A hammer drill, also known as a "rotary hammer" or "roto-hammer", (see also rotary hammer drill) is a rotary drill with a hammering action. The hammering action provides a short, rapid hammer thrust to pulverize relatively brittle material and provide quicker drilling with less effort. Lower power units are usually titled as "hammer drills." Higher power units, usually labeled "rotary hammers," tend to be larger and provide bigger impact forces. Modern units allow the hammer and rotation functions to be used separately or in combination, i.e., hammer mode, drill mode, or both. When used in the hammer mode, the tool provides a drilling function similar to a jackhammer. The definitive origin of the first hammer drill is a matter of discussion. Some claim James D. Smith developed the product in 1975 after he found that a hand-held drill did not have the ability to go through the sandstone he was using during his years as a stonemason,[citation needed] however Hilti had a rotary hammer on the market in 1967 [[1]] and the Milwaukee Electric Tool Corporation claims that in 1935, it was selling a lightweight 3/4-inch electric hammer drill [[2]].
Hammer drills are well suited for drilling holes in masonry or stone. They are also used to drill holes in concrete footings to pin concrete wall forms and to drill holes in concrete floors to pin wall framing. The hammering action helps to break up the masonry so that it can be removed by the drill bit's flutes.

[Types of hammer drills

A hammer drill can either be a drill (such as the one illustrated here) or it can be a replacement chuck that is installed on a power drill. When the drill is set to the "hammer drill" position it allows the chuck to move backward and forward over a short distance, allowing a spring to be compressed. When the spring is compressed, the chuck assembly is pushed as far back as it can go against a small eccentric flange. Any rotation of the chuck causes this flange to push back and reverse the direction of travel. The combined action of the spring, eccentric flange and rotation push the chuck back away from the body of the drill at regular intervals. Under normal operation this sequence causes the whole assembly to "kick" with each turn, jack-hammering the drill bit into the material as it rotates.

A hammer drill has a specially designed clutch that allows it to not only spin the drill bit, but also to punch it in and out (along the axis of the bit). The actual distance the bit travels in and out and the force of its blow are both very small, and the hammering action is very rapid -- thousands of "BPM" (Blows per Minute) or "IPM" (Impacts per minute). Although each blow is of relatively low force, these thousands of blows per minute are more than adequate to break up concrete or brick, using the masonry drill bit's carbide wedge to pulverize it for the spiral flutes to whisk away. For this reason, a hammer drill drills much faster than a regular drill through concrete or brick.
Hammer drills almost always have a lever or switch that locks off the special "hammer clutch," turning the tool into a conventional drill for wood or metal work. Hammer drills are more expensive and more bulky than regular drills, but are preferable for applications where the material to be drilled—concrete block or wood studs—is unknown. For example, an electrician would use a hammer drill for attaching items (such as an electical box) to either wood studs (if used as a conventional power drill) or masonry walls (if used as a hammer drill).
The sparking produced by the motor components of most hammer drills is normal.
[] Rotary hammers

Ramset 342 Dyna Drill and Chipping Hammer, shown with chipping chisel.

Rotary hammers are similar in that they also pound the drill bit in and out while it is spinning. However, they use a piston mechanism instead of a special clutch. This causes them to deliver a much more powerful hammer blow. One can drill bigger holes much faster. Rotary hammers have such force, in fact, that the usual masonry bits are no longer adequate. Their smooth shanks would be pounded loose from the tool's chuck in a few seconds. Therefore, they require special bits which can lock into the rotary hammer and continue on spinning while smashing away.

A number of "special shanks" have been developed by various manufacturers. Over the years a fair number of these proprietary systems evolved, but the remaining shanks in use today are: SDS, SDS-MAX, and SPLINE SHANK.
Rotary hammer drills have an oil filled gearbox, which allows them to operate durably despite the large forces and shocks they receive and the grit-filled environments where they are often used.
Apart from their main function of drilling concrete, the rotary action can be switched off and use is made of just the percussive force. Chisel and point accessories are used for small chipping jobs.
The type of work they do means that they need to have a clutch which cuts in when the drill bit jams. This stops the violent wrenching motion that a drill without a clutch would cause when stopped suddenly from full speed, saving both drill and operator from damage.
Jams are most often caused by hitting reinforcing steel or by a worn bit. In both cases the drill must be disengaged from the bit and the jammed bit backed out of the hole with vise grips or monkey wrench.

A worn drill bit will still drill a horizontal hole, although of a slightly smaller diameter than one created when it was new. When a drill like this is used to drill holes down into a concrete slab, the flutes are so worn that they can no longer lift the dust out of the hole; the concrete dust packs up in the hole and jams the bit.