In my part I need to place a threaded hole. The hole should be 5/16-18 thread. 

When I create the threaded hole and 3D print the part, the hole is way too small. Do I need to cut a hole 1st and then thread it to the sizing I need? If so, what size do I cut extrude first to create the initial hole? Then just use the threaded hole tool as I expect.