Using stamping punch force variation for the identification of changes in lubrication and wear mechanism

The growth in use of Advanced High Strength Steels in the automotive industry for
light-weighting and safety has increased the rates of tool wear in sheet metal stamping. This is
an issue that adds significant costs to production in terms of manual inspection and part
refinishing. To reduce these costs, a tool condition monitoring system is required and a firm
understanding of process signal variation must form the foundation for any such monitoring
system. Punch force is a stamping process signal that is widely collected by industrial presses
and has been linked closely to part quality and tool condition, making it an ideal candidate as a
tool condition monitoring signal. In this preliminary investigation, the variation of punch force
due to different lubrication conditions and progressive wear are examined. Linking specific
punch force signature changes to developing lubrication and wear events is valuable for die wear
and stamping condition monitoring. A series of semi-industrial channel forming trials were
conducted under different lubrication regimes and progressive die wear. Punch force signatures
were captured for each part and Principal Component Analysis (PCA) was applied to determine
the key Principal Components of the signature data sets. These Principal Components were
linked to the evolution of friction conditions over the course of the stroke for the different
lubrication regimes and mechanism of galling wear. As a result, variation in punch force
signatures were correlated to the current mechanism of wear dominant on the formed part; either
abrasion or adhesion, and to changes in lubrication mechanism. The outcomes of this study
provide important insights into punch force signature variation, that will provide a foundation
for future work into the development of die wear and lubrication monitoring systems for sheet
metal stamping.