The Thermal cutting techniques all have interchangeable nozzles at the tip of the cutting head
Nozzel has several differten purposes for the three differens thermal cutting methods, Laser cutting , Plasma cutting  and Flame cutting . This machine part that is most frequentc replaced due to different kind of wear.
The primary purpose for the nozzles is to aim the gas flow towards the cutting area. A small deviaiton in gas flow direction or cross section form can have great negative impact of the cutting result. This gas flow disturbance is often caused by...
The themal expansion effect and/or internal stress caused by earlier sheet rolling processes can result in the cutted part to rise from the sheet. If additional cuts are made clos to a previus cut or a transpartions of the cutting head is programmed to pass over one, a collision can occur with the work piece and the nozzle causing the gas flow to be disturbed.
For Laser cutting an collission can also cause the centering of the laser beam to be offset. Even though the hole might still be not physically damaged the off centering will result in the gas beam to mis aligned.
Splatter of molten metal during piercing can cause one of more of the channels to clog.