Who's keeping tabs? Here’s how to get a drop cut with Powermax plasma.
Why do tabs happen?
I was contacted by a person using a Powermax85® on a CNC and he asked for help. When he cut plate metal into thin strips, he drove the plasma torch beyond the plate’s edge to finish the cut. Makes sense, however, the strips didn’t just drop away from the plate. There was a tab of metal left on the bottom edge of the plate that kept it attached. The problem was even worse on thicker metal and stainless steel. He wanted to know if there was a way to eliminate these tabs, which were slowing him down.
I’ve seen this happen before and it relates to the shape of the arc as it moves forward through the cut. His observations were correct, as these tabs are smaller with the mild steel and larger with stainless. It’s because when cutting mild steel with an air plasma arc, the tip of the arc will have a slight tailback, meaning the arc at the bottom of the plate will trail behind the arc at the top of the plate by roughly 5 degrees. This tailback is more pronounced on stainless steel, commonly between 15 and 30 degrees, assuming you are operating at the recommended (not necessarily the fastest) cut speed and height. It’s the longer arc lag that creates the larger tab.
It’s also true that the thickness of the metal impacts tab size. The tab is larger on thicker material because the arc voltage is higher, and when you stretch the arc off the end of the material, the voltage gets even higher. When Powermax® units reach a certain voltage (...I believe 180 volts for the Powermax85), the system will shut off the DC power, and the arc will extinguish. On thick material this will happen before the bottom of the arc catches up to the top. Then, 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. That’s why the tabs happen. Now let’s talk about what you can do.
Five things to try
- Program the cut exactly to the edge of the plate so that the torch nozzle stops 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.” On machines with better acceleration you often need to program a “plasma off delay” so the machine stops with the nozzle centered on the edge but keeps the plasma arc on, allowing the bottom lag to catch up. Be aware though that this will leave a slightly wider top kerf and a bit of a divot. The “plasma off delay” function has several different names based on your CNC software. I recommend checking with the manufacturer to determine what this function is called and if you even have it.
- Place a scrap piece of metal the same thickness as the material being cut at the end of the plate so the plasma will run 1/4″ or more into the sacrificial metal before extinguishing. This makes for a clean-cut right to the end and always a drop cut.
- Experiment with slower cut speeds. If you do this, expect low speed dross, a wider kerf, and more heat input.
- If cutting on a water table, lower the water and you will get less of a tab.
- Try using more power! A 125 amp or larger plasma will easily drop cut 3/4″ stainless. Most industrial CNC plasma operators use 200 to 400 amps for 3/4″ cutting with no drop cut issues. With an 85 amp plasma, you are at the maximum of its rated production thickness on 3/4″.
Still have tabs?
If you are having issues on materials thinner than 3/4″, and especially on steel (say between 1/2″ and 5/8″), then you could have one of the following issues:
Wrong cut height
Remember, you always want to ensure the arc voltage setting you are using on the height control will allow the torch to run at the physical cut height listed in the cut charts. Usually this is 0.06″.
Poor air pressure or flow
Be sure the inlet air pressure remains above 90 PSI. The only way to be sure this is happening is to install a gauge on the air inlet fitting and read it while air is flowing to the torch. This is especially critical when the compressor is at the lowest pressure on its cycle. See this month’s Technical Tip on page 10 for more on this. If you are controlling the air flow manually or through the serial port, try using the Auto Air Flow feature instead.
Worn or non-genuine consumables
Inspect your set including your O-ring and make sure you are using genuine consumables. Although aftermarket parts can be cheaper, tolerances are all over the place which can lead to cut issues and torch damage.
...I hope this helps others. If you're interested in learning more, check out this blog post on cut quality.