3. Results
In Fig. 2, measurements of the punch stroke, load and torque are shown. The punch reached the bottom dead
center at 0.4 s, and was released at 1 s. Therefore, evaluation of friction coefficients was carried out while the
punch was at the bottom dead center (from 0.5 to 0.9 s). The velocity of the punch was at a constant value from its
initial position until 4 mm above the bottom dead center. Therefore, average velocity of the punch was defined as
the velocity until 5 mm above the bottom dead center.