One of the most common and frustrating problems in plasma-arc cutting (PAC) is short service life of parts. This problem hurts the fabricator both in increased consumable expense and in machine downtime for changing parts and troubleshooting. Most companies keep some record of parts life based on pierce counts, arc-on time, or the number of plates processed. The operator is usually the first to know when parts aren't lasting as expected. Here are some technical tips to help the operator or maintenance troubleshoot a service life problem.
WARNING! Read all safety information in your operations manual before operating or repairing PAC equipment. PAC systems use high voltage and direct current (DC) electricity. Electric shock can injure or kill.
The electrode and nozzle fail prematurely, causing a deterioration in cut quality, failure to pierce, or sudden loss of arc in the middle of a cut.
The electrode carries the negative DC charge from the power supply. It is comprised of a copper holder that contains an emissive element of hafnium or tungsten— metals with high melting points that will sustain an arc. The emitting element is slowly eroded away by the heat of the arc, and the high velocity plasma gas stream. During normal wear a small concave pit is formed in the end of the part which steadily wears away, a few thousandths of an inch at a time, to a depth of 0 .040 to 0.125 in. When the pit becomes too deep, the arc attaches to the copper holder and melts it. The electrode "fails" when it will no longer initiate and sustain an arc. It is a good practice to remove the electrode before it fails.
The nozzle focuses the plasma jet. The hole in the nozzle should be perfectly round and concentric. Both the diameter and length of the hole are critical: any damage to the orifice will affect the shape of the arc and, therefore, the quality of the cut piece. The plasma arc passes through the nozzle without contacting the copper material because the walls of the nozzle are protected by a boundary layer of swirling gas. If the arc does contact the nozzle it will melt some material away. Normal wear for a nozzle is slight chamfering or enlarging of the hole on the leading edge of the orifice. Some damage occurs to the face of the part during each pilot, causing heat discoloration around the orifice. Deposits of hafnium oxide can build up on the interior surface causing disruption of gas flow. The nozzle "fails" when it will no longer produce a straight arc and a good clean cut.
Normal parts life for state-of-the-art air and oxygen plasma systems is 1-2 hours of arc-on time and several hundred pierces. Some systems can reach 1,000 or more starts before a parts change is necessary.
The first step in solving any parts life problem is to examine the parts thoroughly and determine which part failed. The parts usually provide visible clues to the root cause of failure.
There are 3 possible cases:
Case 1: Electrode bad and nozzle bad
If inspection of the parts' service reveals that both electrode and nozzle are severely worn, it is likely that the electrode caused failure of the nozzle. Since the electrode is upstream, it will cause damage to the nozzle when molten material is blown out of the end of the part and deposited into the nozzle interior. If run long enough, all parts will fail in this way.
If the electrode has a deep wide pit and the copper has turned straw colored, blue, or black from overheating, the likely cause is low coolant flow. In extreme cases the end of the electrode may be melted away.Verify the flow rate of the cooling medium. In water-cooled torches check the cooling water flow rate with a bucket test at the return to the coolant tank. If it is not to spec, check for pump problems, kinks, leaks, plugged filters, or other restrictions. In gas cooled torches check for low gas flow.
Small pock marks all over the end of the electrode with corresponding damage to the interior of the nozzle indicates low gas flow. Low gas flow allows uncontrolled arcing between the nozzle and electrode. Check the gas flow rates to the torch. The best way to do this is with a flowmeter (0- 400 cfh) and hose placed on the outlet of the torch with the system in test. If not available, a quick check is to feel the gas flow at the outlet of the torch with only plasma gas turned on. You should feel a swirling flow of gas that actually has a suction force.
If the electrode has a heavy layer of black residue, check for gas contamination. One quick check is the paper towel test. Hold a clean paper towel under the torch with gas flowing through the system. There should be no sign of moisture or contamination.
Case 2: Electrode good and nozzle bad
If the electrode appears virtually new and the nozzle is severely damaged, the most likely cause of failure is double arcing of the nozzle. This occurs if the arc contacts the nozzle and erodes copper material from the orifice.
Damage to the interior of the nozzle such as a slot or "keyhole" indicates low pressure in the plasma chamber. This allows the arc to attach to the nozzle. Check for leaks in the gas lines by pressurizing the lines and using soapy water on all fittings.
Damage to the exterior of the nozzle often indicates a problem with torch-to- work distance. First check the pierce height; it should be two times the cut height to avoid metal spatter. Piercing too low is the number one cause of premature nozzle failure. Check for proper operation of the torch height control. If the torch pierces when it is pushed against the plate, or scrapes the plate during a cut, the nozzle will be instantly destroyed.
If the nozzle looks extremely hot, straw colored, blue-or black in color, check the shield gas flow. The shield gas helps to cool the nozzle and protect the front end of the torch.
Case 3: Electrode bad and nozzle good
If the nozzle is in good condition but the electrode has a deep concentric pit, the plasma gas flow rate may be too high. When the plasma gas swirl is too intense, the element erodes quickly. This causes a rapid deep wear pattern. Check the volumetric flow rate of the plasma gas.
A fourth case also exists: if both parts look virtually new but the torch failed to "fire," and a new set allows the torch to start, the problem is not parts life; it's hard starting. Often, perfectly good electrodes and nozzles are discarded because they failed to fire. Hard starting is most often caused by excessive plasma pressure during preflow when the torch is igniting. Usually the torch "spits and sputters" and struggles to start.
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