the responses I did some testing today while I am working on a few projects. Jim said to increase pierce height to book specs. I see none in the book. It gives a torch work height of 1/16th of an inch, or 0.0625", which is where my cut height is set at. I was under the impression that pierce height was twice the cut height. I have tried piercing as high as 0.14" and it still happens. I have a samson / plasmacam table, and have the ohmic sensing. I can turn the torch off and do a test run, and stop it while it is at pierce height. It is always spot on. Same thing with the cut height.

So, should I take the cut height higher? How high do you guys normally go for piercing 1/4"??

As far as the pierce delay, I will try making it longer. At 0.4 seconds it is through the plate before it starts moving, maybe it's just not finished cutting the divot per say. I think you guys are right about the problem being with the pierce, just not sure how high to go (as I have heard too high ruins the nozzle) or how long to stretch it out. Book says 1 second on delay, I will play with that when I get a chance. I think the molten puddle of crap blows back up on to the nozzle when it starts to move, as it flows across the plate in the opposite direction of the torch movement when it takes off. Very apparent after the cut is done and it's all cooled off, you can see where the junk blew away from the torch when it started moving laterally.

As far as it only being on the nozzle, there is occasionally some goobers on the shield, but they usually just fall off during motion. I think the nozzle being cooled by the air flow makes it want to stick there for some reason.

Air contamination: I have clean dry air, have a canister filter and a large desiccant filter right before the machine. I checked them both when I noticed the problem, they are good to go.

Thanks for the input guys, I will keep you posted see what I come up with. Cutting some 3/8" plate tomorrow will see how that looks.
Shane

[beefy]

i would do more than just pierces, more like small pieces, you need movement.

Depends on what you are trying to do. Elimination is often a way of finding out what can be causing something. By doing nothing more than a pierce (at pierce height) you are checking if it happens at this point.

Once you add other variables into the equation you have to figure out which one caused the problem.

If it can be confirmed it happens at the pierce, as Jim seemed to be indicating, then look into that area further.

Keith.

[I Lean]

Book-spec pierce height for my Powermax 45 is 250% of cut height, so .15". For 1/4" I run a little higher than that, .17". If I cut 3/8" I bump it to .22". I haven't seen any negatives from piercing that high, in fact I get much longer consumable life doing it this way.

[jimcolt]

Shane,

The Powermax1250 does not list the pierce heights.....I forgot that we made improvements to the cut charts when we introduced the new Duramax torch (used on thePowermax1250 replacement....the Powermax85), however it does say to "double" the cut height for piercing thicker materials than about 1/8". From my experience doubling the cut height to .12" is not enough to get the best consumable life. Here are my recommendations based on my experience with the 1250 torch:

- gauge materials through 1/8" Pierce at the same as cut height
-1/8" through 1/4" Pierce at .15"
- 1/4" through 1/2" Pierce at .20 "
-1/2" through 3/4" Pierce at .24"

Use the pierce delay times listed for each thickness, however if it appears the torch moves in x y or z directions before the molten metal is ejecting out the bottom of the material....increase the pierce delay time accordingly. Different machines start the time cycle at different times in the part program....so results can vary!

If you pierce high to save the consumables...be aware that on some machines the z axis will not have adequate time to index from pierce height to cut height before the x and y motion gets the torch off the lead in and into the actual part profile. A longer lead in length will solve this, or a faster z axis speed setting (not all machines have this capability). If the torch enters the part profile too high, the first part of the cut will be distorted or will have a section with bad angularity. If you are trying to cut round holes....the hole will be out of round due to the torch being too high.

Plasma cutting is a bit of a balancing act with cut speeds, cut heights, cut power and part program geometry. The better cnc machines have "canned" much of this good technology in the software, others require more operator expertise and experimentation for best performance.

Jim Colt

[Shane Warnick]

Just wanted to check in on this and thank everyone for their time and advice in regards to the issues above. Seems the pierce height was too low and the delay was too short. I played with it for a few weeks, and watched a lot of pierces (with my shade 14 goggle ). I made a few observations, the most interesting is the vertical ejection of molten metal from the pierce when the torch starts to move. Now keep in mind that the pierce is complete at this point and there is definitely material ejecting out the bottom of the plate. I have even lengthened pierce delay out super long to see if it changed and it had no effect. It always goes up and away from the direction of initial travel, but does not always exit at the same angle. Sometimes is goes almost straight up, and damned if it doesn't stick to the nozzle or shield when it does. I have also noted that the "puddle" from the pierce is always blown away from the direction of travel, and I am not sure if the molten metal being ejected is from the puddle or from the kerf of the cut when the torch starts to move. It's so random and brief that I am unable to tell with my naked eye and I don't have a benford 2100 series super slow mo hidef camera to record it, so I will just call it a little bit of both and be happy. Thankfully increasing pierce height seems to have decreased it or at least changed the angle that it doesn't hit the torch very frequently and is more of an annoyance now than a battle to keep the nozzle and shield clean. Good thing is the plasmacam has no problem indexing down to cut height rapidly, and I just increased lead in length slightly to make sure it's down to cut height before it is to the important stuff. I also played with the move on pierce, and found that it really decreases the pierce times on the thicker stuff. I was cutting some 1" last week, and it still looks like Mount Vesuvius erupting when I am piercing but it cuts the time to pierce by about 40%. So only needs 3 seconds instead of 5.5 seconds . Anyways, thanks again for all the input everyone stay safe and don't put your fingers where they don't belong.
Shane

[jimcolt]

Glad you figured it out. 99% of the time pierce height and/or pierce delay timing is the cause of nozzle orifice and shield damage. Those of us with Plasmacam machines with advanced height control (using ohmic plate sense) are fortunate to have a good height control that can accommodate the proper settings required to make this work.

The height controls on $100k industrial machines have all of the same features (well....they have for over 20 years) as Plasmacam with the difference normally being that these industrial machines often have fully automated settings...operator does not need to be as knowledgeable.

With the Plasmacam height control all of the functionality is there, however you need to make the correct settings for machine control and height control (as well as a few others) every time you change thickness and power (amperage)....or else either cut quality or consumable life will suffer. Most machines under $20k do not have the flexibility and performance in regards to height control that the Plasmacam has.....properly operated of course!

Don't forget....once you have all of your settings that work best for a particular material.....save it in the configurations file. Next time you cut the same material there will be no need for experimenting....click on the correct config, and cut!

Jim