Forstner Bits Bennington VT
North Adams, MA
Hoosick Falls, NY
North Adams, MA
A Forstner Bit can drill holes other bits only dream about.
by Bruce Kieffer
Forstner-style bits can go where no other drill bits dare. The reason is simple: A Forstner bit is rim-guided while other bits are center-point guided. That means you can use all or only part of the bit’s diameter to drill overlapped, angled or partial holes. Forstner bits also shine when it comes to drilling clean, precise 90-degree holes and large-diameter holes. Carbide-tipped Forstner-style bits excel at drilling the precise, flat-bottomed holes required for European-style hinges. Plus, unlike other drill bits, the rim-guided Forstner bit can drill into end grain without deflection.
Forstner bits do have their drawbacks and are not a substitute for your everyday twist- or brad-point bits. For one thing, they are more expensive: A 1/4-in. Forstner bit costs between $3 and $23. They’re also designed pri-marily for use in a drill press, although Forstner bits that are 1 in. or smaller can be used in a hand drill if the bit’s center point is firmly engaged. Forstners also require a very slow rpm rate and thus are slow cutting. Finally, they are very difficult to sharpen.
The knife-edge is the most traditional Forstner design and does a great job of cutting nonstandard holes. That’s because it has the fewest rim interruptions. Its rim is ground to create an inside bevel with a sharp knife-edge. The rim scores the wood before the chip lifters remove the wood from the hole, creating a very smooth, flat-bottomed hole with no tear-out.
The knife-edge rim’s “rubbing” cut generates a lot of heat from friction. Because of this, the style is found primarily in 1-in.-dia. or smaller bits, whose rim speed is relatively slow. To avoid overheating, follow the speed recommendations that come with each bit.
A new Forstner rim design is the wavy edge. Manufacturers claim their wavy-edge bits run cooler than knife-edge Forstners. The idea is that the complex grind of the wavy edge creates ultrasharp points along the rim that help break up the wood chips for easier passage through the bit’s throat openings.
The wavy-edge rim performs just as well as the knife-edge does on 90-degree holes. However, holes drilled by a wavy-edge bit on a board’s edge without the center point engaged have slightly rougher sidewalls than those made by the knife-edge bits. And starting a hole at an angle produces a bit of chatter until the center point engages.
These bits, with sawlike teeth ground into the rim, are great for drilling 90-degree holes. The rim teeth cut, rather than score, the wood, generating less heat and allowing a faster feed rate than knife-edge bits do.