Laser drilling can be divided into three modes: direct drilling, percussion drilling and trepanning (in order of increasing hole quality).
With direct drilling, the hole is produced in a single shot (laser pulse). This considerably limits the maximum achievable hole depth but is a very fast technique. However, direct drilling in general gives a considerable degree of hole taper.
With percussion drilling, a train of pulses is fired at a fixed location on the workpiece. Each consecutive pulse removes a small layer of material deepening the hole. This makes this technique very suitable for small diameter, high quality holes with high aspect ratios (depth to width ratio). Percussion drilling is quicker than trepanning (see below), but it is of limited use for larger diameter holes.
With trepanning, firstly a hole is quickly drilled to pierce the material and the drilling laser beam is then moved around the perimeter of the desired hole. Therefore, the hole is effectively cut rather than drilled. Trepanning can give very good quality holes, particularly if larger diameters are required. However, trepanning is obviously slower than both direct and percussion drilling.
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