Powermax drop-cuts: How do I get my parts to just drop away from the plate?

By Jim Colt, Hypertherm
Posted on 18/04/2019 in SPARK the blog, Plasma cutting

A guy contacted me, asking for help with his Powermax85 table set-up and I’m sharing my response because it might be helpful for others. He asked about eliminating the “tabs created on the bottom edge of the cuts” and how to get his “torch to travel beyond the plate’s edge to finish the cut.” He also wanted parts to “just drop-away from the plate” and noticed this issue was worse on stainless and thicker metal.

Assuming you’re cutting at the recommended cut speed and torch height, it’s typical for a plasma arc to have a slight tailback, meaning the arc at the bottom of the plate will trail behind the top half of the arc as it moves forward. When cutting mild steel with air plasma, this tailback is roughly 5 degrees, but it’s more noticeable on stainless steel at 15 and 30 degrees. It’s this tailback or arc lag that created the “tabs” he saw in his shop, keeping his parts attached to the main plate.

These tabs tend to be larger with stainless because as mentioned the arc tail trails further behind than it does with mild steel, but metal thickness will also impact tab size. In general, tabs are larger on thicker plate since higher arc voltage is required to cut it and that voltage can spike when the arc has exceeded the end of the plate. If your Powermax voltage surpasses what’s safe for the system, the power supply will automatically turn off, so the bottom half of the arc can’t catch-up with the top, thus creating a tab.

Okay, that’s why tabs happen. Now let’s talk about what you can do.

Cut to the edge: Program the cut so the torch nozzle stops exactly on the edge of the plate. On systems with low acceleration and de-acceleration numbers, the de-acceleration alone could be adequate to let the arc bottom catch-up on its own.

On machines with faster 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, for the bottom lag to catch-up. Be aware though that this will leave a slightly wider top kerf and a slight divot. The plasma off-delay function is called different things based on your CNC software brand. I recommend checking with the manufacturer to determine if you have it and what it is called.

Utilize scrap: Place scrap metal, the same thickness as the material being cut, at the edge of the plate. Run the plasma arc past the plate’s edge onto the sacrificial metal, about a ¼ inch (6.35 mm) or more before turning the arc off. This makes for a clean cut right to the edge.

Cut speeds: Experiment with slower cut speeds, but also expect low-speed dross, a wider kerf, and more heat input into the metal.

Water height: If you have a water table, try lowering the water in the table.

More power: Try using more power, if you need it. Most industrial CNC plasma users use 200 to 400 amps for ¾ inch (20 mm) cutting, but a Powermax125 will easily drop cut ¾ inch (20 mm) stainless.

If you are having issues on metals thinner than ¾ inch (20 mm), you could have one of the following issues:

Wrong cut height: Make sure your arc voltage setting on the height control will allow the torch to run at the physical cut height listed in the cut charts. This is typically 0.06 inch (1.5 mm).

Poor air pressure or flow: Confirm inlet air pressure is staying 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. If you are controlling the air flow manually or through the serial port, try using the Auto Air Flow feature instead.

Worn consumables: Visually inspect your consumable set, including the o-rings. Obviously, if there is wear or damage, replace immediately.

Aftermarket consumables: Make sure you are using genuine Hypertherm consumables. Although aftermarket parts can be cheaper, tolerances are all over the place which can lead to cut issues and torch damage. Also beware of counterfeit consumables. You can visit our counterfeit consumables page for more information on how to make sure your consumables are genuine.

If you still have questions, contact Hypertherm’s Technical Service Team.

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