Plasma cutting has become a very popular method of cutting for many metal fabricators because of the speed and precision of the cutting process. Today, plasma cutting systems have the capability of producing precise and high-tolerance cuts quickly and effectively. However, the entire process centres around one important aspect i.e. piercing.
Following proper piercing techniques can help to improve productivity and lower cost of operation. And, following improper piercing techniques is the main cause of the premature failure of parts in the plasma torch.
Steps of a Pierce
A pilot arc is produced when the plasma torch fires and a spark of high-voltage combine with electric current. The pilot arc creates a circuit from the torch’s electrode to the material being cut. The arc will transfer to the conductive metal and start to pierce if the plasma torch is within the transfer distance. Every plasma torch has a maximum limit of transfer distance, which is also the maximum pierce height of the torch and if the plasma torch is further than this distance, the pilot arc is formed in the air and if it is much lower, then the arc will transfer to the workpiece and start piercing.
The various stages of the pierce are:
The pilot arc that has been transferred, heats the metal to a very high temperature and the molten material is blown away. The molten material spurts out from the point of pierce axially over the top of the workpiece in the initial stage of the piercing process.
When the plasma jet enters deeper into the workpiece, a round hole is formed, which starts directing the molten metal towards the plasma torch.
When the plasma arc breaks through the workpiece’s bottom. The molten material and sparks are expelled out of the hole that has been pierced. When the arc grows to its fullest strength and the pierce is finished, the plasma cutter moves and the torch begin the cutting process. If the plasma torch moves very quickly the arc will not penetrate the workpiece fully and if the plasma torch is very slow, the arc keeps cutting the material until it extinguishes.
Some common problems that are encountered while piercing are bridging, double arcing and snuffing, molten metal sprayed back to the plasma torch and these may cause the plasma torch and the consumables to be damaged and result in failure or the torch, which in turn will lead to inferior cut quality.
And some of the techniques that can be used for proper piercing are – to use a creeping pierce, use initial height sensing and not eyeball the pierce height, pierce at a high cut height and cut low, avoid piercing beyond the limits of the plasma system and reduce the number of pierces.
Piercing is a very critical aspect of plasma cutting and all the factors must be considered by the operator required for proper piercing before going ahead with the piercing process.