Laser drilling of micro-holes with small diameter beyond the limits of focused spot by using a sieve plate or a cover plate

To break through the limits of the focused spot diameter to achieve holes with smaller diameter, a method is proposed by using a sieve plate or a cover plate. The experiments were conducted on 2-mm-thick stainless steel workpieces by using a millisecond pulsed laser. Four variables including pulse frequency, focal plane position, sieve diameter, and distance were chosen as the parameter variables. The entrance and exit diameters were measured with a confocal microscope. And, the taper was calculated. The results show that the focal plane position and distance were significant influencing factors. Negative focal plane position and a short distance could get smaller-diameter holes with lower taper. However, the pulse frequency had no significant influence on the entrance diameter and taper. A small sieve diameter was useful to get smaller-diameter holes with lower taper; however, it needed to apply a thicker cover plate to obtain a small diameter hole. The smallest holes obtained by using a sieve plate or a cover plate were at least 10 % less than the focused spot diameter and 20 % less than the diameter by laser directly drilling.