Tool Feature: Cordless Impact Drivers

Your first visit to a tire shop was likely punctuated by the loud BRAP, BRAP, BRAP of a pneumatic impact wrench. Auto mechanics love those tools because they provide loads of torque without jerking the tool when the lug nuts snug up.

Now, you can enjoy similar performance from battery-powered impact drivers. Besides their compact size and high torque, you'll also love how these tools finesse softer fasteners, such as brass screws, without shearing off their heads. So should you dump your cordless drill in favor of one of these bad boys? Let's not go that far. But an impact driver in your shop nicely complements that drill. Here's what you need to know.

Two things an impact driver is not
Despite similar-sounding names, don't confuse an impact driver with these tools:
* Impact wrench. Found primarily in auto shops, this unit—pneumatic or battery-powered—functions like an impact driver, but has a square socket-drive instead of the ¹⁄₄ " hex chuck. It provides even greater torque for driving nuts, bolts, and other large fasteners.
* Hammer drill. This tool proves almost as noisy as an impacter, but for a different reason. Internally, a small hammer delivers a punching action down the length of the bit--like tapping a screwdriver on the end of the handle—as the chuck turns. This percussive force excels at drilling holes in concrete, but offers little more driving torque than a typical drill.

Like a typical drill, an impact driver relies—at least initially—on the tool's motor and gearing to turn the chuck for drilling or driving. When the going gets tough, though, an impacter switches modes to engage the hammer-and-anvil system, shown below, to kick its rotational power into overdrive. In impact mode, the chuck rotates much slower but with much greater torque. (Watch a free video that demonstrates this action in motion.)

Impact drivers bring a lot of pluses to your shop, along with a few minuses.
Pluses:
* 3 to 4 times greater torque than a drill of equal voltage, yet lighter and smaller.
* Requires less arm strength for tough drilling and driving tasks.
* Fewer stripped screwheads.
* Great for assembling machines and tool stands, and unsticking stubborn screws and bolts.
* Short, powerful turns in impact mode make fastener-depth control easier.
* Drilling with small-diameter bits (less than ³⁄₄ ") works like with a cordless drill because the impact function doesn't kick in unless the wood proves tough.
* Perfect for drilling with large-diameter bits because impact function prevents binding that's common to drills.

Minuses:
* Impact hammering creates loud noise, so have hearing protection handy.
* High speeds with 18-volt models—prior to impact mode kicking in—make for a steep learning curve.
* They only work with ¹⁄₄ " hex-shank bits—a detriment for small-diameter drilling (less than ¹⁄₂ ") because these bits tend to be of lesser quality.
* Difficult to gauge screw depth when working in "blind" situations, such as poorly lit areas inside cabinets or inside pocket holes.
* High torque can twist off screws if pilot holes are too tight or you drive screws too deep. It can also damage ordinary drilling and driving accessories, as shown below, so look for those made specifically for impact drivers.

Although an impact driver probably should not be the first cordless tool you buy, a 10.8-or 12-volt model definitely has a place in a woodworking shop, especially for building anything with screws. Its power-to-weight ratio proves second to none, and the slower chuck rotation in impact mode, compared to 18-volt models, provides unparalleled finesse when driving screws to precise depth. But for day-in and day-out drilling duties, we still prefer a standard drill, cordless or electric.

If you build outdoor projects, such as arbors, pergolas, or decks, where you drive a lot of long screws or lag bolts, opt for an 18-volt impact driver. You'll appreciate its added power, and once you become familiar with using it, you can even use it in your shop.

Our advice: When the batteries start to fail on your current cordless drill, consider buying an impact driver with batteries compatible with your old drill. For the cost of the new impacter—about the same as a cordless drill—you'll have both drilling precision and raw power for your shop. You can also buy a cordless impact driver without batteries for about 40 percent less. Or if you'd like to add a new drill as well as an impacter, get a kit that includes both tools with two batteries and a charger. These typically cost about $30 to $50 more than a single drill or driver kit.