Training Articles
Solving PAC Cut Quality Problems -
Dimensional Inaccuracies
By Dave Cook, Centricut Technical Services
Director
This article was published in "Welding Design & Fabrication"
- March 1999
Solving PAC Cut Quality Problems
Many fabrication shops spend a lot of time and money reworking
parts that were cut on the plasma machine to remove dross or correct
dimensional inaccuracies. Some of these cut quality problems are
caused by mechanical and electrical problems of an old or poorly
maintained cutting machine; others are related to the plasma process
itself. With so many variables in the cut quality equation, how
does a plasma torch operator begin to troubleshoot a cut quality
problem? Here we discuss the critical process variables that affect
dimensional accuracy of a plasma cut piece. By carefully controlling
these variables, the operator can minimize or eliminate dimensional
problems and the associated costs of secondary operations or scrap
parts.
Figure 1 shows a cut-away view of a plasma cut piece and illustrates
several elements of cut quality. The dimensions of a part are
most affected by two of these elements: kerf width and bevel angle.
Kerf is the void created by the plasma cutting process,
or the amount of metal removed by the plasma arc. The plasma arc
is dynamicÑit changes in size and shape depending on amperage,
voltage, gas flow and velocity of the moving torchÑand so as the
plasma arc column changes so does the kerf. Nozzle size also has
a direct affect on kerf width since the nozzle orifice constricts
the plasma gas jet to a particular diameter. (Nozzles are sized
according to amperage rating: the bigger the hole the more power
it can handle.) A good rule of thumb for estimating the kerf width
is to multiply the nozzle orifice size by a factor of 1.5. For
example a 200 amp air nozzle with an orifice of .086", will have
a kerf width of approximately .129".
CNC controls for plasma arc cutting have an adjustable
parameter called kerf compensation. The operator or programmer
enters a value for kerf compensation that is usually equal to
the kerf width, (some controls use _ the kerf width). The CNC
then automatically calculates to compensate for the width of the
cut, keeping the kerf on the waste side of the part. To find the
proper kerf compensation value, operators often use a trial and
error method. They first estimate the kerf using the rule of thumb
or physically measure it to get an initial value. They then cut
test pieces, measure them, adjust the kerf compensation up or
down, and repeat the process until the part measures correctly.
Kerf too wide (Part too small). This problem can be caused
by a worn nozzle, high torch standoff (arc voltage), excessive
amperage, inadequate gas flow, or low speed. Each of these variables
will cause the arc column to grow, widening the kerf. An incorrect
(small) kerf compensation value will also cause an undersized
part. Kerf too narrow (part too big). This problem can be caused
by low torch standoff (arc voltage), inadequate amperage, excessive
gas flow, or high speed. These variables cause the arc column
to shrink, narrowing the kerf. An incorrect (large) kerf compensation
value will also cause an oversized part.
Bevel angle is the angle of the cut edge. A cut with 0°
bevel is a straight cut, perpendicular to the plane of the material.
Most plasma torches use a clockwise swirling flow of plasma gas,
which produces a straighter cut on the right hand side of the
kerf with respect to forward torch motion. Typical bevel angles
for conventional plasma torches range from 1-3 degrees on the
"good" side of the cut and 3-8 degrees on the "bad" side of the
cut. High tolerance plasma cutting systems can achieve even lower
bevel angles. Although some bevel is inherent in the plasma process
due to the shape of the gas jet as it exits the torch nozzle,
it is possible to minimize it. Bevel angle greater than 5 degrees
may indicate a problem with PAC machine parameters.
(Excessive) Positive bevel (top of part smaller than bottom).
This problem may be caused by a worn nozzle, high torch standoff
(arc voltage), inadequate amperage, or excessive speed. All of
these variables cause the arc to lag which causes more energy
to contact the top of the kerf than the bottom. As a result, the
kerf is wide at the top and narrow at the bottom. Improper cut
direction around the part may also cause excessive positive bevel
angle. A part with excessive positive bevel all around it may
also have a hard bead of high-speed dross at its bottom edge.
Negative bevel (bottom of part smaller than top, undercutting)
This problem can be caused by low torch standoff (arc voltage),
excessive amperage, or low speed. These parameters cause the arc
to remove more material at the bottom of the plate. Usually a
consistent negative bevel around the part is accompanied by low
speed dross.
Irregular bevel (positive and negative bevel on the same piece)
This problem usually indicates that the nozzle has failed, the
torch is out of square or the electrode and nozzle are misaligned.
These variables cause the arc to deviate from a straight path
through the material. Often one side of a square part will have
a positive bevel and the opposing side a negative. The cross section
of the part looks like a parallelogram rather than a rectangle.
Sometimes the cut surface may not be flat, but rather concave
on one side and convex on the other. These are all signs of severely
worn or misaligned parts.
These 6 recommendations will help the PAC operator to improve
the dimensional accuracy of plasma cut pieces:
- Adjust all PAC machine parameters to exact recommended settings
- Set the kerf compensation to the correct value
- Check torch squareness to the plate regularly
- Monitor speed and voltage; make slight adjustments as necessary
as parts wear
- Check consumable parts at pre-established intervals; change
before cut quality deteriorates
- Use in-process q.c. checks to monitor cut quality
