Tweco PC250 - Manual (Page 15)

OPERATING TIPS
A. Piloting
Piloting is harder on parts life than actual cutting because the pilot arc is directed from the electrode to the tip
rather than to a workpiece. Whenever possible, avoid excessive pilot arc time to improve parts life.
B. Torch Standoff
Improper standoff (the distance between the torch tip and workpiece) can adversely affect tip life as well as shield
cup life. Standoff may also significantly affect the bevel angle. Reducing standoff will generally result in a
reduced bevel angle.
C. Edge Starting
NOTE Edge starting is not recommended for machine type operations as most of the operations use the standoff
method of starting and finishing on the work piece. For edge starts, hold the torch perpendicular to the workpiece
with the front of the tip near(not touching) the edge of the workpiece at the point where the cut is to start. When
starting at the edge of the plate, do not pause at the edge and force the arc to "reach" for the edge of the metal.
This effect will cause reduced tip life. Establish the cutting arc as quickly as possible.
D. Direction of Cut
In the Torch, the plasma gas stream swirls as it leaves the torch to maintain a smooth column of gas. This swirl
effect results in one side of a cut being more square than the other. Viewed along the direction of travel, the right
side of the cut is more square than the left (Refer to Figure below). To make a square-edged cut along an inside
diameter of a circle, the torch should move counterclockwise around the circle. To keep the square edge along
an outside diameter cut, the torch should travel in a clockwise direction.
E. Dross
When dross is present on carbon steel, it is commonly referred to as either "high speed, slow speed, or top dross".
Dross present on top of the plate is normally caused by too great a torch to plate distance. "Top dross" is nor-
mally very easy to remove and can often be wiped off with a welding glove. "Slow speed dross" is normally pre-
sent on the bottom edge of the plate. It can vary from a light to heavy bead, but does not adhere tightly to the cut
edge, and can be easily scraped off. "High speed dross" usually forms a narrow bead along the bottom of the cut
edge and is very difficult to remove. When cutting a troublesome steel, it is sometimes useful to reduce the cut-
ting speed to produce "slow speed dross". Any resultant cleanup can be accomplished by scraping, not grinding.
15
Side Characteristics of Cut
A-00512
Left Side
Cut Angle
Right Side
Cut Angle