Plasma cutting machine maintenance

Murphy's Law says the plasma cutting machine will break down when you need it most—usually right in the middle of a big plate-cutting job. At up to $200 per hour fully burdened cutting-machine downtime gets expensive.

Many shops do not have a regular preventive-maintenance schedule for their plasma-cutting systems. After a few months of neglect, the machine will no longer perform as-designed. Mechanical parts will prematurely wear, causing rough machine motion. This may lead to out-of-tolerance parts and poor cut quality, particularly at higher cutting speeds. When machine components fail, trouble-shooting and servicing can take days.

 

 

Checklist for preventive maintenance

Here is a checklist that serves as a good starting point for a preventive maintenance program.

• Clean the torch body
  Remove the torch parts and examine the inside of the torch. Check for signs of mechanical damage to threads. Clean the inside of the torch with electrical contact cleaner and a cotton swab. Disconnect the torch from its mounting tube and slide the tube back to reveal the torch-lead fittings. Check for leaks or damage to any of the connections. Blow out any accumulated metal dust.
  
  
• Clean the torch leads
  Wipe down or blow off the entire length of the torch leads to remove accumulated metal dust and dirt. Metal dust can cause dissipation of the high voltage needed to start the plasma arc. Check for kinked or worn hoses, exposed wires, cracked fittings, or other damage. Check high-frequency shielding for proper connection to earth ground.
  
  
• Clean out the power supply
  Blow out any accumulated metal dust using clean, dry, shop air. Metal dust can cause damage to power-supply components, especially PC boards. Contactors, relays, and spark-gap assemblies can also malfunction due to excess buildup of metal dust. Check air filters on the power-supply housing; replace as necessary.
  
  
• Check torch-cooling components
  With water-cooled torches, check the coolant stream in the tank for signs of aspirated air or reduced flow. Make sure that the return flow is at the specified gallons per minute. Check that flow switches on the return line function properly—insufficient coolant flow can cause the torch to overheat. Check coolant filters and pump screens and clean or replace as necessary. Check coolant resistivity using a conductivity meter, if available. Resistivity should not exceed 10 micro-ohms for most systems. Flush and replenish coolant every 6 months.
  
  
• Check water quality
  Secondary water quality is particularly important with water-injection torches. Water hardness should not exceed 8.5 ppm or 0.5 grains. Hard water causes mineral deposits to build up on nozzles, leading to shortened life. Use a commercial water softener if necessary. Water quality in water tables is also important. If the water in the table is heavily contaminated with slag and metal dust, it can cause hard-starting of the plasma torch. It may also cause rust accumulation on the cut pieces.
  
  
• Check plasma
  Gas quality is critical to maintaining good parts life and cut quality. To check air quality, hold a clean paper towel under the torch while purging air through the system in the TEST mode. Check for water, oil mist, or particulate contamination. Check filters weekly; empty moisture traps whenever they begin to accumulate water.
  
  
• Clean machine components— rails, gears, racks, and such.
  Use a degreasing agent and an abrasive pad to remove grease, dirt, and metal dust. Lubricate gears with dry lubricant such as graphite powder. If bearings have grease fittings, lubricate them. Do not lubricate rail sections— lubricants will attract contaminants that lead to excessive wear.
  
  
• Level and align rails
  Check joints where rail sections meet with a piece of tool steel or other precision straight edge, feeling for misalignment. Rail alignment will prevent drag on drive motors. Distances between the rails should be constant across the entire length of the system.
  
  
• Align and adjust gears and bearings
  Gears should not overlap above or below the rack. Adjust gear alignment to remove play between gears and racks. Make these adjustments for rail and cross drives. Alignment bearings should have minimal play between them and rail or crossrail surfaces. These bearings are usually mounted on an eccentric. Adjust until no light can be seen between bearing and rail surface. Do not over-tighten. With drive gears disengaged, roll the beam across the rails to check for binding. Adjust accordingly until the beam rolls free with minimal vibration and little resistance.
  
  
• Check the squareness of the torch with respect to the table and workpiece
   Collisions can knock the torches out of square with the workpiece.
  
  
• Check torch-mounting device
  This can cause vibration that will translate into a wavy cut.
  
  
• Check safety limits, which need to operate properly to ensure operator safety and prevent damage to the machine. Move the machine to each limit to test switches. Make sure the machine stops when each switch is reached. Inspect mechanical stops to make sure that they are in proper working order.
  
  
• Tune the drive motors and control
  Drive-motor tuning may be necessary if the motors appear to be out of synch—the torch may not return to its home position, or inaccuracies in cut pieces may develop, particularly in combination moves where both x and y drives are operating. Speeds for each axis need to be equal for proper torch positioning. Motor drift must also be minimized. As these adjustments differ from system to system, consult the manual or an authorized representative for your particular machine to tune the drive package.