art the farther the front sidewall edge 32 extends from the respective connection joint 30 with the main blade 2 the more flexible the first and second sidewalls 26 28 become due to the increasing weight of the sidewall as it extends farther from the connection joint 30 thus creating a greater moment arm about the connection joint 30 turning to fig9 in order to maintain the integrity of the box blade 1 specifically the rigidity of the main blade 2 and especially the first and second sidewalls 26 28 relative thereto a strengthening gusset 40 is incorporated with each connection joint 30 and between the sidewall and the main blade 2 to stabilize each sidewall relative to the main blade 2 each gusset 40 is formed as a substantially conic section i e a partial cone having a base portion 42 attached to and extending radially from the front surface 4 of the main blade 2 to an apex portion 44 spaced therefrom and attached on the inner surface of the sidewalls 26 28 the apex portion 44 may extend substantially the length of the inner surface of the sidewall to be situated substantially near the front edge 32 or forward edge of the respective sidewall the conic section gusset 40 is provided with a contiguous outer surface 46 and an outer supporting edge 48 which is joined to the respective front surface 4 of the main blade 2 and the inner surface of the.