Center Counterbores Over Existing Holes Precisely
When I need a counterbored hole, I normally drill the counterbore first, then the hole. But sometimes I need to drill a hole through two pieces at once and counterbore both pieces. Then, I decide later that I need a larger counterbore. Or, I just forget and drill the hole first.
How can I center a 1⁄2 " counterbore over that hole? Make a boring guide by drilling a hole with a 1⁄2 " Forstner bit in a scrap of 1⁄4 " plywood. To center the guide over the hole, put a standard 1⁄2 " countersink into the workpiece hole. Then slip the boring guide over the countersink and clamp the guide. After popping out the countersink, you can now drill the counterbore with a Forstner bit.
—Jan Svec, WOOD® magazine staff