The quality of the plasma cut edge can be compared to that of cutting using the oxy-fuel process. However, the plasma process cuts by melting the material and therefore a characteristic feature is the greater degree of melting towards the top of the metal resulting in poor edge squareness, top edge rounding or a bevel on the cut edge.
As these problems are associated with the degree of constriction of the arc, torch manufacturers are continually developed new torch designs to improve arc constriction and produce a more uniform heating at the top and bottom of the cut to achieve an acceptable cut quality.
To help understand cut quality, its best that the characteristics of the finished cut are looked at in close detail, the image shown below will help us explain this.
Cutting or Torch Angle
Generally when cutting with a plasma torch, the torch should be held perpendicular to the piece being cut. For mechanically mounted torches, a square can be used to insure that the torch is perpendicular to the plate.
The distance between the torch tip and the work piece during the cutting process will have an effect on the bevel angles. The greater the distance, the greater the bevel angle will be. Typically, smaller hand cutting systems (40 amps and under) are designed to drag the tip on the plate.
For higher amperage hand cutting systems, use of a drag shield cup, a standoff guide, or a cutting guide will help keep a consistent tip to work distance for best results. For automated applications, a device known as an Arc Voltage Control (AVC), can be used. The AVC, also known as a Stand-Off Control or torch height control which will monitor the arc voltage. This voltage directly relates to the tip to work distance.
Thinner plate or larger pieces of plate metal can have some deformities that will cause the torch to work distance, over the surface of the plate, to change. Uneven cutting tables or warpage can cause the plate to be higher or lower in relation to the torch as well. The AVC will constantly monitor the arc voltage. A change in this voltage means a change in torch height. The AVC will raise or lower the torch to keep the stand-off distance constant.
While plasma cutting a void is left behind which is called The Kerf in the material being cut. This kerf width must be factored into the cutting process so that the cut production part’s outside dimensions are not undersized and internal dimensions are not oversized. Cutting amperage and cutting speed play a factor in determining the kerf width.
The orifice of the cutting tip is sized according to the amperage. The greater the amperage being used, the larger the tip orifice will be. Typically, the kerf width will be about 2 - 3 times the orifice size of the cutting tip. Cutting speeds are based on the selected amperage for a given material thickness, the arc will always be cutting the leading edge of material directly below torch. Reduced the cutting speed will result in all available material beneath the torch is melted and blown away, the arc will start to reach forward or to the side in order to remain cutting.
This will wear the tip causing the orifice to widen. This will result in a wider kerf width, creating more dross and generally giving you a poor cut quality.
In an ideal cut, the bevel angle or angle of the cut surface would be perfectly square. The plasma cutting process does result in a slight angle which is called a Bevel Angle, on both the cut and scrap side of the work piece. This is why direction of cut is important.
When the plasma gas flows, it has a swirling action as it leaves the cutting tip’s orifice. This swirl is generally in a clockwise direction which results in one side of the material being cut being squarer than the other.
This means it is very important to consider the travel direction in relation to the piece being cut. In the image below, an circle shaped object is being cut. The inside cut (A) is done in the anti-clockwise direction and the outside cut (B) is done in a clockwise direction.
So remember, if you are making a circular cut and plan to keep the inside round piece as your finished work, move in a clockwise direction. If you plan to keep the piece from which the circle was cut from then move in an anti-clockwise direction.
The formation of dross on the bottom of the plate can be caused when cutting parameters such as speed, amperage, arc voltage, gas pressure/flow and type of gas are not correct for the metal type and thickness being cut.
Most commonly, incorrect cutting speeds are to blame for excessive dross. High cutting speeds can results in “high speed dross” that can be very hard to remove without grinding.
“Low speed dross” can be easily removed with a brush or chip hammer.
Top Edge Rounding
Is when the top edge of the cut face has eroded away and is not square which is created from the plasma cutting process. It is generally caused when cutting with excessive current or standoff distance.
This can be a common occurrence when cutting thickness materials.