An employee was helping to train two new employees on how to change hammers in a hammer mill. The employees conducting the training were removing the hammers, which requires pushing a steel rod through the hammers and collars to push out another rod that holds the components in place. Employee 1 was removing a collar that was stuck when Employee 2 pushed the rod in. Employee 1's left middle finger was pinched between both rods, and his finger was partially amputated. The mill was locked out at the time of the incident.