Tungsten Carbide Micro Drills and Micro DrillingⅡ

In part 1 we know that micro drilling means a special drilling cycle and precise rotation of micro drills which is made to spade shape of diameter less than 50μm. Here we talk about the first important geometric characteristic of tungsten carbide spade-type micro drills.

There are several important geometric characteristics of tungsten carbide spade-type micro drills. First, the point of the micro drill is not a point at all. Even on conventional twist drills, the end is not truly pointed. Instead, the end of the micro drill consists of a cutting edge (called the chisel edge) made by two intersecting planes which also define the two primary cutting edges of the drill. The chisel edge removes material primarily by extrusion and cutting at high negative rake angle. The specific cutting energy along the chisel edge is relatively large compared to the drill's primary cutting edges. The chisel edge also adds to the drilling complexity because of the lack of a point. As the rotating micro drill first contacts the work piece (remember the micro drill has a very small structural rigidity) anything on the surface, including micro roughness and material slope, will cause the drill to walk on the surface as it is trying to begin removing material. Walking is characterized by an eccentric motion of the drill as it turns perhaps coupled with a non-time- varying bending of the drill about its longitudinal axis. Depending on the feed per revolution of the drill during hole start-up, the drill may begin drilling at a slant with the drill deflected like an end-loaded cantilever beam (which it is with superimposed column loading). If permitted to continue, the drill will quickly break. If the drill is strong enough to survive the large stress imposed in it due to micro drilling at a slant, the resulting hole will be slanted rather than normal to the work surface.

(To be continued. This article is divided into several parts and this is part 2, for part 1 please refer to http://news.chinatungsten.com/en/tungsten-information/79981-ti-10250)

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