A method and apparatus for sensing a pierce-through condition of a material made by a piercing force in which a shield (15) surrounding a source of the piercing force is supplied with a gas supply (40) to create a pressure within the shield means. A decrease in pressure caused within the shield by a pierce-through condition created by the piercing force is then detected. The method and apparatus is also for detecting the distance between a nozzle assembly (10) for a machining process and a workpiece (16) to be machined in which gas is supplied to a shield surrounding a nozzle assembly. An increase in pressure in the shield is detected as an open end of the shield (15) approaches a workpiece. The method and apparatus is also for obtaining and maintaining a predetermined gap distance between the nozzle assembly (10) and a workpiece (16) by further detecting when the pressure within the shield means reaches a pressure corresponding values and ranges. The thickness of the workpiece is ascertained by comparing an established nozzle assembly position with a predetermined reference position.