Training Articles
Plasma Arc Cutting - Cut Quality Problems
By Dave Cook, Centricut Technical Services
Director
This article was published in "Welding Design & Fabrication"
- November 1998
DROSS - Many fabrication shops spend a lot of time and
money grinding, chipping, and sanding parts that were cut on the
plasma machine to remove dross. With so many variables in the
cut quality equation, how does a plasma torch operator begin to
troubleshoot a dross problem? By controlling the critical process
variables, the operator can minimize or eliminate dross and the
associated costs of secondary operations.
PAC Cut Quality Problems / Dross
Dross is re-solidified oxidized molten metal that is not fully
ejected from the kerf during cutting. It is the most common cut
quality problem of plasma cutting. Dross may form as a thick bubbly
accumulation along the bottom edge of the plate (low speed dross),
a small hard bead of uncut material (high speed dross) or a light
coating along the top surface of the plate (top spatter).
Dross formation is dependent on many process variables including
torch travel speed, standoff distance, amperage, voltage and consumable
condition. It is also affected by material variables such as thickness
and type of material, grade, chemical composition, surface condition,
flatness, and even temperature changes in the material as it is
cut. However, the three most critical variables to consider in
dross formation are cutting speed, amperage, and standoff distance.
Low
speed dross
If the cutting speed is too slow, the plasma jet begins to look
for more material to cut. The arc column grows in diameter, widening
the kerf to a point where the high velocity portion of the plasma
jet no longer ejects the molten material from the cut. As a result,
this molten material begins to accumulate along the bottom edge
of the plate in a thick globular form. This is called low-speed
dross. At extremely low speeds the arc extinguishes because there
is not enough metal to sustain a transferred arc. Increasing the
amperage or decreasing the standoff (while keeping material thickness
and speed constant) have a similar effect on the cut as slowing
down the cut speed. Both of these changes cause more energy from
the plasma jet to contact a given area of the material in a given
period of time. Excessive amperage or low standoff can also cause
low-speed dross. (Some low speed dross in the corners of a plasma
cut is normal since velocity does not remain constant through
a sharp turn.)
To eliminate low speed dross:
- Increase the cut speed in 5 ipm increments
- Increase the standoff in 1/16 increments or 5 volt increments
- Decrease the amperage in 10 amp increments
- If none of these measures improve the cut, consider a smaller
nozzle size
High speed dross
If the cutting speed is too fast, the arc begins to lag back
in the kerf leaving a small hard bead of uncut material or rollover
dross along the bottom of the plate. This high-speed dross is
more tenacious and usually requires extensive machining to remove.
At extremely high speeds the arc becomes unstable and begins
oscillating up and down in the kerf causing a rooster tail of
sparks and molten material. At these speeds the arc may fail
to penetrate the metal or extinguish. High standoff or low amperage
(for a given material thickness and cutting speed) can also
cause high speed dross since both of these changes cause a reduction
in the energy of the plasma jet.
To eliminate high speed dross:
- Check the nozzle first for signs of wear (gouging, oversize
or elliptical orifice)
- Decrease the cutting speed in 5 ipm increments
- Decrease the standoff in 1/16 increments or 5 volts increments
- Increase the amperage (do not exceed 95% of the nozzle
orifice rating)
Top spatter dross
Top spatter is an accumulation of re-solidified metal that
sprays along the top of the cut piece. It is usually very
easy to remove. A worn nozzle, excessive cutting speed, or
a high standoff is usually the cause. It is caused by the
swirling flow of the plasma jet, which at a certain angle
of attack flings molten material out in front of the kerf
rather than down through it.
To eliminate top spatter dross:
- Check the nozzle for signs of wear
- Decrease the cutting speed in 5 ipm increments
- Decrease the standoff in 1/16 increments or 5 volt increments
Dross Free Window
Between the extremes of high and low speed dross is a window
of dross free or minimum dross cutting. Finding this window
is the key to minimizing secondary operation requirements
on plasma cut pieces.
The window varies with plasma gas used: For example, nitrogen
and air plasma gasses have a relatively narrow dross free
window on carbon steels while oxygen plasma has a wider
dross free window. (Oxygen plasma gas reacts with carbon
steel to produce a finer spray of molten metal, each droplet
having a lower surface tension. This molten spray is more
easily ejected from the kerf.)
The dross free window is also affected by material type.
For instance cold rolled steels cut cleaner than hot rolled
and pickled steels cut cleaner than non-pickled.
To judge the Optimum Cutting Speed:
- The first method involves making a series of test
cuts at various cutting speeds and choosing the speed
that produces the cleanest cut. Lag lines (small ridges
in the surface of the cut) are a good indication of
cutting speed. Slow cutting speeds produce vertical
lag lines that are perpendicular to the plane of the
plate. Fast cutting speeds make slanted s-shaped lag
lines that run parallel to the plate along the bottom
edge. By examining the lag lines the operator can determine
whether an increase or decrease in speed is needed to
find the dross free window. Many operators have the
tendency to slow the machine down at the first appearance
of dross, but often an increase in speed is necessary.
- The second method is to watch the arc (through the
appropriate welding lens!) during the cut and dynamically
change the speed to produce the optimum arc characteristics.
To do this, observe the angle of the arc as it exits
the bottom of the work-piece. If you're cutting with
air plasma gas, the arc should be verticalas it exits
the bottom side of the cut. With nitrogen or argon/hydrogen,
a slight trailing arc is best, and with oxygen plasma
gas, the best cut speed is one that gives you a slight
leading arc.
QUICK REVIEW:
- Eliminating Dross Formation
- Low-speed Dross a Increase the cut speed in 5 ipm
increments
- Increase the standoff in 1/16 increments or 5 volt
increments
- Decrease amperage in 10 amp increments
- High-Speed Dross
- Check the nozzle for signs of wear
- Decrease the cutting speed in 5 IPM increments
- Decrease he standoff in 1/16 increments or 5 volt
increments
- Increase the amperage (but do not exceed 95% of
the nozzle's rating
- Top Spatter Dross a Check the nozzle for signs of
wear
- Decrease the cutting speed in 5 IPM increments
- Decrease he standoff in 1/16 increments or 5 volt
increments
