How to Guide a Forstner Bit With a Portable Drill
All you need for this trick is a flat piece of scrap lumber or plywood—at least 3/4 inch thick — and a couple of clamps. Essentially, you’re making a small jig that prevents the bit from walking when drilling the real hole in the workpiece.
Using Your Jig to Enlarge a Hole
Forstner bits, like spade bits, have a small point at their center that serves as a pivot point for the larger body of the bit. If you want to enlarge an existing hole, there’s no wood for the point to engage, so the bit can’t center itself. As a result, the bit really tends to walk (more like run) off-center when you try to make a larger hole. You can solve this problem by using the same jig technique described above. Carefully center the hole in the jig over the existing hole in the workpiece, clamp the jig securely, and make your new hole. As always, it’s a good idea to clamp your workpiece to a backer board to minimize tear-out, especially with large-diameter holes.
