Conventional solid metal plate cutting technologies are well known in the art. These include oxy acetylene, plasma, water jet and laser. Each has their area of application. This disclosure is specifically restricted to plasma arc cutting of metals which include steel and aluminum.
Computer guided machines or Numerically Controlled (NC) machines appeared in the 1970s, machines which followed coordinates given in a plain English language known as Numeric Control or NC programs. Plasma cutting itself appeared in the 1940s as an extension of electric arc welding, carrying the very high currents in a plasma gas which melted rather than burned the metal.
The rapid drop in the price of computers after 2000 meant that computer controlled shape cutting has become affordable and standard in workshops. However the application of automated cutting of metal fabrics has met with two problems.
The first problem is height control, a problem even for nominally flat material as the plasma torch is sensitive to mm changes in height and must be kept with a narrow range, around 6 mm to 12 mm. With fabricated metal products, the height of the material can change dramatically, often by nearly the whole material thickness of say 14 mm but where cutting is still required.
Traditional NC two axis (XY) plasma cutting machines use a feedback device to control the Z axis. This feedback device utilizes plasma arc voltage to determine and maintain torch height, adjusting the height up if the voltage drops and adjusting the height down if the voltage increases. The feedback device operates independently of the NC control and is turned on or off. The feedback device presumes that the surface height changes slowly and smoothly and has problems with height changes resulting from sharp edges and holes. Expanded metals in particular can vary suddenly by 14 mm in surface height. Traditional NC THC (Torch Height Control) devices cannot react quickly enough for the sharp slopes. As the feedback device operates independently from the NC control, it cannot detect holes in the material being cut and therefore causes the plasma torch to plummet into these holes, causing mechanical damage to components (e.g., cutting torch) of the NC cutting machine.
Secondly, there are large holes in fabricated material as part of the fundamental design of the material. Worse, while generally regular, in a given NC path, these are unpredictable. Holes are a disastrous problem for traditional NC plasma cutting machines designed for continuous cutting of solid plate. There is little or no provision for running off the plate or into a hole. This can damage the torch of the NC cutting machine.
Plasma torches produce arcs which reach 10,000 C where the arc conducts to nearby metal. Movement off a metal surface usually means the arc will fail or in the worst case, blow back into the plasma torch, destroying the torch. Plasma is usually used to cut solid, flat material like steel plate. It is rarely if ever used to cut materials with holes. A high frequency high voltage pilot arc is used to test for conduction to metal and if successful, an attempt is made to establish a high current Direct Current plasma arc of between 10 and say 400 amps at a voltage between 100 and 180 volts DC. This technique is well established in the art.
It has to be mentioned too that the problems of cut width or kerf are still present even though cutting such difficult material. As plasma cut width is not trivial and ranges from 2 mm to 5 mm, the desired cut path has to be offset by half the width of cut on the side of the torch which is in the scrap. This can be a complex calculation which is a standard part of NC path control and has to be solved for accurate cutting.