Assume that the surface of a table on which a workpiece is mounted is flat, and the thickness (plate thickness) of the workpiece is substantially uniform. In this case, a measured height of the top surface of the workpiece at a desired position may be regarded as the height of the top surface of the workpiece. When a hole is machined out in the workpiece, the depth of the hole is generally designated on the basis of the surface of the workpiece. Accordingly, if the position (height) of the top surface of the workpiece is determined, a hole which is superior in depth-direction accuracy can be machined out.
There may be a variation in the plate thickness of the workpiece. In this case, the height of the top surface of the workpiece may be measured at every place to be machined. Alternatively the height of the top surface of the workpiece may be measured at a plurality of places, and an average value of the measured values may be regarded as the height of the top surface of the workpiece when the workpiece is machined.
When a hole is machined out with a mechanical drill, a surface height Zn of a workpiece at a place to be drilled is detected, and compared with an average value Zm of surface heights at N positions, which have been drilled immediately before the aforementioned place to be drilled. When the absolute value of the difference (Zn−Zm) is smaller than a predetermined value δ, drilling is performed. When the absolute value is not smaller than the value δ, it is decided that swarf or the like is caught between the workpiece surface and a substrate pressure plate, and the machining is suspended. Drilling is resumed after such a cause is removed (Patent Document 1).
In recent years, lasers have been used as means for machining small-diameter holes. When a hole is machined by use of a laser beam, the outer shape of the laser beam is shaped into a desired shape by an aperture, and converged by a lens so that an image of the aperture is formed, for example, on the surface of a workpiece. In this case, the allowable value for height variation of the image position is not larger than 30 μm.
Patent Document 1: JP-A-9-277140
The time of machining by a laser is an order of milliseconds. It is therefore not practical to measure the height of the surface of a workpiece in every machining time as shown in Patent Document 1. Even when the height of the top surface of the workpiece from the surface of the table is measured only at a plurality of places, the measuring time becomes longer than the machining time. Thus, the total machining time becomes impractically long.
The plate thickness of a workpiece where holes will be machined by a laser is generally substantially uniform. It has been therefore believed that the machining quality will be uniform if the height of the top surface of the workpiece is measured at one place. However, there has occurred a variation in machining quality in spite of the uniform plate thickness of the workpiece.
The present inventors discovered the following fact. That is, there may be irregularities of about 10-30 μm high in the table surface which has been regarded as flat. The height of those irregularities is close to the allowable value for height variation of the image position. Thus, there occurs a deterioration in machining quality.