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Drilling and Boring Tools

Powerbore, Auger, Forstner, and Multi-spur Bits

Powerbore, Auger, Forstner, and Multi-spur Bits

Powerbore bit. Stanley's Powerbore, Photo 6, left, drills a cleaner hole than the space bit. Available in diameters from 3/8" to 1" in 1/8" increments, these bits handle end-grain drilling particularly well. The long, brad-type point guides the bit, and makes it easy to center on a mark. The bit cuts relatively quickly, but doesn't carry chips out of the hole. So, if you're drilling a deep hole, withdraw the bit frequently to clear the chips. Speeds in the vicinity of 500-750 rpm will work fine.

Auger bit. Before power drilling, there was the brace and auger bit. Many tool dealers still sell augers with the traditional tapered square shank end for use with a brace. You can also buy a straight-shank version to fit the chuck on your power drill.

Augers, like the one shown in Photo 7, above, bore smooth holes. They're well-suited to deep-hole boring because of their length: A 1/4" auger is nearly twice as long as a 1/4" twist drill. And you can buy even-longer ship augers and pole augers. While some augers are sized in inch fractions, you'll still find many identified by the traditional number system. Don't let it throw you, though. The number simply refers to the bit diameter in 16ths of an inch. So, a bit marked 10 would be 10/16", or 5/8".

Most augers self-feed with a screw tip. When power-drilling with an auger, don't run the bit at more than 600-700 rpm.

Forstner bit. A true Forstner bit, named after its inventor, has only a small center point, shown on the left in Photo 8, above. The outer rim guides the bit instead of the center point, enabling the Forstner to cut holes with nearly flat bottoms and smooth, true sides. That also means you can cut any arc of a hole on a workpiece -- the center doesn't have to be on the stock. The small point makes the bit difficult to center on a mark, however.

These are expensive bits, but to many woodworkers, they're the ultimate drilling or boring tool. Sizes generally run from 1/4"-2". For maximum accuracy, you'll want to use the Forstner bit in a drill press. Clear the chips often, and run the bit at a moderate speed to prevent heat damage to the cutting edge. Try 700 rpm for bits less than 1/2" diameter, 500 rpm for bits up to 1", and 250 for those larger than 1". Carbide-tipped Forstner bits are available.

Multi-spur bit. Though this one is often called a Forstner bit, it isn't. The teeth around the multi-spur bit's rim (shown at the right in Photo 8, above) are the difference between it and the Forstner type.

These bits are expensive, but cut cleanly and without splintering. Multi-spur bits work well when drilling into a workpiece on an angle. They do a great job with overlapping holes, too.

Sizes range up to more than 4". You should consider any large one a drill-press-only bit. Even the smaller sizes are much easier to keep under control with the drill press. In hardwood, you can run bits 1" or smaller at about 500 rpm. Slow down to 250 rpm for bits from 1" to 4".

Continued on page 4:  Holesaw and circle cutter; Specialty bits



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