Prediction and minimisation of sheet thickness variation during deep drawing of micro/milli parts

An important problem encountered in the deep drawing processes of sheet metals is represented by the sheet thickness variation that can influence and affect the quality of drawn parts, may cause stress concentration in different zones of drawn parts or may conduct part fracture. The variation of sheet thickness during cold formation can have a greater importance when the drawn parts are made with small dimensions from metal sheets having small thickness. The present paper analyses experimental and numerical simulation results concerning the thickness variation in the case of micro/milli-cylindrical drawn cups made from sheets, called foils, having thicknesses from 0.05 to 0.20 mm. A mathematical model for predicting the thickness variation is also developed, based on the variation of the unit radial forces developed in foil thickness, as a function of part geometry and material yielding trajectory. On this basis it is possible to control and minimise the thickness variation in the following part zones where such phenomena can generate negative effects: part wall where the variations of part diameter, wall inclination and wall curvature are the negative effects caused or influenced by the foil thickness variation and wall–bottom connexion zone where a negative effect can occur if the foil thickness variation results in the loss of material integrity.