Why Does a Plasma Cutter Cut a Bevel?

How a Plasma Cutter Cuts

• Plasma cutters cut by sending an electrical arc through a cutting gas as it passes through a constricted opening in the tip of the cutter. The electrical arc elevates the temperature of the gas such that it enters a plasma state. This gas passes through the cutter's tip at high speed while the electrical arc is transferring energy to the metal being cut. This causes the gas to cut through the metal, blowing the molten scrap through the cut.

Beveling

• All plasma cutting causes some type of beveling, though it may vary in degree based on various cut factors. Beveling is caused by the motion of the gas as it exits the cutting tip. To create the plasma arc to cut the metal, the cutting gas must be shaped into a vortex. Consequently, the arc will have a direction of rotation, which causes beveling to be larger on one side of the cut than the other.

Types of Bevels

• There are three types of bevel -- positive, negative and irregular. Positive bevel is a cut where the top edge is smaller than the bottom edge. Most cuts will have a small degree of positive bevel. Negative bevel is a cut where the bottom edge is smaller than the top edge. This may be caused by a low arc voltage, excessive amperage or low speed, which will all cause the arc to remove more material from the bottom of the cut. A combination of positive and negative bevel in the same piece is caused irregular bevel. This usually indicates a problem with the cutting nozzle that has caused the arc to take a crooked path through the cut.

Ways to Avoid Beveling

• One way to decrease beveling is to cut at the proper speed and height for the material to be cut. Another is to provide constant air pressure and volume of clean, dry air. Beveling can be increased by worn out or clogged cutting tips, so tips should be checked regularly.