Cutting precise, large-diameter holes with a portable power drill or drill press can be done with a variety of drill bit attachments, including a hole saw, spade bit, or Forstner bit. While each one has its advantages, the Forstner typically drills the cleanest holes. But Forstner bits can also be tricky to use in a portable (handheld) drill, as they're better suited for use in a drill press. When used in a portable drill, a Forstner bit has a tendency to "walk," or drift, away from the center, especially at the beginning of the operation.
There's a simple solution to this problem, and it can help whether you're drilling into virgin material, or if you're trying to enlarge an existing hole with a Forstner bit.
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 little jig that prevents the bit from walking when drilling the real hole in the workpiece.
- Cut a piece of scrap lumber to a usable size, leaving enough room for clamping when it's time to secure the jig to your workpiece.
- Set up your drill with the Forstner bit you will use on the workpiece. Make sure the bit is extended enough so that it can drill all the way through the scrap lumber.
- Clamp the scrap lumber to a sacrificial surface. You'll be drilling through the scrap, and you want the sacrificial surface to serve as a backerboard to minimize tearout.
- Begin drilling the hole through the scrap piece. It's ok if the bit walks a little at first; it will cut a clean hole once the main cutters engage the wood. Keep the drill speed slow for the best control.
- Drill all the way through the scrap, keeping the bit as straight (plumb) as possible. Back out the bit and unclamp the scrap.
- Clamp the jig to your workpiece in the desired location. Drill straight through the hole in the jig to complete the hole in your 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 backerboard to minimize tearout, especially with large-diameter holes.