Training Articles Journal - Centricut

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CLASSROOMDon’t Cut Your ProfitsWith PlasmaA productive and efficient mechanizedplasma-arc-cutting operation canmake the difference between profitsand losses on a metal-fabricating job. Ahighly skilled operator produces morecut pieces, of higher quality, with lessscrap material and less rework downthe line. He saves money on torchesand consumables by using them correctly,and minimizes machine downtimethrough simple preventive measures.Here we discuss 10 commonmistakes that companies make in theirplasma-cutting operations, and how toavoid each of them.1. Using consumable parts untilthey blow. Look in your used-partsbucket and you will probably see partsthat have been run to failure. Usingseverely worn consumables can notonly ruin a good piece of metal, it cancause expensive torch failures andunnecessary downtime. The experiencedoperator can often tell by soundor arc color, or by subtle changes intorch height, when parts have worn.The best way to judge the condition oftorch parts is to periodically check thecut-edge quality and check torch partswhen the cut begins to deteriorate.Keep a record of the average parts lifeover time (in number of starts or arcontime) and develop guidelines forexpected parts life based on the amperage,material, and thickness.Once average parts life is establishedthe operator will know when to checkand replace the parts, preventing afailure.2. Changing consumable partstoo frequently. Look in your usedpartsbucket and you may also findparts that are still usable. It is a commonand expensive practice tochange-out consumables too frequently.When an operator changesthe parts, he needs to know what tolook for. If a nozzle is gouged or if thehole is worn out-of-round it should bereplaced. To tell if an electrode isspent, check the pit in the electrode element—theelement is the silver-coloredinsert held in the copper:hafnium for air and O2, tungsten for N2or Ar-H2. Generally, the pit should notexceed 3 ⁄32 inch for air and O2 and 1 ⁄8inch for N2 or Ar-H2. Gas swirlersshould only be changed if a close examinationreveals dirt or grease in theholes, cracks, arc burns, or excessivewear. The same holds true for shields,which should only be replaced if theyshow signs of physical damage. Oftenshields can be cleaned of metal spatterand reused.3. Using the wrong parts and parametersfor the job. Consumableselection depends on the material andthickness being cut, the amperage andplasma gas used, and other parameters.The operator’s manual will definewhich consumables are appropriate forvarious types of cutting. Using incorrectconsumables can lead to shortenedparts life and reduced cut quality.It is particularly important to run partsat the correct amperage. The best cutquality and parts life is usually achievedwhen amperage is set to 95 percent ofthe nozzle rating. Insufficient amperagecauses sloppy cuts; excess amperageshortens nozzle life.4. Assembling the torch incorrectly.The torch should be assembledso that parts align correctly andfit together snugly. This ensures goodelectrical contact and the correct flowof gas and coolant. When changingparts, keep consumables on a cleanshop rag to prevent dirt or metal dustfrom contaminating the torch. Whenapplying o-ring lube, use just enoughto put a shine on the o-ring. Excesslubrication can cause clogging of thegas swirler and metal-dust contaminationin the torch. This can lead to28Welding Design and Fabrication ■ November 2000
uncontrolled arcing in the plasmachamber and, ultimately, torch failure.Grease should never be applied tothe torches—it can cause destructivearcing and burning within the torch.5. Neglecting routine torch maintenance.Torch threads must be keptclean and seating areas should bechecked for contamination or mechanicaldamage. Clean any dirt,metal dust, or excess o-ring lubricantout of the torch using a cotton swaband electrical contact cleaner or hydrogenperoxide.6. Not checking gas and coolantflow. Check the flow and pressure ofgas and coolant every day. If flow is insufficient,consumables will not coolproperly and parts life will be reduced.Inadequate flow of cooling water, due toworn pumps, clogged filters, lowcoolant level, etc... can cause parts andtorch failure. Constant gas pressure isimportant to maintaining the cutting arc.Excess gas pressure is a commoncause of hard starting, and can alsocause rapid deterioration of electrodes.Likewise, plasma gas must be keptclean to prevent short consumable andtorch life. Compressed-air systems areespecially prone to oil, moisture, andparticulate contamination.7. Piercing too low. Even slightvariations in torch height can affect theangularity of the cut surface. Torchheight during piercing is particularlyimportant. One common error is topierce too low. This causes moltenmetal to spatter the front of the nozzleand shield. Arc snuffing can occur ifthe torch pierces when touching themetal, or drags along the surface whilecutting. If the arc is snuffed, the electrode,nozzle, gas swirler, and sometimesthe torch are destroyed. Piercingat a height of 1.5-2 times the recommendedcut height protects the torchand parts.8. Cutting too fast or slow. Cuttoo slow and cut pieces will developlow-speed dross—a large, bubbly accumulationof dross along the bottomedge. Slow speeds may also cause awidening of the kerf and excessiveamounts of top spatter. Cut too fastand the arc will lag backwards in thekerf, causing a beveled edge, narrowkerf, and a small, hard bead of drossalong the bottom edge of the cutpiece, that is difficult to remove.9. Stretching the arc. Arc stretchingcan occur at the beginning and endof the cut if the arc has to stretch, orThe torch should beassembled so that parts aligncorrectly and fit together snugly. Torchthreads must be kept clean andseating areas should be checked forcontamination or mechanical damage.deviate from a straight, perpendicularpath, to find metal. Arc stretching cancause the arc to cut into the side wall ofthe nozzle. When performing an edgestart, the plasma arc should be startedwith the nozzle orifice centered directlyover the edge of the workpiece. This isimportant to remember in punchpress-plasma operations where thearc is started from a punched hole. Inthis application, the arc should bestarted off the edge, not the center ofthe punched hole. Arc stretching canalso occur at the end of the cut if thetorch is programmed to run off theplate with the arc on, or if the lead-outfollows the kerf of previously cut metal.Timing of the arc-off signal and programmingof the lead-out can minimizethis effect.10. Crashing the torch. Torch collisionswith the workpiece can be preventedby programming the shape-cuttingmachine to travel around, ratherthan over, cut parts. Torch-height sensorsalso offer protection from torchcrashes by correcting for variations inthe material. However, voltage-regulatedheight controls can fail to protectthe torch. For example, torch diving oftenoccurs at the end of a cut if the torchfollows the kerf for too long. The torchheightcontrol dives to compensate forincreased voltage as the arc stretches.Careful programming of the lead-outand torch-height control function canminimize this. Finally, breakawaytorch-mounting devices can help preventdamage to the torch if a collisiondoes occur.November 2000 ■ Welding Design and Fabrication29
Page 7: HOW CUTTING SPEED AFFECTS CUT QUALI
Page 13 and 14: Stage 1 Stage 2 Stage 3 Pierce too
Page 15 and 16: Illustration 1Illustration 2Illustr
Page 18 and 19: CLASSROOMWritten By: David J. CookC
Page 21 and 22: Plasma Cost of OperationBest and Wo
Page 23 and 24: adjusted by monitoring and controll
Page 25 and 26: There are 3 possible cases:• Case
Page 27: Figure 2the electrode to remove the
Page 31 and 32: Figure 6Moisture on StartFigure 7Co
Page 33 and 34: Metal Minimum Recommended Approxima
Page 35 and 36: The third consideration is themost
Page 37 and 38: and-pinion drives are the most comm
Page 39 and 40: PC-based controls now offer speedsu

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