To bore holes that are centered on the surface of a board, use the shopmade center-drilling jig shown above. The illustration provides suggested dimension.
Use a straight piece of 1-by-1 stock for the jig arm. You can make such a device any length you choose, but cutting it to the length shown allows it to accommodate even the widest stock used in a typical project. Mark the center of the top face of the arm and bore a hole through it for a guide bushing. The hole should be 1/8 inch larger in diameter than the bushing, which should be slightly larger than the holes you wish to make with the jig. Press the bushing into place.
Next, turn the arm over and mark a line down the middle. Mark ooints on the line 3/4 inch from each end, then bore holes halfway through the stock at these points, making them large enough to hold a 3/8-inch grooved dowel. Dab some glue into the holes and insert the dowel.
To use the jig, position it on the stock and pivot the arm until the dowels are up against the opposite edges of the workpiece. Holding the jig with one hand, fit the drill bit into the bushing and bore the hole.
For a hole centered on the edge of a board, first secure the workpiece edge-up in a vise. Then position the jig on the edge of the stock with the dowels flush against its opposite faces.
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