Estimation of Punching Clearance in the Manufacture Process of Iron Cores

The iron core of non-oriented electrical steel sheets is manufactured by punching, and the punching clearance greatly affects the amount of iron loss. Empirically, it is known that increasing the punching clearance, while extending the life of the die, introduces greater plastic deformation at the end of the iron core. This plastic deformation increases iron loss and decreases motor performance, making clearance a condition that must be controlled. On the other hand, punching clearance is proprietary information of the manufacturer and is not disclosed unless it is produced in-house. Against this background, we have developed an inspection method for estimating punching clearance, to better control the quality of iron cores. First, ring specimens were prepared using die with different clearances (0.6%–16.4%). Magnetic property evaluation for these ring specimens showed a linear relationship where iron loss increased with increasing clearance. The electron backscattering diffraction (EBSD) analysis pointed to the changes in strain introduction at the machining edge. Discussion of these data resulted in a calibrated relationship between clearance and kernel average misorientation (KAM). In addition, similar tests were performed on several grades of non-oriented electrical steel sheets to obtain a versatile calibration curve that takes in to account the effect of hardness. The results and discussion of this study demonstrate a new estimation technique that is expected to contribute to motor production management.