Cordless Drills and Drivers
(Page 2 of 3)
December/January 2005
By Steve Maxwell
Impact drivers. Driving screws isnt always as easy as it should be because driver tips tend to slip out of engagement with the screw head, spinning around and ruining the screw and driver bit in the process. This is called cam out, and its a problem that an impact driver reduces quite effectively.
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If you already own a drill for boring holes and youre looking for the ultimate screw-driving implement, then an impact driver is right for you — no question about it. All models look like small cordless drills except they have no chuck. Instead, the business end of the tool has a spring-loaded locking ring that accepts the hexagonal shank of a standard screwdriver bit. Pull the ring back, slip the bit in place and let the ring go. The bit now is locked and ready for use.
The best trait of impact drivers is the way they spin. Pull the trigger without any load on the tool, and the driver rotates just like an ordinary variable-speed drill. But when the screw bites into the wood and gets hard to turn, the impact drive mechanism causes the driver bit to automatically reset itself in the screw head several times per second — making a rat-tat-tat sound similar to the noise made when a mechanic loosens the wheel nuts on your car, and the difference this makes is tremendous. That sound is the driver rotating the screw a partial turn forward and a little turn backward while delivering a high amount of torque to the screw.
This rotation pattern along with the absence of the chuck means you can't bore holes, but it's great for driving screws. The impact driver mechanism is more efficient than an ordinary drill at turning a driver bit slowly under high load conditions. This is why impact drivers can be smaller, lighter and more compact than a drill driver of similar power. Once you've driven screws with an impact driver, you'll never want to go back to an ordinary drill. Impact drivers cost more than ordinary drills or drill drivers. A 12-volt model costs about $130, and up to about $270 for an 18-volt impact driver.
Voltage and Batteries
Once you've chosen a drill and/or driver, you'll also need to decide how much power you want. If you want a general-purpose model for drilling and screw driving — a machine thats powerful enough to handle most jobs, yet light enough to hold easily — then consider a 14.4-volt drill driver; less than that and it will lack sufficient power for some common applications. With a 14.4-volt drill, you wont have any trouble boring shallow 1-inch-diameter holes in wood and driving dozens of 3 1/2-inch deck screws on a single charge. For driving screws, a 12-volt impact driver delivers roughly the same performance as a 14.4-volt drill driver because of its high-torque turning ability.
