Trre OnIGINAL HONAE WOOOWORKING RruN ... - Wood Tools

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abiliry to hold a cutting edge. That's the first thing to remember about drills - that the rotational speed (rpm), feed rate, bit rype, hole diameter, and workpiece properties must be balanced to avoid heat build-up or tool failure. For deep holes especially, backing the bit out periodically to clear chips will help dissipate the heat before it's a problem. Keep in mind also that the forces involved change rapidly for larger holes. It's the area of the circle, not the diameter, that represents the material you're removing. For example, a 1"-dia. hole is four times larger than a 1/2" hole, though its diameter is only hvice as great. For safery's sake, large-diameter bits have to rotate at significantly slower speeds than smaller drills do. TWIST DRILLS: IllE BASIC M0DEL Twist drills are easily the most common rype around today, in part because they are so versatile. Most are made from high-speed steel, which can withstand high temperatures without "burning." They can be used to drill almost any metal, plastic. wood, or composite material. but in larger sizes (over l/2") Because rwist drills are designed for use with metal, they feature a slighdy blunted nose that won't deform under pressure against a hard suface. The tradeoff for this feature is a tendency to spin away from the intended hole center, a habit known as walking or skating. Recently, manufacturers have been grinding new tip profiles on rwist drills to make them stay put when starting a hole. Called "pilotpoints" or "bullet-points," these do improve a bit's precision, but their complex geometry usually means do-it-yourself sharpening isn't an option.'Whatever their shortcomings, standard rwist drills can at least be sharpened easily on a bench grinder or with an inexpensive drill sharpening accessory. Titanium-nitride (TiN) coatings on some bits add hardness and a self-lubricating properry but this gold-colored microthin coating improves metal-drilling only, with little or no difference in wood. W0OD-BORlltlG BfiS '<strong>Wood</strong> is softer and machines differently than metal, and specialized bits they have evolved for drilling it. can get pretty expensive, especially For instance, drill bits designed relative to other rypes ofbits. for wood have a center point that TWIST DRILT BIili DRITL BIT ANATOMY Chip llfter Auger blt WOOD BORING BITS liil-coated bit Standard bit Bullet-point bit Brad-poirt bit Spade bit Self-feed screw polnt (not for drlll press use) Cutting spur Chlp clearance flute Twlst drlll contacts the workpiece immediately to keep the bit placement precise. Also, cutting spurs score the wood at the perimeter of the hole so the fibers at its edge are cut cleanly rather than torn. Forstner blt Forstner bit Muftispur bit Ship auger
SPEGIAL-PURP$;E BITS Gountercink bits w/ pilot drills Self-centering Brad-point bits can be found in regular carbon steel, high-speed steel, and even carbide-tipped versions. Grinds vary slightly among manufacturers, but all feature a sharp center point and rwo winglike spurs that are machined directly out of the drill's spird flutes. They cut aggressively and leave a clean hole, but as with nvist drills, sizes over 1/2" in diameter can get pricey. Spade bits, in sizes frorn 1/4" to 71/2" and priced from about $3-$6, solve the cost problem but leave a rougher hole than most other bits. The flat steel heads of these bits are ground with two short spurs and a Iong triangular center point. The simple design creates a scraping rather than cutting action, so the wood fibers are torn rather than cut. Spade bits see a lot of use in rough carpentry - counterboring bolt holes for a deck frame, for example, or drilling through studs for wiring. At the other end of the precision range is the Forstner bit. Like other wood-boring bits, Forstners have a center point for locating the bit, but from there the differences grow. Rather than one or rvvo cutting spurs that score the perimeter of the hole, these bits feature two large cutting rims that together cover about half the hole's circumference. This large contact area allows Forstner bits to do drilling acrobatics you can't pull off with an ordinary bit - overlapping holes, partial Plug cutter 35mm hingB-bodng bit Masonry bit (industdal-drttyl arcs at the edge of a board, drilling If depth and not diameter is the at steep angles, and more - though objective, ship augers typically offer some tricks require a drill press. at least 72" of usable length with a Some large Forstners feature a "saw tooth" rim, designed to dissi- continuous spiral flute to help clear chips. Common sizes run pate heat and cut more aggressively. The multispur bit, a similar design but with only one chip lifter and gullet, is not a true Forstner bit. Among these two you'll find a wide range of diameters (up to 4") and prices (from $3 to over $100 each). 3/8" to 71/z",with prices up to about $50. SPECIAI.PURP(X;E DRILITS Some applications or materials call for special-purpose drill bits. Holes for wood screws can be modified or produced with a cone-shaped BIG DRILLING WITHOUT BIG SPENDING Prccision drill bits machined ftom solid steel rod get costly in laEer sizes. Some affodable altematives include, clochrise from left, hole saws, an adiustable circle cutter or t'fly cutte/' lfior ddll prcss use onlyl, and an expansion bit for a bit brace. 54 WORKBENCH ! MAY I IUNE 1999
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Trre OnIGINAL HONAE WOOOWORKING RruN ... - Wood Tools

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