The shape of the arc from a plasma torch
In plasma cutting, a high-temperature, highly pressurized, highly charged gas is forced through a small nozzle orifice inside the torch. The expanding plasma jet is narrow near the orifice and broader as it moves away from the nozzle.
The gaseous mixture of highly charged ions seeks electrons to neutralize its positive charge. The electrons are drawn from the workpiece and from the atmosphere. If the workpiece is not present, the arc gradually loses its temperature, its pressure, and its electrical charge, tapering itself along the way.
The plasma arc is narrow at the top, wider in the middle, and narrow again at the bottom.
At first glance, it looks as if a short arc would cut with the best edge quality since the plasma is the hottest, most forceful, and most electrically charged at the top. But a short arc length also creates a considerable bevel at the kerf.
If the arc is too long, the bevel is reversed, the narrowest point is at the bottom of the kerf.
The best quality edge is usually in the middle.
Plasma Arc Voltage Control
A torch height control system is used to maintain the optimal height for plasma cutting, and one method used today is by sensing the cutting voltage. The cutting voltage is proportional to the distance between the torch and the workpiece, so it can be used to adjust the standoff height. Also known as arc voltage sensing, AVC, this method is not completely without difficulties.
1. When the CNC sends a command to start cutting, the THC moves the torch down, touching the workpiece. This is the rock-bottom point, where a high intensity spark is created. This spark creates a pilot arc, and the the torch now moves upwards to the pierce height.
2. This small volume of plasma will, in turn, light the main flow of gas. Plasma cutting begins and the metal gets pierced. Molten material jumps out of the workpiece, forming a pool. Since the molten metal cannot yet be removed by the force of gravity, the torch needs to continue moving upward to a programmed molten pool jump height while piercing.
3. Once the molten pool is drained, the torch now moves down to the programmed cut height. The THC unit now sends a signal to the CNC to start the XY Motion.
4. The torch continues to move at the same cutting height. Arc voltage is constantly fed back to the controller unit, which uses a PID control loop to keep the arc voltage constant.
Height control is automated based on the measured voltage of the plasma arc. The voltage is lower for shorter distances and higher for greater distances. The arc is extinguished when the the power supply cannot produce a voltage for the arc to travel the distance. So, a good Torch Height Control is crucial for uniformly good quality cuts.