For utility shop furniture or other quick-and-dirty projects — especially temporary items — we often don’t care much about the screws. They can be visible or not, proud of the surface or not; sometimes, it just doesn’t matter.
But for most of our work, if a screw is going to be visible we want it to look good, seated smooth and flush with the surrounding surface whenever possible. Sure, you could forcibly drive a flathead screw flush, but that almost always tears the wood surface fibers and looks terrible. In hardwood, you might not even be able to set the screw flush.
This is where countersinking (or its close relative, counterboring) come in. When you top off a pilot hole with a countersink, the screw head seats perfectly: all its surfaces contact the wood with the same amount of force, making for a strong attachment and a clean look.
When we want those screws to disappear entirely, a properly sized deep-drilled countersink can accept a wooden plug that can then be cut and sanded flush for a smooth surface and an accented appearance.
Handling these tasks, and more, is the job of a dedicated countersink. But there are a lot of them out there: one piece or a combination of drill bit and countersink, single-fluted or with multiple flutes (or none at all) and a wide range of chamfer angles make for tricky selection. As with just about all woodworking tasks, countersinks work best when correctly matched to the job at hand.
Types of Countersinks
One-piece countersinks come in two main flavors, the most commonly recognized being a fluted countersink, which has a short shank with an elongated cylinder topped with a conical tip. (The shape always reminds me of the old Apollo command module.) The most typical fluted countersink used to have four cutting edges on the cone, although one-piece fluted countersinks today more commonly have an odd number of cutting edges — usually one or five.
Fluted countersinks come in several sizes, able to cut conical holes intended to fit screws of varying head dimensions. A pilot hole is first drilled for the screw, followed by countersinking the hole to allow a flathead screw to seat flush with the work surface. It’s always best to use a countersink with a body diameter that closely matches that of the screw head; using one too narrow won’t create a big enough countersink to accept the screw head, and using one too wide runs the risk of creating a crater-sized countersink that completely swallows the screw.
A fairly recent type of one-piece countersink called a cross-hole countersink has no flutes. Instead of sharpened cutting edges, the bit has a hole bored through the cone at an angle. As the bit spins, the trailing edge of the hole makes the cut. Used the same way as fluted bits (pilot hole first, countersink second), cross-hole countersinks cut very smoothly and remove material quickly, and they excel at ejecting chips as they drill. These bits generally cost a bit more than their fluted cousins.
Getting the Angle
Countersinks are available with cones shaped to create six distinct chamfer angles — 60°, 82°, 90°, 100°, 110° and 120° — designed to match a particular screw’s head angle. The two most commonly used angles in woodworking are 82° and 90°. Those two particular angles are based on the standards to which the screws are manufactured and used. Screws with 90° heads are made to the ISO Standard; they’re usually metric and far more common overseas. Most recognized in North America is the Unified Thread Standard of 82°, which matches just about all woodworking screws you might encounter in the U.S. and Canada. However, both standards of countersinks are readily available here.
Since chances are good that you’ll only use screws with 82° heads, does it make a difference which countersink you buy and use? Well, that depends. With the 90° countersink, because of the 8° difference the very bottom of the screw head makes contact with the wood first, leaving the sides and top of the screw unsupported. The screw must be further tightened to bring the rest of the head into contact with the countersink, creating a greater force on the wood at the bottom. Essentially, what you’re doing is forcibly elongating the countersink by overdriving the screw. With the 82° countersink, all surfaces of the screw head make contact with the wood at the same time and with an equal amount of force.
Common sense will tell you that matching the screw to the countersink is the correct way to go. This is especially true with hardwood, as forcing the screw tighter in an effort to properly seat the screw into the countersink puts a lot of stress on the head — try it with a brass screw, and the head will likely snap off. For hardwood, matching the countersink is always best. For softwood, however, not nearly as much stress is created when seating an 82° screw in a 90° countersink, and the difference may be unnoticeable.
With the bits I’ve described so far, countersinking is a two-step process: pilot hole first, followed by the countersink. It’s effective, but time consuming. However, a wide variety of combination countersinks do both tasks at the same time. Going by several names — countersink drills, pilot screw countersinks and piloted countersinks are just a few — these bits wrap a fluted countersink around an adjustable drill bit. (Crosshole countersinks aren’t used for these.)
The original idea for these was around years ago with piloted countersinks made of a single piece of flat steel with the ends rolled to form a shank. They looked like arrowheads. There was nothing adjustable about these — you either had the right size or you didn’t — and the holes and countersinks they made weren’t much to look at, but they did the job quickly and in a single step. They sold by the thousands, and chances are good that anyone who’s been woodworking for a few decades has a couple rattling around the bottom of a toolbox somewhere.
Today’s versions produce better results and are far more versatile. Styles vary, but each works basically the same way. Each countersink is really a combination tool, and it consists of a separate drill bit centered in a through-hole in the body of the countersink and held in place by either a setscrew or a collet action of the countersink body. The drill bits can either be straight or tapered, their drilling length adjustable simply by loosening the setscrew or collet, sliding the drill bit to the desired length and retightening. Most of these countersinks can be used with a stop collar that sets the depth of the countersink, and most come in sets that include varying sizes of matched countersinks and drill bits to accommodate the screws being used.
Nearly every major tool manufacturer that offers drills (as well as a few who don’t) produces basic, inexpensive combination countersinks of this type. Many of these are constructed in a “flip” style, with a double-ended cutter. When it’s snapped onto the driving body in the countersink mode, you can drill both pilot hole and countersink. Snap the countersink off, flip it around and snap it back in to expose the appropriate driver bit to set the screw.
Three typical examples are the Rockler Pro Tapered Countersink, the CleanStop countersink from Woodworker’s Supply and the Jack Rabbit from Jack Rabbit Tool. Each has some unique features.
The Pro Tapered comes in a set of three complete countersink combinations, sized for #6, #8 and #10 screws. The bits are nicely tapered to match the profile of traditional woodworking screws, and each drill bit has a flattened shank for secure locking with the setscrew. A depth stop with two setscrews is included for each bit, and these countersinks have the largest main shank of 3/8″. For cutting action, there are two extra-long flutes that extend up the countersink body to help eject chips through the depth stop. The three-piece set sells for about $60, but each countersink is also sold individually, with #5 and #12 countersinks also available.
The CleanStop shares a similar pair of long flutes for chip ejection, but it uses straight drill bits instead of tapered. Four combination countersinks sized for #6, #8, #10 and #12 screws come in the set, but only two depth stops are included: they must be shared among the four countersinks. The depth stops are unique and feature a two-piece design with a polymer cap attached to the front of the metal stop. The cap spins freely but stops when it contacts the workpiece to help prevent marring or burning the worksurface. The main shank is a 1/4″ hex design. The four-piece set sells for about $60, but countersinks are also sold individually. The Jack Rabbit set includes the same four sizes as the CleanStop, but it uses a single body. Each countersink is a separate collet that screws into the driving body, tightening on the drill bit as it’s put in place. The business ends sport four short flutes that do not extend past the depth stop in typical use. A single brass depth stop is included for the driving body.
The Jack Rabbit countersink is a “quick-change” combination, using separate drive shanks sporting both Phillips and square-drive ends. In use, you drill the pilot hole and countersink, then just snap the driving body off the shank and drive the screws home in a quick one-two action. The set, which includes a pair of wrenches for tightening the collet-style cutters, two double-ended driver shanks and a magnetizing ring for the drivers, sells for about $50 as of 2010.
All three sets include a storage case, as well as Allen wrenches. Replacement drill bits are available for all.