I’m not entirely sure what the difference is, but a cursory reading leads me to believe that a hammer drill has a back-and-forth motion (like hammering in a nail) whereas the impact driver’s impacts are left-to-right (in line with the screw’s direction of rotation).
Is that a correct understanding?
Yup.
The hammer drill spins while knocking at the material. It either chips it, or causes the bit to bite in. The impact driver is like putting a socket and wrench on a nut, and then tapping the wrench with a hammer. Neither one needs to apply much force: its death by 1000 taps.
Yes. They are also essentially two different tools. An impact is not normally used on screws, but nuts & bolts. The quintessential kind is used to remove car lug nuts. Also impact guns are most often driven by compressed air. A hammer drill is, well, a special type of drill. It has a standard drill bit chuck and is usually electrically powered (though large ones can be pneumatic) and adds a ‘hammering’ motion like you describe. These are often used for drilling into hard surfaces, namely concrete and masonry.
An electric screw gun is just a drill-type driver for putting in screws more fast & easily, but not into hard materials. It may have a clutch-slipping drive that engages when pushed against, but it doesn’t add any additional force other than rotational torque.
Electric impact drivers, both AC and battery, are regularly used to drive deck and lag screws. A 3" wood screw will go in a lot faster with an impact driver than with a drill/driver. I use mine for wood screws larger than #8.
But are these actually called ‘impacts’? I always just called them screw guns. And given the way screws work I don’t think you’d *want *to add any pounding down-force to them, just torque.
My impact driver is driven by a mallet. My hammer drill has a special kind of slip clutch.
I wouldn’t use either term without clarification if it was important.
The impact drivers, not impact wrenches, have no downward force other than what you apply to keep the bit in the screw head. They will drive a screw much faster than a drill/driver. I’m talking about models made by Hitachi, Milwaukee, Bosch and the like.
http://www.amazon.com/gp/aw/d/B000JEYJ1S
Look at the guts here.
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