Compact, lightweight, and powerful, they're the perfect tool for running screws — but wear your earplugs

Think of a cordless impact driver as a cross between an impact wrench used by auto mechanics and a conventional cordless drill. Though I had heard about these tools, I had never seen anyone using one, so I was skeptical. I drive a lot of screws as a remodeler, and I thought nothing could be faster or more effective than my cordless drill. But when I was given the chance to test several of these tools for JLC, it didn't take me long to realize that a cordless impact driver is a superior tool for running screws. The tool's impact action makes driving 3/8-inch lags feel like you're driving drywall screws, and because impact drivers are about a third smaller than cordless drills, they fit in many places where a drill can't go. Plus, unlike my bulky 18-volt drill, they hang easily from my toolbelt.

The Tools

Generally, impact drivers are available in the same voltages (9.6 to 18) as cordless drills. To take advantage of the tools' compact size, I limited my test to 12-volt models from DeWalt, Hitachi, and Makita. I also tested 14.4 drivers from DeWalt, Makita, and Milwaukee. Even though these tools are in the middle of the pack in terms of voltage, I didn't find any fasteners they wouldn't turn. Their power and ease of use are remarkable.

Chucks and bits. All the drivers I reviewed have hex-shaped chucks designed for standard 1/4-inch hex bits, but you aren't limited to just square and Phillips. Most accessory manufacturers also offer nut drivers and other specialty bits. With an adapter, you can also use standard 1/4- and 3/8-inch drive sockets for lag screws and bolts. Manufacturers suggest using heavy-duty impact sockets for safety.

Although you can use an impact driver for drilling holes by fitting it with a hex-shank drill bit, the impact action makes this slow and inefficient. Don't expect it to replace your cordless drill.

How Impact Works

Unlike a cordless drill that relies on a motor and transmission to create torque, an impact driver uses a hammer-and-anvil mechanism inside the tool housing. As the hammer spins and strikes the anvil, it spins the driver bit in short bursts. This creates enough power to drive and even snap lag screws. Aside from increased torque, the design has another benefit. Anyone who's ever driven a long screw with a drill knows how much torque is transferred back to the operator, and how the bit tends to cam out of the screw head unless constant heavy pressure is applied. With an impact driver, the torque transferred back to the operator is noticeably reduced, and you don't need to apply as much forward pressure to the bit. This not only makes the tool less tiring to use, but it's especially helpful when you're working in confined spaces or hanging from a ladder.

How an impact driver works.A spinning hammer hits against an anvil connected to the chuck. The inertia created by the spinning hammer gives impact drivers their impressive torque and makes driving screws less tiring, but it's also what creates the tool's biggest drawback: noise.

The drivers I reviewed don't start impacting until they approach 15 in-lb of torque. At low torque, the impact driver will actually run short, self-drilling screws faster than a slower-spinning conventional drill-driver. What's cool is that the tool does all of the adjustment. You don't have to flick a switch or change gears when switching from low torque to high. Just pull the trigger, and the tool does the rest.

Noise Factor

This nearly effortless driving action does have one significant drawback: These things are loud. The extra noise comes from the internal hammer striking the anvil up to 3,000 times a minute. While it's not as bothersome outside, earplugs are a must when you're running the tool close to your head. When I was doing some work in a local doctor's office, the noise distracted the staff and patients so much that I had to switch to my cordless drill. Keep this in mind if you work in places where noise might be a problem.

Run-Time Tests

I used the impact drivers for more than two months, on jobs ranging from decks to kitchen cabinets. In addition, I did some run-time tests to see if there were noticeable differences and if the 14.4-volt tools did any better than their 12-volt counterparts. I also wanted to see what would happen to motors and gearboxes when they were subjected to continuous operation.

Before testing, I charged and discharged the batteries four times to condition them. I then put them to the test by driving 21/2-inch galvanized screws and 1/4x4-inch lag screws into pressure-treated lumber. I stopped the test when I noticed a decrease in performance. Amazingly, out of the hundreds of deck screws I drove, I only had five cam out, stripping the screw's Phillips head. Running the screws was almost effortless, though I did go through a few Phillips bits.

The Verdict

After a couple of months of using impact drivers, I was hooked. The ability to effortlessly drive screws from odd angles or while stretching is reason enough to get one. During a kitchen cabinet installation, I was able to screw two face frames together inside a 9-inch wall cabinet where my cordless drill would never have fit. And because the tools are so much lighter than a big cordless drill, I could hang one from my belt all day and not really notice.

My Picks

The best run-time winner and my favorite for running the heavy lags and driving large numbers of deck screws was the Milwaukee. It felt good in my hand and was nicely balanced. For working in confined spaces, the 14.4-volt DeWalt is a good choice; it's the same size as the 12-volt drivers, but it packs a little more punch. For lighter work and intermittent use, I liked the Hitachi because its built-in belt hook kept the tool by my side, whether I was wearing my toolbelt or not. Hitachi's built-in bit storage is handy, too.