Punching Force Reduction with Wave-Formed Tools

Flat-end tools are the most general types used in sheet metal punching and nibbling. They are geometrically simple and easy to sharpen but, on other hand, their cutting forces are relatively large, and hence the cutting process is frequently noisy. In order to reduce both cutting force and noise tools with one-way or two-way shearing have been utilised. The major drawbacks of these tools are the asymmetry of cutting easily causing non-circular holes with round tools, lateral forces with one-way shearing, excessive forming during cutting and more complex tool geometry to maintain. Here a new geometry for a punch is employed. The shearing edge is a sinus curve with several peaks making the cutting edge circularly symmetric and the phenomenon totally balanced. This means smaller forming forces, particularly in cases when also the radial form is concave. The geometry is without doubt more complex compared to the flat-end tool but rather easy to produce by multi-axis milling and electro-discharge machining. In the current work a set of experimental punches has been designed, manufactured and tested. A simple analytical theory for cutting force has also been derived and compared with the test results. The results show that the new geometry produces very precise hole geometry with a lower cutting force compared to conventional flat-end tools. Of course, more theoretical and experimental work is required to optimise the tool geometry including the tool clearance for each pair of material strength and thickness.