Power-driving wood screws gets better

If you're planning an outdoor building project this summer, then you'll probably be driving screws into wood. They hold much better than nails and have dropped in price enough to make using screws for decks, docks and outdoor play sets a no-brainer. This is especially true given the abundance of square-drive Robertson screws we enjoy in Canada.

The main advantage of the Robertson screw head is that it resists "cam-out" better than anything else. This is the dreaded and all-too-familiar event where your screwdriver tip twists out of the screw head, damaging the screw, the driver tip, possibly your hand, and probably the surrounding wood. Cam-out infuriation is the psychological condition that prompted Wall Street Journal writer Rich Hirsch to pen his article Phillips Screws -- Fasteners From Hell? The agony is at least as bad with slot-head screws.

Though good right from the start when Peter Lymburner Robertson first invented the screw head that bears his name in 1908, Robertson-drive wood screws have gotten better, more varied and much cheaper over the last ten years. And though they're superior to any other wood screw on the planet, there's still a problem. With more and more folks driving them with some kind of power drill, even the legendary traction offered by a square driver in a square hole sometimes isn't enough to get the job done. Apply a little too much drill-delivered torque, or use a driver bit that's lost its crisp edges, and even a Robertson screw buckles under the pressure. Anyone who's experienced the grief of a terminally mashed-over screw head sticking 1/8-inch above that brand new deck surface knows the pain.

For reasons I don't fully understand, the Japanese building trade has been especially sensitive to the possibility of fastener cam-out that occurs whenever screws are power driven. That's why they've consistently demanded a different kind of tool than the drills we use here to drive screws. And this insistence has led manufacturers to innovate, a fact that now means we westerners have access to an outstanding, relatively unknown tool for driving screws quickly, even though we were too stupid to ask for it ourselves.

It's called a cordless impact driver, and though it looks something like a hand-held drill, key differences make it absolutely unbeatable for driving screws. Cam-out is practically impossible with an impact driver, even for inexperienced users.

While regular drills drive screws with a constant, rotational motion, impact drivers operate in rapid pulses when the twist load gets heavy. Squeeze the variable speed trigger and an impact driver operates as any ordinary drill, propelling the screw into wood. But when friction increases, it starts to make a sound like the one you here when a mechanic is tightening the lug nuts on your car wheels. This rapid-fire rat-tat-tat sounds irritating, but it indicates something very useful. The tool is alternately driving, then not driving the screw in a kind of quarter-turn-forward/eighth-turn-backward pattern. This back-and-forth motion resets the screwdriver tip into the screw several times a second, an action that means the driver tip just keeps on driving, even when it's worn and rounded. Impact drivers even offer redemption to those nasty reprobates, the foul-tempered, cam-out-prone Phillips screws.

Until recently, cordless impact drivers designed for driving wood screws hadn't received the usual level of R&D investment that other tools have enjoyed. This means that not every cordless impact driver has the oomph to sink, say, 3 1/2-inch long deck screws into a knotty piece of pressure-treated spruce. But that's changing in a way outdoor wood builders need to know about. Impact drivers in the 12 to 14.4 volt range delivering more than 1000 in/lbs. of torque are hitting the market right now, offering the power needed to drive screws into anything.