|GETTING THE BEST OUT OF YOUR PLASMA CUTTING CONSUMABLES.
Shield cups , electrodes and nozzles all wear out that is why they are called consumables. If you know how and why they wear out though you can prevent premature wear.
There are several signs of worn consumables; the experienced operator can often tell by sound or color of the arc or subtle changes in torch height that indicate parts wear. However, the best way to judge the condition of the torch parts is to periodically check the cut edge quality of the metal, and check the torch parts when the cut begins to deteriorate. Keep a record of the average parts life over time (in number of starts or arc-on time) and develop guidelines for expected parts life based on the amperage, material and thickness.
Once average parts life is established the operator will know when to check and or replace the parts--preventing a catastrophic failure. Consumable life in air plasma cutting depends on various factors including cutting amperage, cutting speed, air quality, humidity, air pressure and operator skill.
The orifice in a torch cutting nozzle is designed to constrict the arc to a specific diameter without the arc touching the nozzle. The plasma gas provides a thin boundary layer of cool gas between the arc and the nozzle. If the current level is to high or gas flow is to less the arc may contact the cutting nozzle causing a double arc which will damage the nozzle by increasing its diameter disproportionately. Thus proper constriction will not be achieved resulting in a poorly focused arc and a bad cut finish. If a nozzle is gouged on the inside or outside or if the hole is worn out of round it should be replaced--otherwise it may be reusable. When the nozzles wear out cut quality deteriorates . Gouges on the inside indicate that too high current or inadequate air flow was used. Touching the job , piercing or cutting too fast can damage the nozzle from the outside .
Every time an arc is struck the electrode receives a high current surge that brings it from a cold state to an extremely hot state very quickly. The ionized plasma gas flowing past the electrode is full of charged particles . These particles pull away atoms from the hot surface of the electrode which wears away the emitting element and creates a pit in the surface.
The number of starts and stops is more important than actual cutting time in establishing the life of the electrode. It is important to keep an eye on your consumables to know how to evaluate when they need changing. To tell if an electrode is spent, check the pit in the electrode element (the element is the silver-colored insert held in the copper: hafnium for air and O2, tungsten for N2 or Ar-H2). Generally, the pit should not exceed 2.5mm for air and O2 and 3.2mm for N2 or Ar-H2. Electrodes should be changed in time to prevent possibly serious damage to the torch if the electrode bursts inside.
Electrode holders ,Contact tubes , Swirls should be checked visually and replaced whenever mechanical damage is noticed. 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 diameter. Check the shield cups for cracks, arc burns and slag build up.
BEST CUTTING METHODS
Ensure proper gas flow. If air pressure is to high electrode life will be reduced. If the air pressure is too low the nozzle life will be reduced.
Ensure correct distance between nozzle and the job. When piercing increase distance to max possible allowed by the power source.
Pierce within limits. You can usually pierce upto 35% the clean rated thickness of your system . Standoff, the distance between the workpiece and the tip of the torch, is critical to both cut quality and parts life.
Even slight variations in torch height can affect the angularity of the cut surface. The height of the torch during piercing is particularly important. One common error is to pierce too low. This causes molten metal to spatter the front of the nozzle and shield causing damage to the parts and subsequent cut quality problems.
Arc "snuffing" can occur if the torch pierces when touching the metal, or drags along the 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 correct nozzle diameters as specified by the manufacturer for different current ratings. The best cut quality and parts life is usually achieved when the amperage is set to 95% of the nozzle's rating. If the amperage is too low, it the cut will be sloppy; if it is too high, the nozzle life will be poor.
Change both electrodes and nozzles at the same time. Nozzles can be reused by drilling a larger hole for higher currents or for gouging as specified by the manufacturer.
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 HF 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 slow will cause cut quality problems. 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 beveled 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.
Use 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 can last for months or even years with proper care. Torch threads must be kept clean and seating areas should be checked 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 filters and hose cables regularly for leaks and rectify. Drain the air filters and clean the elements thoroughly every week . The flow and pressure of gas and coolant for water cooled torches should be checked 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 filters, low coolant level etc is a common cause of parts and torch failure.