A drill / driver applies rotational force, a hammer drill applies percussive force, and an impact driver applies both rotational and impact force, making it superior for directing force downward. It does this by means of an internal spring, hammer and anvil. As the motor turns the shaft, a spring simultaneously compresses and then releases, driving a hammer against the anvil at a rate of more than 50 strikes per second.
An impact driver delivers significantly more torque than a drill / driver, and is less likely to twist your wrist when the fastener stops or binds. It’s also much less likely to strip the head of the fastener, though you can shear it off. Impact drivers have a spring-loaded, quick-release chuck that accepts 1/4″ hex shank impact-rated driver bits and sockets. You can easily drive lag bolts up to 1/2″ by 6″ (longer for smaller diameter bolts) into structural lumber. Some models incorporate a switch that enables you to set the speed / torque level (low, medium or hard) to match the task at hand. You can purchase an impact driver as a tool only (if you already have extra batteries from the manufacturer), in a kit that consists of one or two batteries and charger, or as part of a kit containing other power tools. If most of the work you do is small-scale, where you’ll be using 8- to 10-gauge screws in shorter lengths, opt for a 12V model.
To avoid wood splitting, particularly split-prone wood, and when screwing near the end of a board, it’s best to drill a pilot hole before sinking the screw.
Impact drivers can deliver considerable impact force. If your impact driver has a torque/speed switch, use it to match the size of screws you’re sinking – lower torque for smaller diameter and shorter screws.
Impact drivers don’t have clutches. Learn to feather the trigger by pulling on it in short spurts. This protects the screw from the full torque of the impact driver, making it less likely to shear off the head of the screw.
If your impact driver comes with a single battery, buy a second. You’ll always have a fresh power supply ready to go.