Hex vs. Key

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Hex is measured flat to flat, not point to point. So, it is the same measurment as the inside circle. Before broaching a 3/8" hex, drill and ream a hole to 3/8". Most broaches are usually .0005-.001 oversize so 3/8" hex stock will be a slip fit into a 3/8" hex broached hole. A hex can easily be held in a 3 jaw chuck or in a hex collet. The "interrupted cut" is usually never a problem when turning it down on a lathe. Just don't take too heavy of a cut at once.