Permissible internal pressure in dies for cold extrusion may be increased by the use of one or two shrink rings. The design essentially boils down to defining the diametrical interference fit of the die/ring assembly. There are two methods of design: analytical one based on Lame’s solution and numerical one based on finite element modelling (FEM). None of these methods includes roughness of the interface surfaces of the die insert and the shrink ring. However, when designing the pre-stressed micro-dies, the interface roughness cannot be disregarded and the classical design of the diameter interference must be corrected. In this paper, a semi-physical modelling of interface roughness (SPMIR) is used as novel method which determines the interference correction value. A set of FEM models created on the base of Abbot-Firestone curves, determined from the roughness profile, enables determination of a contact surface stiffness curve and further the interference correction. Relative correction of the die diameter interference increases with the diameter decrease, which might be recognized as a pre-stressed micro-die assembling scale effect. In the experimental part, three miniature dies, with interface diameter 2.8 mm and different levels of interface surface roughness, Ra=0.16, Ra=0.43, Ra=0.60, have been manufactured by WEDM. Relative interference corrections calculated by the SPMIR method reached respectively 17.6, 27.9 and 43.3 %. A practical design advice has been formulated as follows: interference correction based on the interface surface topology is recommended for micro-dies with the interface diameter less than 10, 15 and 25 mm, for the three levels of the surface roughness investigated.
|Number of pages||6|
|Publication status||Published - 15 Nov 2017|
- semi-physical modelling
- surface roughness