Direct friction measurement in draw bead testing

The application of draw beads in sheet metal stamping ensures controlled drawing-in of flange parts. Lubrication conditions in draw beads are severe due to sliding under simultaneous bending. Based on the original draw bead test design by Nine [1] comprehensive studies of friction in draw beads have been reported in literature.

A major drawback in all these studies is that friction is not directly measured, but requires repeated measurements of the drawing force with and without relative sliding between the draw beads and the sheet material. This implies two tests with a fixed draw bead tool and a freely rotating tool respectively, an approach, which inevitably implies large uncertainties due to scatter in the experimental conditions.

In order to avoid this problem a new draw bead test is proposed by the authors measuring the friction force acting on the tool radius directly by a build-in piezoelectric torque transducer. This technique results in a very sensitive measurement of friction, which furthermore enables recording of lubricant film breakdown as function of drawing distance.

The proposed test is validated in an experimental investigation of the influence of lubricant viscosity on the limits of lubrication in drawing of stainless steel sheet.