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Winter 2013-2014

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Tool Talk: Forstner Bits

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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.

Bit Types



Knife Edge
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.

Source:
Lee Valley, (800) 871-8158, www.leevalley.com, 1-in. Forstner bit, #06J71.16, $7.



Wavy Edge
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.

Source:
Freud Diablo, (800) 334-4107, www.freudtools.com 1-in. Precision-Shear Forstner bit, #FB-007, $12.


Sawtooth/Multispur
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. Because it can handle a higher rim speed, the sawtooth rim style is most common on bits larger than 1 in. diameter. These bits don‘t perform well when drilling without the center point engaged. The teeth create too many rim interruptions, so the bit is not truly rim guided. Starting a hole at an angle produces chatter and rough sidewalls with heavy tear-out. 

Source:
Hickory by Oldham, (800) 828-9000 www.oldham-usa.com, 1-in. Forstner bit, #HW1013, $10.


Buying Advice

For this article, I tried more than 100 different Forstner-style bits from various manufacturers. Four basic styles are available: knife edge, wavy edge, sawtooth and carbide spur (see “Bit Types,” pages 26 and 27). All the bits I tested drilled standard 90-degree holes just fine. But only some could drill the angled or partial holes that Forstners are known for. If you want to drill these specialized holes, stick with the knife-edge or wavy-edge design. These bits have an almost continuous rim, with few interruptions. The more interruptions in the rim, as in a sawtooth bit, the poorer the bit's performance when drilling holes without the center point engaged. These interruptions have an advantage, however: They generate much less heat, so the bits can run faster and drill holes more quickly than bits with a more continuous-rim design can. The sawtooth bit is also easier to sharpen than other Forstner style bits are.
Cost and Performance
U.S.- and European-made Forstner bits can cost up to four times as much as bits imported from other countries. But I found that generally the more expensive bits held their sharpness longer and drilled more accurate holes. One factor in the price is the steel used to produce the bit. The more expensive Forstner bits are made of durable high-speed steel. Carbon-steel bits cost slightly less, but their steel will not hold an edge as long as high-speed bits will. A carbide-tipped bit also costs more, but it's the bit to buy if you drill lots of 90-degree holes in abrasive materials, such as MDF, particleboard or melamine. These abrasive materials will quickly dull even high-speed steel bits. One last bit of buying advice: Most of these bits are available in sets. Buying a set can save you money. Plus, the sets come with a handy storage case to protect the delicate edges of your Forstner bits.

10 Tips for Using Forstner Bits



PHOTO 1:

Obey the Speed Limit
Adjust the speed of your drill press to match as closely as possible, but not exceed, your Forstner bit's recommended speed. Most Forstners have different speed ratings for hardwoods and softwoods.

 

PHOTO 2:
Keep ‘em Safe
A clean, sharp rim is critical to a Forstner bit's performance. Rolling loose in a drawer will damage the bits' delicate rims. A simple wood block with holes drilled for the bit shanks does the trick.


 

PHOTO 3:
Drill Angled Holes by Hand You can make angled holes using a Forstner bit in a hand drill.  The trick is to start at 90 degrees until the rim and the center point are engaged, and then tip the bit while keeping it spinning. This works best with knife- or wavy-edge bits that are less than 1-in. diameter.


 

PHOTO 4:
Hog Out a Mortise Forstner bits can cut overlapping holes. This is an especially useful technique for hogging out the majority of material in a mortise. It keeps the chisel work and elbow grease to a minimum.


 

PHOTO 5:
Pocket Screws without a Jig Drilling angled pocket screw holes is a snap with a Forstner bit and a drill press. Shallow pocket screw holes like this are often used to fasten a table's top to its apron.

 

PHOTO 6:
Big, Clean, Precise Holes Forstners can't be beat for drilling precise, large-diameter holes up to 4 in. They are a favorite of clock makers. But be warned: These large-diameter bits are expensive—more than $150 for a 4-in. model. If you're only drilling the occasional large hole, you're probably better off with a fly cutter or a hole saw.

 

PHOTO 7:
Drilling on the Edge Partial holes on a board's edge are required for figure-eight-style fasteners used to attach a tabletop to an apron. A Forstner is the only drill bit that can drill this style of hole. It's the perfect choice for this task.

 

PHOTO 8:
No Tear-Out Drilling Forstner bits excel at drilling clean, precise holes for wood plugs. Because the rim scores the wood fibers ahead of the cutters, the Forstner bit virtually eliminates tear-out.

 

PHOTO 9:
Best Bit for a Cup Hinge Carbide-tipped Forstners are the choice for drilling European-style hinge-mounting holes. They can drill into melamine or MDF all day long and stay sharp.

 

PHOTO 10:
Forstner Bit to the Rescue Ever drill your pilot hole before your counterbore hole? I have. A Forstner bit came to my rescue! Drilling a larger hole over a smaller hole can only be done with a Forstner bit.