Shear cutting is one of the most widely used manufacturing processes in the production of sheet metal components. The reasons for this are the high output volume combined with low costs per part. The profitability of this process is significantly influenced by the lifetime of the active elements and the occurrence of unexpected process disturbances. While there are already many publications on the former, there are only few examinations on the wide spread process disturbance of slug pulling, which describes the phenomenon where the cut-out part is pulled upwards again during the punch return stroke. In particular, the different forces on the slug that cause this phenomenon have not yet been measured individually and independently of one another in one single tool. Thus, a shear cutting tool was developed that enables the measurement of the individual forces on the slug depending on various process parameters. Following, single stroke experiments were carried out to determine these forces and establish relationships between the process parameters, the characteristics of the slug and the measured forces. Finally, the interaction of all partial forces depending on selected process parameters is discussed in order to classify the relevance of every single force with regard to the occurrence of slug pulling. This understanding of the process is necessary in order to make a well-founded decision when designing future tools or selecting available remedial measures to avoid slug pulling.
Frictional forces in sheet metal blanking are central in different aspects, e.g. in wear prediction, validation of simulation models or in so called slug pulling. The latter is a phenomenon where the slug is pulled out of the die by the punch after the sheet metal is separated. This leads to process disturbances reaching from a blocked belt feeder up to severe tool damage caused by the simultaneous cutting of the slug and the sheet metal strip. A sufficiently high frictional force between the slug and the die prevents this effect. Despite its importance, this force and its causes have not yet been investigated in detail. A method was developed in this paper to measure the frictional force between slug and die. A shear cutting tool with an integrated piezoelectric load cell and an inductive position sensor was used on a stamping press to cut sheet metal made of CuSn6 (R350, thickness 1 mm). The die clearance, the punch edge radii and the lubrication conditions were varied. A larger die clearance resulted in a lower frictional force while a larger punch edge radius increased it significantly. Lubrication reduced the frictional force, especially for small die clearances. Finally, the cause of the frictional force was investigated by identifying the relevant springback modes of the slugs. This was carried out by correlating the slugs' deflection, oversize, and clean cut height with the frictional force. Especially the slug oversize, i.e. the difference between the slug's diameter and the die's inner diameter, revealed a strong correlation. Calculations showed that the deformation in radial direction is the main cause of the frictional force between slug and die. It suggests that the slug oversize is a good measure for the magnitude of the frictional force.
The primary goal when manufacturing components in a blanking process is a high output to achieve good cost efficiency. Therefore, availability needs to be as high as possible. However, several process disturbances like slug pulling increase downtime and thus counteract this aim. Slug pulling is influenced by different forces that trigger the slug being pulled and those that hamper this effect. The predominating hampering force is friction between the slug and the die. Consequently, the influencing factors for this force have to be understood to reliably prevent slug pulling. In this publication, the influence of the die channel geometry on the occurring frictional forces and the part quality when blanking the non-alloy quality steel 1.0338 are investigated. Therefore, experiments with a variation of die channel geometry and punch diameter combined with force measurement are performed. Furthermore, a numeric simulation model based on the experimental results is used to investigate various die channels. The results enhance the knowledge about correlations between process parameters, slug properties, like slug deflection, and frictional forces and help to reliably prevent slug pulling.
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