PLASMA CUTTING Replacement Parts

Extending the Life of Plasma Consumables This article is reproduced with permission of Canadian Machinery and Metalworking.

Electrode with pitted Damaged cutting tip Damaged Safety Damaged Shield cup surface sleeve with cracks, arc burns, and slag Shield cups, electrodes and cutting tips all wear out—that is why they're called consumables. If you know how and why they wear out, though, you can prevent premature wear.

Consumable life on air plasma cutting torches depends on many factors, including cutting amperage, cutting speed, air quality, humidity and operator skill. For maximum performance, the tip and electrode should be changed at the same time. To extend the life of these consumables, it helps to know how they work. Here is a closer look.

CUTTING TIPS The orifice in a torch-cutting tip is designed to constrict the plasma arc to a specific diameter, without the arc touching the nozzle. The plasma gas provides a thin boundary layer of cooler gas between the arc and the nozzle. If the current level is turned up too high or the gas flow rate is too slow, the arc may contact the cutting tip. If this happens, then current flows to the work piece through the cutting tip itself—not just through the plasma arc. This is called double arcing and it damages the cutting tip. You will know when this occurs because it leaves gouges in the cutting tip. Also, the orifice will no longer correctly constrict the arc, which means a poorly focused, weakened arc and a sloppy cut.

ELECTRODES Every time an arc is struck, the electrode receives a current surge that brings it from Electrode Life Gas No. of Max. Depth a cold state to an extremely hot state very quickly. The ionized plasma gas flowing past the Starts of electrode is full of charged particles. These particles pull atoms away from the hot surface of Electrode the electrode, which wears away the emitting element and creates a pit in the surface. Air 150-300 3/32 in. O2 150-300 3/32 in. The number of starts and stops is more important than actual cutting time in N2 600-1800 1/8 in. determining the lifetime of an electrode. For air and oxygen, you can expect a service life of Ar-H2 600-1800 1/8 in. 150 to 300 starts; for nitrogen and argon/hydrogen, 600 to 1,800 starts.

In N2 and Ar-H2 cutting, the electrode will last longer than the nozzle, while in air and O2 cutting, the nozzle may outlast the electrode. So, it is important to keep an eye on your consumables and to know how to evaluate whether they really need changing.

When the nozzle wears out, cut quality deteriorates. Gouges on the inside indicate overpowering (the current is set too high for the nozzle size) or inadequate gas flow. If a nozzle is gouged on the outside, it's from double arcing, piercing, or touching the work piece (drag cutting is okay, but only with very thin metal and low current). Either way, it needs to be replaced.

To know when an electrode is spent, take a good look at the pitting in the electrode element. If you're using oxygen or air plasma, the pitting should be no deeper than 3/32 in. If you're using argon or nitrogen, it shouldn't be any deeper than 1/8 in. If the pits are deeper than these guidelines, it's time to change the electrode.

When you check the safety sleeve, look for cracks and arc burns. Also, check for dirt, metal filings or grease in the holes. If you find any of this, change the sleeve. Safety sleeves last quite a while---you may change the nozzle 20 times before you need to change the sleeve.

On the shield cups, check for cracks, arc burns, and slag buildup that will not come off. If you find any of this, change the shield.

Information for this article was contributed by DovaTech, Ltd. Beecher, Illinois.

Stoody Industrial & Welding Supply, Inc. (SIWS) is not associated with STOODY Company or any of the manufacturers identified herein.