What Is a Forstner Bit?

manufacturing industry

Drill bits are used in conjunction with hand drills or drill presses to make holes in woods, plastic, and metals. A Forstner bit is a particular type of drill bit that is often used in furniture making and other woodworking projects. They are usually more expensive than standard twist bits, but they have a few unique features that make them desirable.

Most drills are guided through the material by a central point, but Forstner bits are primarily guided by the outside rim. This means that they can be used to drill pocket holes, which are holes drilled at an angle; partially overlapping holes; and holes that are on the edge of the material. Note that the central spur in the bit pictured below is used to locate and demarcate the center-point as opposed to being the drill guide.

In addition to the previously mentioned benefits, Forstner bits create square-bottomed holes which may be required for certain applications or projects. They are very efficient at removing large amounts (hogging) of material in short periods of time. Forstner bits are available in a wide array of diameters typically ranging from 1/4 inch up to over 3 inches (6 - 75mm). Like other drill bits, they are often sold in sets with an array of sizes.

The Forstner bit was invented by Benjamin Forstner, a gunsmith. It was originally widely used by gunsmiths, who prized it for its ability to bore a smooth-sided hole. This type of bit is not commonly used in modern guns, however, which are usually made using the appropriately named gun drill.

The nature of the Forstner bit can make it difficult to move the bit through wood. For this reason, it is more frequently used in drill presses rather than in hand drills. As mentioned, the central point of the bit is not designed to guide the bit; as a result, the force required to push the bit into the wood can cause the spur can "wander," meaning that it does not stay steady in one place. It is helpful for anyone using a Forstner bit to remember not to depend on this center spur to guide the bit into the wood.

forstner bit

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4
why is it that no one sells a 13mm forstner bit? is this due to supertition, like buildings that have no 13th story? I need a 13mm bit, and can't find one anywhere.
- anon47968
3
Some forstner bits can be combined with a countersink bit to make a "Super-Bit" so you can drill mortise holes and countersink them in one operation.
- westernlog
2
While drilling, forstner bits have a strong tendency to pull themselves down into the workpiece. When used in a drill press, they will yank the workpiece up toward the chuck with great force, especially when the bit reaches the bottom side of the workpiece. The workpiece must be positively restrained for forstners. Spiral drill bits have a lesser tendency, but will grab the work if fed too rapidly into the workpiece. Straight-fluted bits, like reamers and rod bits have no tendency to self-feed into the work.
- anon38599
1
Forstner bits can also be used in wood with lots of voids and/or other imperfections. Since a forstner bit creates its own guide, imperfections in the wood will not lead the bit astray.
- concordski

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