Noun
(engineering, machining) A cylindrical iron or steel bar used as a lever; thus:
A short bar put through the hole in a socket spanner or box spanner to provide leverage.
A bar used to loosen or tighten some types of chucks and tool holders on machine tools such as lathes or milling machines.
A set of two tommy bars is the standard tooling for the spindle nose threads of some minilathes and minimills.