An experimental investigation on ultra-precision instrumented smart aerostatic bearing spindle applied to high speed micro-drilling

High speed drilling of small diameter holes ranging from 0.1 to 1 mm in printed circuit board (PCB) drilling industry has been largely carried out by using tungsten carbide twist drills and high speed spindles. In high speed drilling, however, the tool wear leads to increased cutting forces and results in poor hole quality as well as the hole location inaccuracy. Both hole quality and location accuracy are further considered as significant issues in drilling multilayer and heavy copper layered PCB workpieces. This paper presents the experimental development of an instrumented smart spindle, which can examine the tool wear by measuring the axial shaft displacement (known as the axial displacement) on the fly and thus applicable to real-time drilling processes. Cutting trials using the instrumented smart spindle and a Kistler dynamometer (9345A) on the customized multilayers PCB workpiece have shown a good correlation between the axial displacement and axial drilling force measurements. The axial displacement is capable of being used to detect the drilling engagement with each copper layer. Furthermore, the instrumented smart spindle can additionally be used to prevent a thrust bearing from being over-loaded and most importantly to detect tool wear in process.