Odd issue not cutting through at edge of material

[motoguy]

Ran into an issue cutting some 3/4" stainless a couple of weeks ago. I was cutting a line, "ripping" some SS railing down to a smaller size. I made some practice cuts on a scrap piece, and kept running into an odd issue. For some reason, the plasma would leave a small uncut section at the end of the material. Here are some photos of the stainless bar. Ignore the angularity issue, which I fixed, as well as the couple of lines where I was had THC voltage incorrect.

You can see how the beam cuts cleanly through the top, and even the bottom, but will -not- cut through that small (.2 or so) section near the end. It stops in nearly the same place every time. This was about a 3" wide part. I changed my overcut length. Changed my cutting height. Changed the cut speed (down to 50% book rate). Made the cut file up to 6" long (so it wouldn't be near the end of the drawn line while cutting, hence no EOC rules or changes). NOTHING I could do would get it to finish this cut at the bottom!

There didn't appear to be a big arc lag, and certainly not once speed was slowed down to 50% book. The arc appeared vertical at that point, but would still "jump" over that section at the end. It's like it would get to that point, and then 'skip' over to the edge of the material.

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I then went to 1/2" mild steel, where I KNOW I did not have this problem in the past. It's cut 1/2" like butter. Sure enough, same issue! The SS was a rush job, so in the end I just ripped the bars down, pried it apart at the uncut "nub", and ground the nub down. I meant to look into the issue, but forgot to do so.

Which brings me to yesterday. Cutting a 12"x144" sign out of 1/2" CorTen steel. To my surprise, the letters would not drop out. Once I wiggled them out, I found the same issue was back again. There was a small uncut "nub" where the beam crossed the kerf at the lead-in/lead-out point. I had to wiggle each of these letters out of their holes, and then clean up the cut area to remove the "nub". I changed overcut length. Changed lead out length. No change. Same thing each time. Here are some photos showing what I'm talking about, at the lead-in/lead-out area:

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You can see each of those letters has a small uncut "nub" where the beam crossed the kerf line, at the lead-in/lead-out point. Again, no changing of overcut, lead-in/lead-out, etc changed this. It still appeared each time. Cut quality was good otherwise.

Same thing occurred on some 3/4" mild steel that I ran a test cut on.

Does anyone know why this is happening only at the end of material (not necessarily end of cut), on such a variety of materials? It's not a speed issue...I slowed speeds until the arc was completely vertical / slow moving, with no lag. Still happened. Changed overcut lengths. Still happens. Changed lead-in/lead-out, no change. On the straight line cuts (on the bars), I changed the drawing, so that the computer thought it was cutting (up to a) 6" line, across this 3" material. I thought maybe if it was an EOC issue, this would keep it from presenting itself. I figured the arc would shut off once the voltage error was detected (when it passed outside the material). No change.

I ran the files with all software-driven features removed or disabled (THC, rules, overcuts, lead-in/lead-out, etc), thinking maybe it was something to do with CommandCNC or Linux Sheetcam (I hadn't cut thick materials since the Linux upgrade), but no changes there.

Ideas?

[JakeEmery]

This is actually pretty common on SS and thicker material. What you are seeing is the effects of the lag of the arc on the cut. If you watch the arc with a welding helmet you will see the arc gets pulled behind the torch. This is even more true with the SS materials. This will show up in two places when you are CNC cutting - jumping the kerf from a previous cut path, and cutting interior profiles like holes. If your CNC has the ability there is a trick to clean this up. For the last 1/2" of the part slow the cut speed down and this will allow the arc to get back under the torch and will leave a cleaner exit on the cut. This will help both on cleaning up the tab of a interior part like a hole, or if you are getting kerf jumping where there is still a tiny corner still attached to the skeleton (the old have to wiggle it to release it issue).

I see you emailed into our service group - and figured it was easier to answer here as it is a great question and a lot of people could learn from this.

Regards,

Jake Emery - Hypertherm Tech Service

[jimcolt]

When cutting steel with an air plasma, assuming you are operating at the optimum (not fastest) recommended cut speed, at the recommended physical cut height (always adjust recommended voltage so the physical cut height is correct) expect that the plasma arc will have a slight tailback, meaning the arc at the bottom trails behind the top roughly 5 degrees.

On stainless steel, under the above conditions expect that bottom of the arc trails back at least 15 to 30 degrees.

Both of these issues are affected by thickness, with roughly the same tailback angle, however the dimension between the top and bottom of the arc gets bigger. When you program a part that is designed to allow the torch to run off the edge and extinguish, expect a tab to hold the pieces together at the bottom, with stainless and its larger tailback angle providing a larger tab at the bottom. Again, the thicker you cut, the worse this issue gets.

Plasma cutters do this on thicker materials as the arc voltage is higher, and when you stretch the arc off the end of the material (long arc) the voltage gets even higher. When the Powermax units reach a certain voltage (I believe the Powermax85 has a threshold voltage of about 180 volts) they will shut off the DC power, and the arc will extinguish....this (on thick material) will happen before the bottom of the arc catches up to the top, based on the thickness and lag angle. If a Powermax is running on rather low line voltage and cutting thick material it is possible for this threshold voltage to be even lower, making the bottom tab even bigger.

Best ways to allow for a drop cut when you are cutting to an edge:

1. Program the cut exactly to the edge so that the torch nozzle stops exactly centered on the edge. On some machines with low acceleration and de-acceleration numbers the de-acceleration is adequate to allow the bottom of the arc to "catch up" to the dop, dropping the cut piece. On machines with better acceleration you often need to program a "plasma off delay" that allows the machine to stop motion (with the nozzle centered on the edge) but keeps the plasma arc on with a timer, therefore allowing the bottom lag to catch up. This will leave a bit of a wider top kerf and a bit of a divot on the edge. "Plasma Off Delay" has a number of different names based on the CNC / Software on your machine, check with the supplier to determine what this function is called, or if you even have it!
2. I have worked with large cutting shops a number of times in the past that had to cut strips of "bar stock" out of plate, and this same issue always occurs. In many cases a "cheater bar" of the same thickness material is placed at the end of the plate, the strips are cut and the program allows the plasma to run 1/4" or more into the sacrificial cheater bar to extinguish. This makes for a clean cut right to the end of the plate and always a drop cut, as the tab created by the lag angle is in the cheater bar.
3. You can reduce the machine acceleration rate and use method 1. above.
4. You can experiment with slowing the cut speed down enough to minimize the arc lag, though expect low speed dross and a wider kerf and more heat input (camber and warpage often are the result).
5. If cutting on a water table, lower the water and you will get less of a tab.
6. Last but not least, brute force can solve the issue. A 125 Amp or larger plasma will easily drop cut 3/4" stainless. Most industrial cnc plasma users would use at between 200 and 400 amps for 3/4" cutting, with no drop cut issues. With and 85 Amp plasma you are at the maximum of its rated production thickness on 3/4".

If you are having issues on materials thinner than 3/4", and especially on steel (say 1/2" and 5/8"), then there could be another issue such as:

1. Cut height too high (remember, always ensure that the arc voltage setting you are using on the height control allows the torch to run at the physical cut height listed in the cut charts, which is usually .06"). Cut height too high will leave the tab that will not allow a drop cut.

2. Air pressure or flow issues. Be sure the inlet air pressure remains above 90 psi....the only way to be sure is to have a gauge installed on the air inlet fitting and read it while air flows at the torch, especially when the compressor is at the lowest pressure on its cycle.

3. Be sure the Powermax is in Auto Air Flow. (if you are controlling the flow manually or through the serial port....ensure the pressure is correct. disable serial and try it in Auto to be sure).

4. Be sure the O Ring is in good shape on the torch body. Be sure 100% of the consumable parts are genuine Hypertherm. (aftermarket parts are cheaper, but tolerances are all over the place...they will cause cut issues and potential torch damage.)

Hope this helps! Best regards, Jim Colt

[motoguy]

Thanks Jim. That all makes sense on the stainless, but it doesn't explain why I am now having issues on half inch mild steel. I have cut half inch mild steel in the past, with no issues whatsoever. Parts typically fall out of my half-inch just fine.

Also, on the half inch parts, notice the issue is not at the end of the cut. The issue is where the arc beam crosses the kerf line at the lead in / lead out junction.

Consumables are all Hypertherm and new. I replaced the O-ring on this as well. Cut height was correct at .06. Air pressure was fine, at 125 psi. Flow was fine as well, as there is about a 4 psi drop when the plasma kicks in. My compressor puts out 150, which is regulated down to 125 on the plasma leg. My compressor is set to kick on at 115, so I should never have below 100 psi at the plasma. Gauge is at rear of plasma unit.

It's the "may be something else if this is happening on 1/2" mild steel" that concerns me. Same cutting parameters as before, but now having this issue on a material that has cut fine in the past (1/2" mild steel).

[jimcolt]

So if everything is set right as you say (height, speed, proper consumables, non-worn consumables) and not only are they set right but you have verified they are right (timed a distance and calibrated speed, measured the actual cut height and tweaked it to .06" by adjusting arc voltage) and that air pressure is in Auto control....and it is doing the exact same thing on the end of steel cuts as on stainless (steel still has a lag but it should be less than stainless) then you should probably contact tech service and have them help you troubleshoot the issue. Something in the air system could be leaking or plugged, the air regulator could be out of spec, I've never heard it before....but maybe the amperage is not correct. Could be a lot of things that could affect this drop cut issue.

I know with my Powermax 85 I always have to set an end of cut delay fairly long for 3/4 parts to drop, on mild steel of 5/8" and 3/4" I need to set this delay as well, 1/2" mild and 3/8" stainless I generally do not need the delay to drop the cut.

Jim Colt Hypertherm

Thanks Jim. That all makes sense on the stainless, but it doesn't explain why I am now having issues on half inch mild steel. I have cut half inch mild steel in the past, with no issues whatsoever. Parts typically fall out of my half-inch just fine.

Also, on the half inch parts, notice the issue is not at the end of the cut. The issue is where the arc beam crosses the kerf line at the lead in / lead out junction.

Consumables are all Hypertherm and new. I replaced the O-ring on this as well. Cut height was correct at .06. Air pressure was fine, at 125 psi. Flow was fine as well, as there is about a 4 psi drop when the plasma kicks in. My compressor puts out 150, which is regulated down to 125 on the plasma leg. My compressor is set to kick on at 115, so I should never have below 100 psi at the plasma. Gauge is at rear of plasma unit.

It's the "may be something else if this is happening on 1/2" mild steel" that concerns me. Same cutting parameters as before, but now having this issue on a material that has cut fine in the past (1/2" mild steel).