I'll never forget the first time I slipped while driving a slot-head wood screw using my Yankee push drill. The bit drilled a 1/4-inch hole right through my thumb. It was the mid-1970s, when I was still banging in finish nails by hand, and having sore–and injured–fingers was part of the job.

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Impact drivers get their name from the pounding action of the hammer connected to the motor against the anvil attached to the bit holder.

Credit: Photo: by dotfordot.com

My fingers started to heal in the early 1980s, about the same time the first cordless drills came out. I thought I had died and gone to 9.6-volt heaven–but the best was yet to come when, some 20 years later, I got my first cordless impact driver. I haven't let go of that tool since.

If you don't know about impact drivers, let me tell you about one of the greatest tools that's ever been invented next to the nail gun, table saw, and power miter saw. Just like an auto mechanic's pneumatic impact wrench, the rotation of the impact driver delivers hammering force, adding a lot of torque without a lot of reaction force. Inside the tool, attached to the back of the bit holder is a two-lugged gear called the anvil. A drive gear with two matching lugs (the hammer) repeatedly strikes the lugs of the anvil when the torque needed to turn the bit exceeds the torque of the tool's rotational force alone. Similar to the blows per minute of a rotary hammer, impact drivers are measured in impacts per minute.

Put simply, an impact driver runs like a very fast drill until you need more torque to drive a screw. Then the hammer hits the anvil about 50 times per second and drives the screw with hundreds of little hits. Finish carpenters' 12-volt drill/drivers ramp up to about 250 inch-pounds of torque, while early impact drivers topped out at about 700 inch-pounds. But some new lithium-ion impact drivers approach 1,500 inch-pounds–a huge difference. You'll rarely strip the head of a screw with an impact driver, but you can shear one off easily.

The Tools

We set out to test the latest 18-volt lithium-ion impact drivers, but we made a few exceptions to round out the field. All but the Bosch are LI-powered, but a few different voltages and tool configurations are in the mix. The 14.4-volt tools include the Hilti SID144-A and Panasonic's EY7540LN2L and EY7542LN2L combination impact/clutch-controlled driver. The 18-volt tools are the Bosch 23618, DeWalt DC827KL, Hitachi WH18DL, Makita BTD141 and BTD142HW compact, Milwaukee 0881-22, Ridgid R8823, and Ryobi P230. The Craftsman 320.28128 is the only 20-volt tool.

Flexible Fuel. The Bosch is limited to nicad batteries, but many of the drivers also can be powered by their brand's nicad or NiMH batteries of the same voltage, including the DeWalt, Hitachi, Milwaukee, Ridgid, and Ryobi. The Ridgid can even use 24-volt LI batteries. Makita's compact model is smaller because it has a half-sized battery with reduced runtime but full 18-volt power. Hitachi just introduced a similar compact version, but it was too late for our test.

Similarities. Functionally, these tools are very similar. All have variable speed triggers, automatic brakes, through-handle direction switches with lockoff position, and headlights. Impact drivers have 1/4-inch hex bit holders that hang on to bits with a ball-bearing detent system for a vibration-proof grip and quick bit changes.

Combination Tool. Panasonic's EY7542LN2L is an impact driver with a separate, nonimpact driver mode controlled by clutch settings. This tool saves you from having to keep a separate drill on hand for drilling holes, as an impact driver is probably one of the worst tools to use with a drill bit. This tool also is safest for driving soft brass screws or steel screws against hardware.