Training Articles
Mechanized Plasma System Maintenance
by Dave Cook, Centricut Technical Services
Director
This article was published 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
- Using Consumable Parts Until They Blow
- Changing Consumable Parts Too Frequently
- Using the Wrong Parts and Parameters for the Job
- Assembling the Torch Incorrectly
- Neglecting Routine Maintenance
- Not checking gas and coolant flow a Piercing Too Low
- Cutting Too Fast or Too Slow
- "Crashing" the Torch
Using the Wrong Parameters & Parts for the Job
Consumable selection depends on the material and thickness being
cut, the amperage and plasma gas used and other cutting parameters.
The operator's manual will define which consumables are appropriate
for various types of cutting. Using incorrect consumables can
lead to shortened parts life and reduced cut quality. It is
particularly important to run parts at the correct amperage.
The best cut quality and parts life is usually achieved when
the amperage is set to 95% of the nozzle's rating. If the amperage
is too low, the cut will be sloppy; if it is too high, the nozzle
life will be poor.
Assembling the Torch Incorrectly
The torch should be assembled so that the parts are aligned
correctly and fit together snugly. This ensures good electrical
contact and the correct flow of gas and coolant through the
torch. When changing parts, keep consumables on a clean shop
rag to prevent dirt or metal dust from contaminating the torch.
Cleanliness during torch assembly is very important and often
neglected. When applying o-ring lube, use just enough to put
a shine on the o-ring. Too much lubrication can cause clogging
of the gas swirler and metal dust contamination in the torch.
This can lead to uncontrolled arcing in the plasma chamber and
ultimately torch failure. Grease should never be applied to
torches--it can cause destructive arcing and burning within
the torch.
Neglecting Routine Maintenance
Torches can last for months or even years with proper care.
Torch threads must be kept clean and seating areas should be
checked for contamination or mechanical damage. Any dirt, metal
dust or excess O-ring lubricant should be cleaned out of the
torch. To clean the torch, use a cotton swab and electrical
contact cleaner or hydrogen peroxide.
Not checking gas and coolant flow
The flow and pressure of gas and coolant should be checked every
day. If the flow is insufficient, consumables will not be cooled
properly and parts life will be reduced. Inadequate flow of
cooling water due to worn pumps clogged filters, low coolant
level etc. is a common cause of parts and torch failure. Constant
gas pressure is important to maintaining the cutting arc. Excess
gas pressure is a common cause of "hard starting," a situation
in which the torch fails to initiate an arc when all other conditions
for normal operation are correct. Too much gas pressure will
also cause rapid deterioration of electrodes. Likewise, plasma
gas must be kept clean to prevent short consumable and torch
life. Compressed air systems are especially prone to oil, moisture,
and particulate contamination.
Piercing too low
Standoff, the distance between the workpiece and the tip of
the torch, is critical to both cut quality and parts life. Even
slight variations in torch height can affect the angularity
of the cut surface. The height of the torch during piercing
is particularly important. One common error is to pierce too
low. This causes molten metal to spatter the front of the nozzle
and shield causing damage to the parts and sub-sequent cut quality
problems. Arc "snuffing" can even occur if the torch pierces
when touching the metal, or drags along the surface while cutting.
If the arc is "snuffed", the electrode, nozzle, gas swirler,
and sometimes the torch are destroyed. Piercing at a height
of 1.5-2X the recommended cut height protects the torch and
parts from damage.
Cutting Too Fast or Too Slow
Cutting too fast or slow will cause cut quality problems. If
the speed is too slow the cut pieces will develop "low speed
dross" a large bubbly accumulation of dross along the bottom
edge. Slow speeds may also cause a widening of the kerf and
excessive amounts of top spatter. If the speed is too fast the
arc will lag backward in the kerf causing a beveled edge, a
narrow kerf and a small hard bead of dross along the bottom
edge of the cut piece. High speed dross is difficult to remove.
The correct cutting speed will produce minimal dross--the result
will be a clean edge that needs little rework before the next
step in the manufacturing process. "Stretching" the arc. Arc
stretching can occur at the beginning and end of the cut if
the arc has to "stretch" (deviate from a straight, perpendicular
path) to find metal. Arc stretching can cause the arc to cut
into the side wall of the nozzle. When doing an edge start,
the plasma arc should be started with the nozzle orifice directly
centered over the edge of the work piece. This is important
to remember in punch press/plasma operations where the arc is
started off of a punched hole. In this application, the arc
should be started off the edge not the center of the punched
hole. Arc stretching can also occur at the end of the cut if
the torch is programmed to run off the plate with the arc on,
or if the "lead out" follows the kerf of previously cut metal.
Timing of the arc off signal and programming of the lead out
can minimize this effect.
Crashing the Torch
"Tip-ups" and crashes can irreparably damage your torch. Torch
collisions with the workpiece can be prevented by programming
the shape cutting system to travel around (rather than over)
cut parts. Torch height sensors also offer protection from torch
crashes by correcting for variations in the material. However,
voltage regulated height controls can fail to protect the torch.
For example "torch diving" often occurs at the end of a cut
if the torch follows the kerf for too long. (The torch height
control dives to compensate for increased voltage as the arc
stretches). Careful programming of the lead out and torch height
control function can minimize this. Finally, breakawaytorch
mounting devices can help prevent damage to the torch if a collision
does occur.
Lesson Review
Ten Common Mistakes to Avoid
- Using Consumable Parts Until They Blow
- Changing Consumable Parts Too Frequently
- Using the Wrong Parts and Parameters for the Job
- Assembling the Torch Incorrectly
- Neglecting Routine Maintenance
- Not checking gas and coolant flow
- Piercing Too Low
- Cutting Too Fast...
- ...or Too Slow
- "Crashing" the Torch
A good operator with well-maintained PAC equipment can save a
shop countless hours of downtime and thousands of dollars in operating
expenses. These savings will result in greater profits for your
cutting operations and your company.
