Training Articles Journal - Centricut

CLASSROOMWritten By: David J. CookCentricut Technical Services DirectorUpgrade and Retrofit aPlasma-Arc-CuttingMachineAre you spending a lot of time andmoney on secondary operations toclean up dross, remove bevel angle,or rework out-of-tolerance parts thatwere cut using plasma? Does yourcutting machine vibrate? Does itbreak down and stay down for daysor weeks? Is your CNC control outdatedand slow? Many of theseproblems can be eliminated throughoperator training and propermachine maintenance; others arebest solved by equipment upgrades.Plasma-system upgradesA new plasma system retrofittedonto an older cutting machine orpunch press can mean improved cutquality, increased cutting capacity,and faster cutting speeds. Today’splasma systems offer significantimprovements in power supply andtorch technology, including microprocessor-controlledpower supplies,sophisticated gas-flow controls, andoptimized torch and parts designs.Consumable-parts life has improvedwith the use of inert start gases foroxygen-plasma cutting. Current- andgas-pressure ramping or steppingdevices soften the plasma arc duringignition and cut off.When selecting a new plasmasystem consider these variables:Material type will determine theplasma gas used. If you only cutstainless steel or aluminum, anolder nitrogen-plasma system maybe adequate. Air plasma may alsobe a viable low-cost alternative. Formild-steel cutting, oxygen plasma isbecoming the industry standard forcut quality and weldability. Air plasmaoffers similar cut quality on carbonsteels, but may produce nitridingalong the cut surface.Material thickness will determinethe size of plasma system needed.Determine the thickest material thatyou regularly cut and select asystem that will cut this material at areasonably fast speed. Manufacturersrate their systems by maximumcut capacity from an edgestart; pierce ratings are typically halfthe cut capacity.Cutting-machine upgradesCutting machine upgrade optionsinclude:Mechanical reconditioning toadd rack and pinion drives is nowthe most commonly used method ofupgrading motion transmission.Installing new gears and racks onolder machines will improve theirperformance. Bearings to supportthe moving elements of the machinein longitudinal (the bridge movingalong the rails) and transverse (thetorch carriage moving along thebridge) axes are critical to smoothmotion and accurate positioning.New drive packages includeservo motors, drive amplifiers, andencoders retrofitted to an oldmachine. PWM (Pulse width modulation)DC servo motors are typicallyused for plasma machines and areavailable as retrofit kits. The result:greater positional accuracy, highertorque motors, and faster accelerationtimes.Torch height control (THC) adds18Welding Design and Fabrication ■ November 2000