Forstner drill bit

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Forstner Bits: Not Your Average Drill Bits

Colt Action 2Sometimes I get curious about the origin of things. In this case, I was curious about how Forstner drill bits came to be. I did a little digging and came up with a tidbit of information on Wikipedia and Seems there was this gunsmith named Benjamin Forstner.

Benjamin Forstner Source:

Benjamin Forstner

If you know anything about guns, you know that during the early years of gun manufacturing, gun stocks and grips were fitted by hand to each gun. This required great skill and accuracy in drilling the recesses necessary to accommodate the barrel and stock of the gun.

To quote Wikipedia:

The Forstner bit was "patented on September 22, 1874, [and] was to make him a rich man. Without the lead screw (which Forstner called the "gimlet-point") and cutting lips of more conventional wood boring bits it would prove especially useful to gunsmiths like himself and other high-end woodworkers. The bit was unsurpassed in drilling an exceedingly smooth-sided hole with a flat bottom. It was better than the Russell Jennings twist bits for boring at an angle and not following the grain of the wood.

Forstner PatentForstner Patent Drawing

Forstner bits are unique in their ability to drill overlapping holes, notches on the edges of a workpiece, and angled holes on the face of a workpiece (pocket holes, for example). The bit will cut whether or not the center spur is engaging the workpiece. This feature is unique to the Forstner bit.

Colt Forstner bits sold by Infinity Cutting Tools excel at drilling clean, flat-bottom holes, even in the edge of a workpiece.

Colt Forstner bits sold by Infinity Cutting Tools excel at drilling clean, flat-bottom holes, even in the edge of a workpiece.

As time wore on, patents expired, and dozens of manufacturers sought to improve on the design. One of those companies is Colt, based in Germany. They manufacture a unique Forstner-style bit in France.

Colt Forstner-Style Bits by Infinity Cutting Tools

Colt Forstner-Style Bits by Infinity Cutting Tools

Unlike your run-of-the-mill Forstner bit, Colt bits have a unique geometry that creates a clean, flat-bottom hole without burning and clogging.

The first thing you'll notice is that the entire circumference of the bit isn't a cutting edge as it is on cheaply made bits. Colt bits have sort of a scoring tooth that quickly scores the outside diameter of the hole. This prevents burning.

Then if you look at the flat cutting edges on the inside that actually do the bulk of the cutting, you'll notice some differences there, too. The cutting edges have notches cut into them. The result is small shavings that are less likely to clog the bit.

Another benefit to this unique cutting edge geometry is that it facilitates sharpening the bit when needed. And because there's less tendency to burn, the bit will stay sharper longer.

Finally, and I know it's a small detail, but the center spur on Colt Forstner bits is shorter than you'll find on a lot of Forstner-style bits. This means there's less of a dimple at the bottom of the stopped holes and there's less likelihood the spur will break through the opposite side of the workpiece when drilling deep, stopped holes (recesses for hinge cups, for example).

Infinity Cutting Tools carries a full line of Colt drill bits. I use them. And I highly recommend them.

Carbide. Another improvement in Forstner bits over the years has been the introduction of carbide for the cutting edges. The Infinity Tools 7-pc. Carbide-Tipped Forstner Bit Set offers a good value. The tough, carbide edges will last many times longer than conventional steel bits. I keep a set on hand when drilling tough materials.

The cutting edges on these bits are tipped with carbide for long life and easy drilling in tough materials.

The cutting edges on these bits are tipped with carbide for long life and easy drilling in tough materials.

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Tip for Using a Forstner Bit With a Portable Drill

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 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 small jig that prevents the bit from walking when drilling the real hole in the workpiece.

  1. 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.
  2. 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.
  3. Clamp the scrap lumber to a sacrificial surface. You'll be drilling through the scrap, and the sacrificial surface will serve as a backer board to minimize tear-out.
  4. Begin drilling the hole through the scrap piece. It's okay 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.
  5. Drill all the way through the scrap, keeping the bit as vertical (plumb) as possible. Back out the bit and unclamp the scrap.
  6. Clamp the jig to your workpiece in the desired location. Drill straight through the hole in the jig and into the workpiece to complete the hole.

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.


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