The process limits in sheet metal forming are determined not only by the material but mainly by the friction and lubrication conditions. Due to products increasing in complexity and size, as well as the use of high-strength steels, the tribological conditions will become even more important in the future. The coefficient of friction is an essential variable for the description of the tribological conditions. Since the interactions within the tribological system are very complex, the friction can only be determined if the entire tribological system is considered.
Usually, it is not possible to investigate the friction during the real process because to both economical and metrological reasons. Therefore, appropriate model experiments have to be developed to simulate the relevant tribological systems in sheet metal forming and to measure the friction coefficient directly.
At the Institute for Production Engineering and Forming Machines a friction test method was developed, which is based on the strip drawing test and which works according to the VDA standard.
Thereby, a 100 mm wide strip of sheet metal is cleaned and oiled with a defined amount of lubricant. The prepared sheet material then passes into the parted tool with two flat dies, which are connected to three load cells. For the assessment of a tribological system (consisting of tools, sheet metal and lubricant), it is suitable to analyze the coefficient of friction as a function of contact normal stress and sliding velocity.
The test stand can run with individual sheet metal strips or directly from the coil. This allows performing a large number of tests within a short time, a statistical validation of the results and a variation of many parameters.
Depending on the requirement flat dies, strip-tension or draw bead tools are used, which can be kept at a defined temperature. The data acquisition system allows measuring and storing the data for friction forces, contact forces, forming forces and total forces as well as the outlet velocity of the strip. The evaluation of the measured data is performed using an evaluation tool based on the MATLAB software, which was developed at the PtU.