While impact wrenches look an awful lot like power drills from the outside, they are not the same tool. Where most drills impart rotary motion to the bit directly, an impact wrench provides it indirectly, hammering the anvil with torque from a rotating weight. These torque spikes are very powerful, and can quickly break even the toughest bolts free.Pneumatic Impact Wrenches
When it comes to power tools, air is at the top of the food chain. Air impact wrenches are usually the most powerful, but they are also the least common for home use. Where other tools require electricity, these wrenches need a complete compressed air infrastructure from a compressor and tank, all the way to hoses and fittings. They are great if you have lots of big jobs, but a hobbyist won't have enough need to justify one.Corded Impact Wrenches
Corded impact wrenches draw on mains power so you can keep using them all day. Unlike air tools, they rely on an infrastructure that you already have so there's less to worry about when it comes to getting it set up. Unfortunately, just as air tools can only go as far as the hose reaches, corded tools can only reach as far as the extension cord.Cordless Impact Wrenches
Battery cordless impact wrenches mean freedom. With the more powerful batteries in these devices, people finally had a tool that could go anywhere they could. Get a spare battery and you can have one charging while you're using the other. If you would like to remove another step from your work, try regular cordless impact wrenches that dock and recharge without changing the batteries. Cutting the cord lets you work on your terms.Using Impact Wrenches
Using impact wrenches to break fixed bolts loose is simple, using them to tighten is something else. When you're tightening fasteners, you never want to use an impact wrench to thread the nut or for the final touches. It's just too powerful; you can crossthread a bolt as easily as threading a nut, and you're almost sure to over-tighten if you ride the impact wrench all the way down.