POSSIBILITIES TO MEASURE CONTACT FRICTION IN BULK METAL FORMING

Friction occurs in all metal forming operations and, in general, it has a negative impact on process parameters, die life, as well as workpiece quality. In cold metal forming processes high interface pressures between die and material take place. These pressures can be the limiting factor of application of cold forming. By reducing interfacial friction, contact pressures can be reduced too. Knowledge of friction amount (factor of friction mu and/or friction ratio m) is essential for calculation of the main process parameters (load, energy), for choosing a proper lubricant but also for numerical modelling of forming operations. There exist a number of experimental methods to determine friction in metal forming processes. Current paper deals with the possibilities to evaluate friction. It analyses and assesses a number of existing friction models in cold metal operations. Proposal of a new friction model for cold metal forming operations is presented and discussed. Measurement of friction force in backward extrusion for different lubrication conditions was also preformed.Original Abstract: Trenje je neizbjezna pojava u svim postupciima plasticnog deformiranja metala i ima negativan utjecaj na parametre procesa, radni vijek alata, kao i kvalitetu radnog komada. U hladnom prostornom deformiranju veliki pritisci djeluju izmedu alata i radnog komada. Ovi pritisci mogu biti limitirajuci faktor prilikom primjene tehnologija deformiranja. Smanjenjem kontaktnog trenja, smanjuju se i kontaktni pritisci. Poznavanje parametara trenja (faktora trenja /gm/ i/ili omjera trenja m) je od velikog znaciaja prilikom proracuna parametara procesa (npr.sila ili energija) i odabira odgovarajuceg sredstva za podmazivanje, kao i numericku simulaciju obrade. Postoje mnogobrojne metode za odredivanje parametara trenja u obradama deformiranjem. Neke od ovih metoda opisane su u ovom radu. Takoder je predstavljen i novi model za odredivanje parametara trenja u obradama deformiranjem. U radu je izvrseno i mjerenje sile trenja u procesu protusmjemog istiskivanja za razlicite vrste maziva.