Have you enjoyed a lively debate about drill bits lately? Do you and your buddies nearly come to blows arguing whether black oxide or gold oxide is a better finish treatment for high-speed steel? Do you spend many frustration-filled hours trying to explain to anyone who will listen that a twist bit is, in fact, an inclined plane carved into a cylinder?
Although a few dedicated tool enthusiasts might answer yes to these questions, most of us don’t spend much time contemplating drill bits. But before you bore your next hole, take a few minutes to investigate the fast-changing, information-rich world of drill bits. Learning about them will help you get better results in your work, prevent you from wasting money on bits or features you don’t need, and extend the lives of your drilling tools.
This story focuses on common drill bits for boring metal, wood and plastic. These include twist bits, spade bits, Forstner bits, brad-point bits and auger bits. A special thanks to Bob Crowther of Irwin Industrial Tool Co. for generosity in sharing his expertise on drills and drilling.
Here’s something you may not know: The fluted metal rods we call drill bits are more accurately dubbed drills, and the machines that spin them are called drill drivers by purists. (Now the formerly obscure term drill driver has crept back into regular usage to describe cordless drills that can also drive screws.) Bits are categorized by general type, hole diameter, length, the type of metal they’re made from and the finish, if any, that coats them. There are numerous other variables as well, depending on the general type.
Most wood and metal drill bits are made from high-speed steel (HSS) or heat-treated carbon steel. Solid carbide steel is employed in making some small hobbyist bits, but the material is too brittle for carpentry, woodworking or metalworking. Carbide tips, however, are common for general-use drill bits. Cobalt-alloy steel is used to make highly durable bits designed for drilling hard alloy metals such as stainless steel.
Twist bits come in the widest variety of finish treatments, including A) bright-finish high-speed steel (HSS), B) HSS with black oxide treatment, C) HSS with gold oxide treatment, D) titanium nitride coating and E) cobalt alloy (solid throughout).
Both HSS and carbon steel bits come in a variety of finishes, including:
- Bright — This is the most inexpensive finish for HSS. It leaves bits vulnerable to rusting, but bright bits can be sharpened without making any accommodations for the finish.
- Black oxide — Heating HSS bits to 950 degrees F creates a black finish that resists rust and corrosion and lubricates the bit.
- oxide — This finish is similar to black oxide, but the HSS is heated to 450 degrees. There is no practical difference between gold and black oxide.
- Titanium — Titanium (also called titanium nitride and abbreviated TiN) is a coating applied using a process similar to electroplating. It can extend the life of a metal-boring bit up to six times because it protects cutting edges from overheating. Titanium coatings do not have any beneficial effect for wood boring, however.
- Cobalt — Intended for drilling hard alloy metals such as stainless steel, cobalt bits are not coated; they’re solid alloys, usually with a cobalt content of about 8 percent. The extra hardness lets manufacturers make the flutes smaller as a percentage of the total thickness, increasing the strength of the bits.
Although they are designed for drilling metal, twist bits also can be used to drill wood and plastic. The twist bit comes in 162 standard diameters of 1/2 in. or less –- and that’s not including metric sizes.
Twist bits are manufactured using four sizing scales: fractional, number gauge, letter and metric. The smallest twist bits (up to 0.228 in.) are designated by numbers that match standard wire gauges. From there, the letters (A to Z) kick in (up to 0.4130 in.). Fractional twist bits (nonindustrial) are sized from 1/32 to 1/2 in. in increments of 1/64 in.
The most common twist bits are called jobbers. Shorter bits called stubbies or machinist bits offer greater resistance to high-pressure drilling in metal. A 1/4-in.-dia. jobber is about 4 in. long. By contrast, a 1/4-in.-dia. stubby is about 2-1/2 in. long. Aircraft bits are larger-diameter twist bits in extended lengths (6 or 12 in.). Only the bottom portion of the bit is fluted.
In recent years, the standard tip profile has evolved from a 118-degree pinpoint to a 135-degree (flatter) split point. A split point has two ends, so it looks like a tiny flat screwdriver head. The split point is an advantage primarily when drilling metal because the bit is less prone to “walking” as you start the cut. The shallower 135-degree angle is not as aggressive in engaging the cut, but it cuts faster once the hole is started.
The advent of the keyless chuck and the quick-change bit holder has broadened the number of shank options for drill bits. In addition to the round cylinder (B), bits for keyless chucks come in flat shapes that are hexagonal (E) or three-side (D). Quick-change ends are forged into some bits (C) and brazed onto others (A).
Spade bits bore 1/4- to 1-1/2-in.-dia. holes in wood and wood-base materials. Whereas other drill bits cut the wood fibers, spade bits scrape them away. Many bits feature a cutting spur at each end of the cutting head to slice through the fibers as they are scraped out, resulting in a somewhat cleaner cut. But even so, spade bits create rough holes prone to tear-out and are therefore best suited for rough carpentry.
Most spade bits are made with heat-treated carbon steel, not HSS. It is possible to find models with oxide or titanium finishes, but these extra-cost finishes provide little improvement for the type of drilling spade bits typically do.
Compared with other bits, spade bits come in limited sizes. For this reason, some carpenters grind down worn bits to custom widths with a bench grinder. In general, you’ll find spade bits in 17 diameters and two lengths (6 and 12 in.) but the selections may be more limited in 12-in. versions.
Spade bits are for cutting 1/4- to 1-1/2-in.-dia. holes in wood and wood products. The centering spur engages and orients the bit, and the flatter beveled surfaces scrape out wood fibers. On some models, outboard spurs cut the fibers for a cleaner hole. Smaller bits included in some spade bit sets consist of a centering spur only and technically are not spade bits.
For extra-deep holes or drilling through hard-to-reach areas, you can find extended-shaft bits as long as 16 in., or you can buy a 6- or 12-in. extension bit holder. The holes in spade bits are for fishing wire through walls.
For drilling a flat-bottom, smooth-side hole, no bit beats a Forstner-type bit. Many drill bits are lumped into the category of Forstner bits, usually erroneously.
The Forstner bit was invented around 1876 by Benjamin Forstner, a gunsmith who hailed from Salem, Oregon. According to Forstner’s obituary, the bits were originally known as “webfoot augurs” or “Forstner flange bits” and were manufactured under contract by the revolver-manufacturing Colt Co. of Bridgeport, Connecticut.
Woodworkers depend on Forstner bits for drilling holes with smooth sides and flat bottoms. While each of the bits above is called a Forstner by its manufacturer, neither one actually is. With its solid cutting rims and smaller center spur, the one on the left is more similar to a Forstner bit than is the multispur bit with saw tooth rims on the right.
Today, true Forstner bits are made and sold exclusively by the Connecticut Valley Manufacturing Co. These high-end bits are desirable to serious woodworkers, but have questionable benefit and are probably too costly for most DIYers, especially since inexpensive multispur bits are available.
Typically made of HSS, Forstner-type bits come in a wide range of diameters as large as 3 in. A typical seven-piece set includes bits from 1/4 to 1 in. dia. Use these bits only with a drill press.
Brad-point bits are essentially twist bits augmented for wood boring. Each of the two flutes terminates in a cutting spur to keep the edges of the entry hole crisp. A center spur is set into the wood surface to allow you to drill holes with much greater accuracy than a twist bit provides. Most brad-point bits are made of HSS. Carbide tips and titanium coatings are extra-cost options, although the carbide tips are more brittle than HSS and make sharpening more difficult.
Typical sets of seven include brad-point bits from 1/8 to 1/2 in. dia. Single bits up to 1 in. dia. are not hard to find. They can be used with either a drill press or a handheld drill.
Auger bits (also called ship auger bits or augur bits) bear a strong resemblance to old-style brace-and-bit drill bits. Some even have the traditional four—side tapered shank. More often, however, they have a hexagonal or flatted shank. Equipped with a screw-point that feeds the bit into the wood, they are used primarily by tradespeople to drill guide holes in house framing for cable or tubing.
Auger bits require more torque than other bit types and will bog down quickly if used with an underpowered drill driver. A 1/2-in. impact driver or a right-angle drill is perfectly suited for use with auger bits. But be sure to attach and use the auxiliary handle – when an auger bit under load jams, it can easily throw the drill and cause injury or damage.
Common auger bits cut holes from 1/4 in. to 2 in. dia. Shaft lengths typically are 8 or 18 in., but exceptions are not hard to find.
The most common and versatile length for twist bits is the jobber type. Shorter versions called machinist bits or stubbies are more resistant to high drilling pressure. Aircraft bits come in 6- and 12-in. lengths. Extended-length spade bits are designed for drilling through exterior walls.
ANATOMY OF A DRILL BIT
A twist drill bit is a relatively simple tool. The major parts include the shank, the flutes and the point. The shank is the solid portion that fits into the driver. The flutes are carved into the bit to create cutting edges. (Standard bits today have two parallel flutes that spiral upward.) In the past, drill points were true pinpoints with a cutting angle of 118 degrees. Most bits made today have a 135-degree split-point tip, which is shaped more like a tiny flat screwdriver head than a pinpoint; this design minimizes walking.