An employee was using an overhead crane to lift 25 pieces of 1/8" x 10" x 20 long flat bar material to block it for loading onto a tractor trailer. During the blocking process, the employee was installing the wood block when the chain in the overhead crane slipped out due to the bow of the material at the lifted height. The 25 pieces of material fell, smashing the employee's left gloved thumb between the wood block and the material. When he removed his hand, his thumb tip was amputated.