FOCUS ON PLASMA CUTTING
Plasma system components
A basic plasma cutter or plasma cutting system includes the following components :
( such as oxygen , high purity air , nitrogen and a hydrogen / argon / nitrogen mixture ) are used as the plasma gas for strong results on a multitude of conductive materials .
The basic plasma arc cutting system consists of a power supply , an arc starting circuit and a torch . These system components provide the electrical energy , ionization capability and process control that is necessary to produce high-quality , highly productive cuts on a variety of different materials . The power supply is a constant current DC power source . The output current ( amperage ) of the power supply determines the speed and cut thickness capability of the system . The main function of the power supply is to provide the correct energy to maintain the plasma arc after ionization .
The arc starting circuit is a high-frequency generator circuit that produces an AC voltage used to create a high-intensity arc inside the torch to ionize the gas , thereby producing the plasma . The torch serves as the holder for the consumable nozzle and electrode , providing cooling ( either gas or water ) to these parts . The nozzle and electrode constrict and maintain the plasma jet .
Modern machine designs
“ Early CNC profiling machines were fitted with single or multiple oxyfuel cutting heads . These were built using large heavy castings and / or structural beams and driven around at relatively slow speeds using gearboxes with large motors – and such machine designs are still available today . Modern high-speed precision plasma cutting machines need to be able to accelerate , decelerate and change direction in a smooth , controlled and vibrationfree manner . This requires a more refined machine design that includes a variety of features ,” commented Kerf Developments . Some of the key components to form modern plasma machine designs include :
■ A rigid bridge assembly that is strong but not excessively heavy , as the task of the bridge is to move the torch assembly around the cutting table by precisely following a pre-defined NC toolpath .
■ A reliable and predictable CNC control and motion system which can position the cutting torch exactly where it needs to be in a smooth and controlled manner . The controller must provide feedback to the user and communicate with the plasma unit to set power settings , speed feeds and gas pressures where appropriate .
■ Precision electronic torch height control which can maintain the exact pierce and cut height for the process . This needs to be vibration-free , as any movement or vibration in the torch will transmit to the cut profile .
■ Safety breakaway device for the torch . This needs to be ‘ rigid ’ but also able to protect the torch over the life of the machine . These are typically either magnetic or maintenance-free pneumatic systems . Whichever style is selected , the torch needs to maintain rigidity throughout the cutting process .
■ Independent downdraft cutting tables that are not connected in any way to the motion system . By remaining standalone , the table ensures that vibration is not introduced into the motion control system whilst cutting .
■ Advanced CAM software that can take base CAD geometry and apply appropriate technology to the part to ensure consistent high-quality cutting . This should include an intelligent selection of cutting speeds for holes , slots and other internal features , together
■ Power supply : A constant current DC power source . The open circuit voltage is typically in the range of 240 to 400 VDC . The output current ( amperage ) and overall kilowatt rating of the power supply determines the speed and cut thickness capability of the plasma cutter . The main function of the power supply is to provide the correct energy to maintain plasma arc after ionization .
■ Pilot arc starting circuit : In most liquid-cooled torches of 130 amps and higher , this is a highfrequency generator circuit that produces an AC voltage of 5,000 to 10,000 volts at approximately 2MHz . This voltage creates a highintensity pilot arc inside the torch to ionize the gas , producing the plasma . Rather than the highfrequency starting circuit , air plasma torches typically use a moving electrode or “ blowback start ” technology to ionize the gas .
■ Torch : This serves as the holder for the consumable nozzle and electrode . It provides cooling ( either gas or water ) to these parts . The nozzle and electrode constrict and maintain the plasma arc
Other elements can include a cutting bench , exhaust system and coolant circulation system .
ISMR December 2023 / January 2024 | ismr . net | 33