Centricut Training Lessons

Mechanized Plasma System Maintenance
by Dave Cook, Centricut Technical Services Director

Article originally appeared in
"Welding Design & Fabrication" - September 1998

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

Many shops do not have a regular preventative maintenance schedule for their plasma cutting systems. After a few months of neglect, the cutting 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. The plasma system may malfunction causing cut quality and parts life problems.

When components in the machine fail, troubleshooting the problem and servicing the machine can take days. Sticking to a regular schedule of preventative maintenance can avert most of these problems. The following checklist is a good starting point for a program of preventative maintenance.

Plasma System Maintenance Checklist:
Clean the torch body
aRemove the torch parts and examine the inside of the torch.
aCheck for any signs of mechanical damage to threads.
aClean the inside of the torch with electrical contact cleaner and
a cotton swab.
aDisconnect the torch from its mounting tube and slide back the tube to reveal the torch/lead fittings.
aMake sure there are no leaks or damage to any of the connections.
aBlow out any accumulated metal dust.

Clean the torch leads
aWipe 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.)
aCheck for any kinked or worn hoses, exposed wires, cracked fittings, or other damage.
aCheck high frequency shielding for proper connection to
earth ground

Clean out the power supply
aBlow out any accumulated metal dust from the power supply using clean, dry, shop air.
aMetal dust can cause damage to power supply components especially PC boards.
aContactors, relays and spark gap assemblies can also malfunction due to excess buildup of metal dust.
aCheck air filters on the power supply housing; replace as necessary.

Check torch cooling system
aWith water cooled torches check the coolant stream in the tank for signs of aspirated air or reduced flow. Make sure the return flow is at the specified gallons per minute.
aCheck flow switches on the return line to make sure they are functional.
aThese prevent operation of the torch with low coolant flow, preventing overheating.
aCheck coolant filters and pump screens and clean or replace as necessary.
aIf available use a conductivity meter to check coolant resistivity.
aThe reading should not exceed 10 micromhos for most systems.
aCoolant should be flushed and replenished every 6 months.

Check water quality
aSecondary water quality is particularly important with water injection torches.
aWater hardness should not exceed 8.5 ppm or 0.5 grains.
aHard water causes mineral deposits to build up on nozzles leading to shortened life.
aUse a commercial water softener if necessary.
aWater quality in water tables is also important.
aIf 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 necessitating more rework.

Check plasma gas quality
Plasma gas quality is critical to maintaining good parts life and
cut quality.
a To check air quality hold a clean paper towel under the torch while purging air through the system in the "TEST" mode.
aMake sure there is no sign of contamination. Shop air systems are especially prone to contamination problems.
aCheck for water, oil mist, or particulate contamination.
aFilters should be checked once a week; moisture traps should be emptied whenever they begin to accumulate water.

Clean machine and surrounding area
aRails, gears and racks should be kept spotlessly clean.
aUse a degreasing agent and an abrasive pad to remove grease, dirt, and metal dust.
aLubricate gears with dry lubricant such as graphite powder.
aIf bearings have grease fittings, lubricate them.
aRail sections should not be lubricated. Lubricants will attract contaminants leading to excessive wear.

Level and align rails
aRails should be periodically checked and adjusted for levelness using an accurate level.
aCheck joints where rail sections meet with a piece of tool steel or other precision straight edge, feeling for any misalignment.
aFiling may be needed to eliminate any difference in alignment not corrected through leveling.
aAlignment of the rails is also important to prevent drag on drive motors.
aDistances between the rails should be constant across entire length of system.

Align and adjust gears and bearings
aGears should be in near perfect alignment to gear racks; they should not overlap above or below the rack.
aAdjust gear alignment until no play is felt between gears and racks. These adjustments should be made for both rail and cross drives.
aAlignment bearings should have minimal play between them and rail or cross rail 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 rails to make certain there is no binding.
Adjust accordingly until the beam rolls free with minimal vibration and very little resistance.

Check torch alignment and mounting
aCheck the squareness of the torch with respect to the table and work-piece.
aTorch squareness is critical to maintaining a consistently square
cut edge.
aTorches are often knocked out of square by a collision with
the metal.
aCheck for looseness in the torch mounting device.
This can cause vibration that will translate into a wavy cut.

Check safety limits
aSafety limits need to be operating properly to ensure operator
safety and prevent damage to machine.
aMove machine to each limit to test switches. Make sure machine stops when switch is reached.
aInspect mechanical stops and make sure they are in proper
working order.

Tune drive motors and control
aDrive motor tuning may be necessary if the motors appear to be out of synch. (The torch may not return to home precisely, or inaccuracies in cut pieces may develop particularly in combination moves where both X and Y drives are operating.)
aSpeeds for each axis need to be equal for proper positioning of torch. Motor drift must also be minimized.
aAs these adjustments differ from system to system, consult the manual or an authorized representative for your particular system to tune up the drive package.

QUICK REVIEW:
Routine Maintenance Of Mechanized Plasma Systems

aClean the torch body
aClean the torch leads
aClean out the power supply
aCheck torch cooling system
aCheck water quality
aCheck the plasma gas quality
aClean machine and surrounding areas
aLevel and align rails
aAlign and adjust gears and bearings
aCheck torch alignment and mounting
aCheck safety limits
aTune drive motors and control

This article was published in
the September 1998 edition of
Welding Design & Fabrication.