The smaller the drill bit, the more likely it is to break. This is what every do-it-yourself enthusiast discovers at some point, meaning a trip to the hardware store to pick up a replacement. While this is annoying for the handyman, in industry it is a serious financial consideration. The Materials Center Leoben (MCL) started searching for solutions for drills which operate under extreme conditions. Diesel injection nozzles made of heat treatable steel, for example, have holes drilled over 100 mm deep yet only 2 mm in diameter. When these injection nozzles are produced on a large scale, tool failure impacts on the unit cost of the fabricated item in two ways: firstly, manufacturing special drills is costly and secondly, changing tools is time consuming and reduces productivity.
The MCL’s materials research revealed that the determining factor for whether or not a drill fractures is the length of the chips. At their narrowest point these chips are only ~ 40 μm thick (about half the thickness of a human hair) and up to several centimeters long. If chips are too long they become entangled in the drill hole and obstruct the drill, which then inevitably breaks.
But why does the length of the chips vary? The question was solved by conducting experimental investigations and checking the results through high resolution SEM images. The steel in which the holes are drilled has microscopically small inclusions of non-metallic materials which arise during steel production. Many of these inclusions are harder than the surrounding steel matrix and it is precisely these particles which help cause the chips to break as desired.
Using finite element simulation of the process of chip formation, it was possible to observe the individual effects of drill geometry (state of wear), friction between the chip and drill (tool coating) and process parameters (cutting speed and advance) on the chip geometry produced (chip thickness) and the stress imposed on the drill (local stresses and temperatures, global forces). Combined with specific experimental investigations, this method of analysis provided a better understanding of the critical process parameters for chip formation and breakage.
Impact
The detailed understanding of the most significant influencing parameters in deep hole drilling now enables the targeted selection of materials and drilling parameters in order to enhance process reliability.