A cutter head correctly balanced will not have running balance unless the head is centered properly on the spindle. This brings out the importance of using heads with good self-centering devices. The faster the spindle speed the more important it is that the heads are centered correctly. Failure of a self-centering device to work properly is often caused by hammering it on or off the machine. This not only springs the sleeve or self-centering cone but the peening action will raise the metal and cause the cutter head to run out of round.
Foreign matter is another cause of cutter head out of balance.
A cutter head that has pitch or gum heavy enough to be seen on the head will not give good performance. Square head bolts, nuts, and washers should be of the same size and weight when running on opposite sides of the head. These should be balanced in pairs or in sets and marked so that balanced sets will be placed opposite each other on the head. Knife Gibbs as used on round head are balanced in pairs at the factory and placed opposite each other on the head. The gibbs and heads should be so marked that when the gibbs are taken out they will go back in the same slot where they were originally.