To form a pair of cracking grooves on the inner peripheral surface of the big-end hole of a connecting rod surely and at low cost.
A pair of rotary form-rolling means 30a, 30b which are pivotally supported in approximately parallel in the slit 20 of a holder 18 are lowered with a hydraulic cylinder 14 and the edge parts 22 of the form-rolling means 30a, 30b are brought into contact with positions opposed to each other of the inner peripheral surface of the big-end hole 26 of the big-end part 38 of the connecting rod 24. Successively, by further lowering the pair of the form-rolling means 30a 30b and rotating the edge parts 22 of the pair of the form-rolling means 30a 30b, the pair of the cracking grooves which are composed of intermittent straight-line-shaped grooves are formed on the mutually opposed inner peripheral surfaces of the big-end hole 26.
YASUOKA TORU
OZEKI YUKIO
SHIDA KOJI
Toshiyuki Miyadera
Naoki Kashima
Yasuo Takubo