Analysis of Pick-Up Development in Punching

The development of pick-up of work piece material on the punch surface with increasing number of strokes during punching of stainless steel sheet is investigated applying Scanning Electron Microscopy (SEM), 3D roughness mapping and measurements of the backstroke force. It is found that pick-up on the punch stem develops gradually backwards from the tip of the punch with increasing number of strokes saturating at a certain level while the radial build-up in the case of dry friction conditions continues until the clearance between the punch and die has been filled up. The rate of pick-up is significantly smaller in case of lubrication and may be very low if an efficient boundary lubricant is applied. Good correlation between the pick-up development and the backstroke force is found.