You probably mean a hammer drill. An impact driver provides large torque, not cutting action. A hammer drill has a sawtooth ribbed plate inside that causes the bit to be driven into the work repeatedly when the drill is operated. There's usually a switch to disconnect the hammer action and run as a normal drill. A lot of consumer brands are ****. I returned about 5 to the store before ending with a Bosch that is still working (though in one the vibration sheared off a component in the electronic speed control). Should be OK just for 4 holes though. (I had maybe 50). With all the hassle, I should probably have sprung for a professional Hilti, or rented if I could have done it all in one day. Personally, I would not get a cordless hammer drill, unless maybe a new Li-Ion model. I'd get corded. Busting concrete for a 3/8" hole takes power. When I wanted to drill holes in a rock in the middle of nowhere, i used a car battery and a 1000W inverter.