Plasma Piercing

Piercing too close to the material is a common cause of premature consumable parts failure. The intention of this document is to arm the machine programmer and operator with knowledge of the piercing process to help understand why it is so important.

What Happens During a Pierce? When the torch fires a pilot arc is produced which transfers to the plate below. This arc heats the material and its high velocity gasses begin blowing out the molten metal. This molten mass sprays out axially from the pierce point over the top of the material. As the pierce progresses a round bottomed hole is formed which directs the molten spray upwards towards the torch. Once the arc pierces the bottom of the plate the molten material is ejected through the pierced hole.

The length of time that the torch is active in piercing is called the ‘pierce delay’. If this is too short then the arc may not have fully penetrated the material. If too long then the flame may be extinguished as it creates too great a gap to sustain the arc.

Under normal conditions the nozzle controls the arc column which has been focused by the swirling vortex of gases passing the electrode. These gasses provide a boundary which prevents the arc from contacting, and hence deforming, the copper nozzle.

Piercing Problems Double Arcing: If the torch is placed too close to the plate pre-pierce then the plasma jets can disrupt the electric field surrounding the arc and pull the arc into the nozzle sidewall. Damage in this way leads to serious cut problems like excess bevel angle, dross or failure to penetrate the material.

Bridging: is a more extreme form of the above. Molten material builds up in a puddle that contacts both torch and plate. This usually involves catastrophic failure of the shield and damage to the nozzle.

Snuffing: is the most extreme form of double arcing, produced when the torch begins to pierce with the torch nozzle or shield pressed against the plate. This leads to uncontrolled double arcing in the plasma chamber of the torch and catastrophic failure of the electrode, nozzle and shield.

Too High: Piercing from too high can cause excessive pilot arcing and hence damage to the nozzle.

Tips and Techniques:

 Pierce high and cut low: the rule of thumb is to pierce at 1.5-2X the cut height. Piercing high prevents double arcing, bridging, and snuffing.  Use a creeping pierce: if your CNC is capable, use a creeping pierce. This feature slowly moves the torch during the pierce operation causing the "rooster tail of molten material to miss the front end of the torch. (Think of the shot glass with the water streaming into one side rather than the middle.)  Don't eyeball the pierce height: Use the initial height sensing if it is available. For manual machines use feeler gauges.  Don't pierce beyond system limits: Pierce rating is typically ½ the cut rating  Avoid piercing: whenever possible use chain cutting or edge starting (for example off the edge of a punched hole) to reduce the number of pierces.