Sheet metal trimming dies – characterisation methods of geometry and surface topography and the influence on wear

Licentiate thesis, 2010

In recent years, the life cycle of car models has decreased from 7 to 5 years and the volume sizes are becoming shorter in the automotive industry. The increasing number of car models introduced to the market means that a fewer number of parts must pay the costs for the tool/die design and manufacturing. In consequence, there is a need for reducing the manufacturing cost and lead time for the tool and die manufacturing. The higher product requirements (reduced weight, increased safety and lower emissions) have meant that a larger number of car body parts must be made in AHSS (Advanced High Strength Steels). This has led to a significantly larger abrasive and fatigue tool wear. The tool wear is therefore expected to increase even more in the future, if the issue is not addressed properly. Today the total maintenance cost of the trimming tools are approximately 70% of the total die maintenance cost in a press shop. The purpose of this thesis is to increase the knowledge of how surface topography and geometry on trimming dies influences tool wear, tool life and part quality. Methods to analyze wear, measure cutting edge radius on production dies and methods to analyze surface topography have been established. The cutting edge geometry has been evaluated with respect to the cutting edge radius and the angle of the upper tool. Experimental testing together with FE simulation has been used. Results show that by using a cutting edge radius of 80µm, rather than 20µm, the tool “wear in phase” can be more controlled. Sheet metal trimming when cutting at high angles (25° - 60°) has been evaluated. Two versions of the geometry on the lower cutting edge was tested, a traditional geometry and a 1.5mm shelf geometry. Results show that the shelf geometry reduces side force, especially for the higher angles. The shelf geometry also reduces the sharpness off the cut edge profile, thus increasing part quality, compared to the traditional geometry. Surfaces on both semi-industrial test dies and dies used in production were analyzed with different surface topography analysis methods. The result shows that the wear process is different on the different sides of the trimming edge. The texture direction of the surface is changed due to wear, with the strongest change on the clearance side of the edge. The roughness on sheet side of the edge is reduced due to a hammering effect from the sheet material. The result on the semiindustrial test dies and the production dies correlates well. Future research will be continued on surface topography and trimming edge geometry and its relationship to used manufacturing methods and tool wear.