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Forstner Bit Selection, Use & Care Print  |  Back

From: Woodcraft

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Forstner bits are exceptionally good choices when you need to produce flat bottomed holes--if for no other reason than producing a napkin holder, for example, that holds salt and pepper shakers. They are also the only drill bits that can drill overlapping holes, which makes them ideal when you make mortises--drill the holes, clean up with a good chisel, and go on the next one. Forstner bits also drill into angled sides vertically, making hole alignment and spacing easier than with any other drill bit.

With all their good features, we don't use them for every job because they're a little bit fragile, compared to other drill bits. Excessive speed, or too much force without a break, can ruin the bits in seconds. The larger the Forstner bit, the faster it can be ruined by excessive speed. What happens is simple: because of the bulk of the bit's body, it heats up rapidly, so that you must keep speeds down (compared to possible speeds with HSS standard drill bits and even those allowable with brad point bits). If the bit body heats up enough, the heat draws the temper from the bit. A blued Forstner bit is ruined.

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That said, high speed steel (HSS) Forstner bits don't lose temper easily, and carbide tipped Forstner bits are even sturdier. Both, though, work far more efficiently within clean holes, so the clean-out requirement remains.

The first job, then, is to limit the speed of the drill you're using, whether a hand drill or a drill press. If you're using a trigger controlled hand drill, set it on the lowest possible speed range, and then pay strong attention to the actual speed of the drill chuck.

bit3.jpgHand drills can't be as tightly speed regulated as can drill presses, so constant attention to their speed is essential: in fact, though, it's really best not to run Forstner bits in handheld drills, because of the lack of speed control (and accuracy--Forstner bits are made for clean, neat, accurate holes, which is seldom what a handheld drill produces, regardless of bit type). The list below shows the safe running speeds of different sized Forstner bits, with the smaller sizes able to run at pretty much the same speeds as twist drills, while the larger sizes--at the top of the lists--must move more slowly.

An important step in the interest of keeping your Forstner bits sharp and in good shape is cleaning the hole bore often. Forstner bits do not have flutes that can pass chips up and out to the surface of the piece being drilled, so it's necessary to lift the bit out of the bore, or almost out of the bore, every 15 to 30 seconds.

Sharpening & General Care[b1] bit4.jpg Forstner bits with dull chippers (see fig. 1) feed poorly and chips easily jam them up. When you get your Forstner bit, it is almost ready to use. Honing the chipper faces will improve their action immensely. You can use a straight or shaped stone, Arkansas, Japanese or diamond, to get them perfectly smooth. Hone only on the flat: you do not want to affect the edge bevel at all.

When the rim (see fig. 1) of your bit gets dull, sharpen it by using a 1000 grit stone, in a half-round or a slip stone style. Finish by honing with a fine stone like a hard Arkansas or a 4000 grit water slip stone. When you are sharpening the inside of the rim of a Forstner bit, never take off any more material than necessary. Coarse scrape it until there's a clean edge. Finish with your 4000 grit slipstone. When the chippers on your Forstner bit get dull, sharpen them by stoning the face of the throat and the bevel. Always stone exactly parallel to the throat, to keep original angles. This means that you must stone exactly parallel to the throat. Any sharpening on a Forstner bit should be done in a way that maintains the original angle.

When your Forstner bit comes from the factory, the chippers are about 0.005" below the rim. Keep chippers slightly lower than the rim so that the rim is severing the tips of any fibers before they are removed by the chippers.

Use care to keep original bevel angles all the way through any sharpening job.