To facilitate a protrusion work of an external face to the outside by raising the internal pressure of a can higher than the external pressure, applying the strike on the external face of the can in this state, bulging the struck mark and forming a bulging part.
At first, a pressurizing device 20 is attached on an open end of a can drum 1, and the inside pressure of the can drum 1 is made larger than the outside pressure. Next, a striking device 30 is arranged oppositely toward a band like part 12 of the upper part of a straight part 5 on which a bulging part 11 is formed on the can drum 1. Then, while the striking device 20 is driven, and moved relatively around the axial line CT, the band like part 12 is evenly struck by the tool 31 and struck marks 11a of the tool 31 are formed over all the range of the band like part 12. The struck mark 11a is made in a recess toward the inside of the can drum 1 just after striking, but because the strength of the struck mark 11a part is reduced by being deformed in this way, it is made not able to resist the internal pressure of the can drum 1 and is protruded to the outside. Then, many struck marks 11a are protruded and a bulging part 11 is formed.
BABA HITOSHI
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