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Plasma Cutting Consumables: How to get the best out of them

Some parts of your plasma cutting machine wear out with usage and need to be replaced; for this reason they are called consumables. The more you know about cutting, the longer the life of your machine consumables. Any tool calls for an experienced user to deliver satisfactory quantity and quality of work. Plasma cutting machines make no exception; not only do they deliver low quality results, they also tend to suffer more damage when they are in the hands of badly trained personnel. Shield cups, (nozzles and electrodes) also suffer from more wear and tear and this can turn out very costly in the long run. Consumables longevity depends on many factors: Being genuine or not, cutting power amperage, Feed Rate, the quality of the cutting gases used, inert gas humidity, Gas pressure, the operator’s experience, everything seems to contribute to lengthening or shortening the life of Consumables. To the experienced operator, there are many tell-tale signs for parts wear: the colour of the arc, changes to torch height and, most importantly, deterioration of the cut surface quality are his main points of attention. Average parts life as far as number of starts is concerned should be recorded; these records are very important, and the more detailed they are, the best maintenance chances they offer: What is the average life of your machine’s nozzle based on the cutting amperage, the kind of the material or its thickness? If you know, you can adjust working conditions to achieve a long and productive life for your consumables.

Use of Genuine Consumables

The optimised design of the genuine consumables is the precondition for reaching highest cutting quality and for their longevity. Topmost accuracy during fabrication and the use of properly selected materials or combinations of materials are required. The user can profit only from those benefits if besides an ideal process sequence always genuine consumables are used. Fake consumables prices, compared to the genuine parts’ ones, are astonishingly low, putting the prospective buyer in an awkward position. The truth is that they never come cheaper in the long run! According to careful and precise measurements, fake consumables life is more than ten times shorter compared to the life of the genuine ones; if you pay more than one tenth of the price, you are losing money. On the other hand, cut quality is rarely as good as the one we achieve using genuine parts, which may mean that we are losing money (and customers) all the way! If we also consider that fake consumables may induce damage to the torches and the machine in general, we see that, using them, we risk proving ourselves "penny wise, pound foolish"!

The Cutting Nozzle

The opening (orifice) in the nozzle is designed to constrict the plasma arc to a certain diametre, without the latter coming in contact with the nozzle. The shielding gases provide a thin low-temperature protective layer between the arc and the nozzle. If the electric current is too high or too low, the arc can come in contact with the nozzle creating a double arc, resulting in damaging the nozzle by enlarging the nozzle orifice. As a result we will not achieve the proper arc diametre and we will have a poorly focused flame jet and a low quality cut surface. If we cheque the nozzle and see it is damaged (gouges on the inside or outside or even a burned orifice, or one that has lost its roundness), it should be replaced; reusing it we risk getting a low quality product. Gouging itself is a rather tell-tale damage, as far as the source of the damage is concerned: Gouges on the inside are a sign for too high current applied or too low gas flow velocity; gouging on the outside is usually due to contact between the nozzle and the work-sheet, to a far too high cutting speed or to persistent piercing.

The Electrodes

On every “Start Up” the electrode is given a start input to initialise an arc. This current brings the electrode to very high temperatures; the ionised gas jet surrounding it is full with ions. These charged particles detach atoms away from the electrode tip and form a pit in it. Repeated Start-Ups and Shut-Downs wear out the electrode more than long hours of continuous work. It is very important to cheque the condition of the electrode and, if necessary, replace it. We can judge its condition chequing the pit in the electrode element’s tip (this is the silver-coloured refractory metal "nucleus" in the middle of the copper electrode - Hafnium for air and Oxygen, Tungsten for Nitrogen or Argon-Hydrogen). Usually the pit should not get deeper than 2.5mm for air and oxygen, or 3.2mm for nitrogen or Argon/hydrogen mixture. Replacing the electrodes in time is critical if we want to avoid major torch damage by the electrode bursting inside it.

Other Consumables

Electrode holders and Contact tubes should be chequed visually and replaced whenever mechanical damage is noticed. Swirls are of utmost importance to remain clean and their gouges free from anything that could stop the unhindered flow of gas. Check for dirt, metal fillings and grease in the holes. Look also for cracks and arc burns. Check the O-Ring for cuts, softness and outer diametre. Check the shield cups for cracks, arc burns and slag build up.

Best Cutting Methods

  • Gas flow should be in the limits prescribed by the manufacturer. Electrode life is reduced when air pressure is too high. On the contrary, in case air pressure is too low, nozzle life will be shortened.
  • For any plasma source/torch type combination there is an optimal distance between the job surface and the torch nozzle; try to follow the instructions closely.
  • When piercing, this distance should be increased. The golden rule should be followed: Pierce high, cut low. If we do not, cutting/piercing quality and consumables life will suffer. The height of the torch during piercing is particularly important. If we pierce too low molten metal splashes upwards and spatters the front of the nozzle and shield damaging the parts; damaged parts result in low cut quality.
  • Arc "snuffing" occurs when the torch pierces touching the metal, or when it keeps in contact with the sheet surface while cutting. If the arc is "snuffed", the electrode, nozzle, gas swirler and, sometimes, the torch, are destroyed. Piercing at a height of 1.5-2X the recommended cut height protects the torch and parts from damage.
  • Use the right nozzles as specified by the manufacturer for different amperage ratings. The best cut quality and parts life is usually achieved when the amperage is set just under the maximum of the nozzle's rating. If the amperage is too low, the cut won’t be square; if it is too high, nozzle life will be shortened.
  • Change both electrodes and nozzles at the same time.
  • Keep plasma gas clean and dry. Use correct air pressure and flow. The plasma gas pressure or flow setting should not exceed the factory recommended settings. Excess pressure in the plasma chamber makes it harder for the High Frequency spark to jump the gap. The pilot arc is effectively blown out before it is fully established. High gas pressure accounts for the majority of hard starting problems.
  • Cutting too fast or too slow will cause cut quality problems and impaired consumables longevity. If the speed is too slow the cut pieces will develop "low speed dross" a large bubbly accumulation of dross along the bottom edge. Slow speeds may also cause a widening of the kerf and excessive amounts of top spatter. If the speed is too fast the arc will lag backward in the kerf causing a bevelled edge, a narrow kerf and a small hard bead of dross along the bottom edge of the cut piece. High speed dross is difficult to remove. The correct cutting speed will produce minimal dross--the result will be a clean edge that needs little rework before the next step in the manufacturing process.
  • Make edge starts wherever possible. Start the arc with the nozzle orifice directly over the work piece. Avoid arc stretching. This happens when the arc has to stretch to reach the job at the beginning or end of the cut. Arc stretching can cause the arc to cut into the side wall of the nozzle.
  • Avoid unnecessary pilot arcs.
  • Remove slag from the shield frequently.
  • Purge the torch after consumables change to blow out moisture if any.
  • Keep the torch and consumables clean. Torches, if properly cared for, can last for months or even years. Torch threads must be kept clean and seating areas should be chequed for contamination or mechanical damage. Any dirt, metal dust or excess O-ring lubricant should be cleaned out of the torch. To clean the torch, use a cotton swab and electrical contact cleaner or hydrogen peroxide.
  • When cutting periodically clean oxides from the electrode and nozzle. This build up disrupts gas flow and shortens life of consumables besides causing difficulties in arc starting.
  • Check air filtres and hose cables regularly for leaks and rectify. Drain the air filtres and clean the elements thoroughly every week. The flow and pressure of gas and coolant for water cooled torches should be chequed every day. If the flow is insufficient, consumables will not be cooled properly and parts life will be reduced. Inadequate flow of cooling water due to worn pumps clogged filtres, low coolant level etc. is a common cause of parts and torch failure.

(Based on an article in and an article published in "Welding Design & Fabrication" - September 1998)





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