Prediction of Draw Bead Coefficient of Friction Using Surface Temperature

Sheet metal stamping involves a system of complex tribological (friction, lubrication, and wear), heat transfer, and material strain interactions. Accurate coefficient of friction, strain, and lubrication regime data is required to allow proper modeling of the various sheet stamping processes. In addition, non-intrusive means of monitoring the coefficient of friction in production stamping operations would be of assistance for efficiently maintaining proper stamping quality and to indicate when adjustments to the various stamping parameters, including maintenance, would be advantageous.

One of the key sub-systems of the sheet metal stamping process is the draw bead. This paper presents an investigation of the tribology of the draw bead using a Draw Bead Simulator (DBS) Machine and automotive zinc-coated sheet steels. The investigation and findings include:

1. 1)
   A new, non-intrusive method of measuring the surface temperature of the sheet steel as it passes through the draw bead. (The method can be used in experimental or in production environments.)
2. 2)
   The temperature rise of the surfaces are shown to correlate with the measured coefficient of friction.
3. 3)
   The lubrication regime is shown to be “Boundary Lubrication” in the draw bead for sheet speeds even in excess of typical production speeds.