Boom for construction machine

A boom for a construction machine provided on a vehicle body of said construction machine comprising:

• a box-shaped structural body formed of a cross section forming a square closed sectional structure by a left side plate and a right side plate facing each other at an interval in a left-right direction and extending in a front-rear direction, an upper plate joined to upper end sides of said left and right side plates by welding, and a lower plate joined to lower end sides of said left and right side plates by welding, and having a foot-side mounting member provided on said vehicle body in a rear end side and an arm-side mounting member provided on said arm in a front end side;
• wherein
• said foot-side mounting member is joined to rear ends of said left side plate, said right side plate, said upper plate, and said lower plate of said box-shaped structural body by welding, respectively,
• said arm-side mounting member has left and right joining plates to which front ends of said left and right side plates of said box-shaped structural body are joined by welding, respectively, and has upper and lower joining plates to which front ends of said upper plate and said lower plate are joined by welding, respectively, and
• said lower plate is constituted by a first front lower plate joined to said lower joining plate of said arm-side mounting member by welding, a second front lower plate joined to a rear end of said first front lower plate by welding, a third front lower plate joined to a rear end of said second front lower plate by welding, and a rear lower plate made of one or a plurality of plate materials joined to a rear end of said third front lower plate,
• said first front lower plate is made of a plate material having a plate thickness equal to said lower joining plate of said arm-side mounting member,
• said second front lower plate is made of a plate material having a plate thickness equal to said third front lower plate and having a plate thickness smaller than said first front lower plate,
• an X-shaped groove is formed between said rear end of said first front lower plate and said front end of said second front lower plate having a plate thickness smaller than said first front lower plate, and a single different-thickness plate is formed by the double-side welding performed to said X-shaped groove from both sides in the plate thickness direction,
• a first backing material is provided on a rear end of said lower joining plate of said arm-side mounting member,
• a second backing material is provided on a front end of said third front lower plate,
• a first V-shaped groove is formed between a front end of said first front lower plate constituting said different-thickness plate and a rear end of said lower joining plate of said arm-side mounting member having a plate thickness equal to said first front lower plate toward said first backing material, and joined from an outer side of said box-shaped structural body by one-side welding at the position of said first V-shaped groove, and
• a second V-shaped groove is formed between said rear end of said second front lower plate constituting said different-thickness plate and said front end of said third front lower plate having a plate thickness equal to said second front lower plate toward said second backing material, and joined from the outer side of said box-shaped structural body by one-side welding at the position of said second V-shaped groove.