Godding, Robert
In the body shops, where the stamping parts are assembled, the punctual geometry measurement (e.g. of welding points)
is state-of-the-art. The installation of various laser-sensors is necessary to check the location of one point at a time.
However, the number of sensors is limited by the working space of the robots. Furthermore, the consequent reduction of
working steps and, thus, the number of assembled parts, implied the enlargement of their size (Figure 2).
Today car side components are front fender panel hood outer door panel
mostly pressed in one single stroke.
Therefore, the measurement of
extensive stamping part areas is
indispensable. However the large
dimensions of the formed parts are in
contradiction to the high resolution
required to detect the comparatively
small deviations as well as the short
time available for the quality control car roof side component
of each part within the scope of the
mass production. For the automated
joining technique applied in body
shop by robots failure-free and
accurate stamping parts are an
essential prerequisite. The stamping
part accuracy has a key position
achieving close tolerances in the
assemblies
© IFUM
Figure 2: Visible stamping parts of a car (large-sized)
However, sheet metal stamping of components becoming more and more complex is characterized by several structural
stamping defects like necking, cracks and wrinkles as well as geometrical deviations due to springback /1-5/ (Figure 3
and 4). Avoiding rejects requires quality control throughout the process chain. Strain analysis is only useful during the
try-out of new tools or after production process parameters have been modified (e.g. the lubrication). Springback should
be measured during the production process.
wrinkles in ES
the wall
spring back
bulges
scratches /
surface-
defects
puckers
Figure 3: Structural stamping defects Figure 4: Shape-deviations of formed parts
2 SPINGBACK ANALYSIS
Sheet metal forming processes involve a combination of elastic-plastic bending and stretching deformation of the
formed part. These deformations can lead to large amounts of springback upon unloading (e.g. removal of the punch
and lifting of the blankholder) due to the redistribution of residual stresses.
The legal demand to reduce the fuel consumption of cars have forced the automotive industry to realize car lightweight
concepts using new sheet materials. Nevertheless especially the application of modern high strength steels (HSS) with
reduced sheet thickness and aluminum alloys have enlarged the difficulties to obtain close tolerances because of part
shape deviations due to springback. The difficulties are still increasing with the automation in production and
assembling since springback causes assembly difficulties with adjacent parts.
Springback is influenced by a variety of factors, e.g. mechanical properties of the sheet material, frictional conditions
(e.g. sheet and tool topography, lubrication), tool geometry (e.g. die radius, die clearance) as well as process parameters
(e.g. blankholder forces) /6-8/. Avoiding springback presupposes homogeneous stress and strain distributions during the
forming process. Therefore, springback can be alleviated by the optimization of material flow between the blank holder
and the die e.g. by the variation of the tool geometry, the blankholder pressure and lubrication. The prediction of
292 International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B5. Amsterdam 2000.
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