The 9/16″diax3/16″deep counterbores are for usage with our speedy locate and fasten systems. These allow our vises and fixtures, and other hardware to be located and bolted down with one fastener. The tolerance on these is rougher than the precision bores (.003″) but try it out, you will find that almost every time the fixture is back in place within .001″, if you need better design around the precision bores.