The invention pertains to a process and equipment used to cut workpieces by thermal means.
At the present three processes are mainly to cut workpieces thermally in industrial applications. These include oxyacetylene torch cutting, plasma cutting and laser beam cutting.
Oxyacetylene torch cutting, can be used only for steel, in the absence of special adaptive measures. Here heating is primarily by means of a chemical reaction. The rate of the reaction is dependent on the diffusion rates of the co-reactants. The cutting speed in turn is dependent on this reaction rate. When construction steel in a greater thickness range is cut, the cutting speed is between 0.5 and 1.5 meters per minute. The cutting kerfs here will vary in size, ranging in widths of up to a few millimeters.
In plasma cutting energy is applied by means of an electrical arc or plasma which exhibits high electrical power of up to and beyond 100 kW. The heated material is removed from the cutting kerf with the aid of the momentum of the plasma beam. At shallower cutting depths, the cutting speeds for plasma cutting are higher than those for oxyacetylene torch cutting. But, the cutting kerfs at cutting depths of 3 to 10 mm are quite wide, which has an influence on the precision of the cut.
In laser beam cutting, energy is applied by a laser beam and the melted or vaporized material is removed from the cutting kerf by means of a stream of gas. Laser beam cutting gives favorably narrow cutting kerfs and consequently high precision at high cutting speed and low thermal loading of the workpiece being cut. At greater cutting depths, in steel more than 10 mm thick, for example, and depending on the width of the cutting kerf, it becomes impossible to achieve the high cutting speed determined by the laser beam acting as the energy transfer medium since at the high flow rates required for the gas stream to remove the molten material a supersonic shock wave separates from the cutting face after a few millimeters of penetration into the cutting kerf, thus reducing accordingly the effective gas speed at the point where it ought to be high. Thus in laser beam cutting the cutting rate for thicker workpieces is limited since greater momentum cannot be transferred with a stream of gas in a satisfactory way. A medium with a greater specific density than gas, a liquid, for instance, with which greater momentum could be applied, cannot be used in laser beam cutting because light at the wavelength of a CO.sub.2 laser is absorbed in all liquids which might be considered for this purpose.