Rotary Hammers vs. Hammer Drills: Which one is right for you?
This guide also includes information on SDS, SDS-Plus, SDS-Max, and Spline drive

A hammer-drill is a beefed-up version of a standard drill, good for light-duty concrete or masonry drilling that requires holes up to 3/8-inch diameter. A hammer-drill can rotate and hammer back and forth while it drills, allowing it to bore into concrete without burning the bit. Because the hammer feature is added to a drill, it takes the same straight-shank bits as a regular drill. Due to the torque produced when drilling concrete, these bits may slip in the chuck. Most manufacturers have hammer-drills in their tool line. A hammer-drill is a good tool for homeowners, or the occasional odd job. They are generally considered to be impractical for contractors.

A Rotary Hammers (or Roto-Hammers) are basically hammer-drills on steroids. Though roto-hammers work in the same 'rotate-and-hammer' manner as hammer drills , they are much more powerful. They use a piston mechanism to provide the hammer action, allowing them to deliver a much more powerful blow. Rotary hammers are good for heavy concrete drilling, specifically for deep holes and those larger than 3/8-inch diameter. One of the benefits of these tools is the chuck/drive system: the most common are 'SDS', 'SDS Plus', 'SDS Max', and 'spline.'

Developed by Bosch in 1975 for hammer drills, the SDS (German, Steck - Dreh - Sitz; Insert - Twist - Stay) uses a cylindrical shank on the bit, with indents held by the chuck. Bits are inserted into the tool chuck by pressing in, and locked in place until a separate lock release is engaged. The rotary force is supplied through wedges that fit into two or three open grooves. Two sprung balls fit into closed grooves, allowing movement whilst retaining the bit. The hammer action actually moves the bit up and down within the chuck since the bit is free to move a short distance. SDS requires the tool and bit have the same shank diameter as the chuck - there are two common sizes:

Some roto-hammers also offer the ability to turn off the drilling part of the action, leaving just the hammer mode. With optional chisels inserted instead of drill bits, this duplicates the action of a jackhammer: excellent for chipping concrete or bricks.

SDS/SDS-Plus: a 10 mm shank with two/four open grooves held by the driving wedges, and two closed grooves held by locking balls. Regular SDS bits have 2 grooves; SDS+ bits have four. SDS and SDS+ bits are interchangeable. This is the most common size and works on hammers up to 8lbs (4kg). The wedges grip an area of 75 mm(squared) and the shank is inserted 40 mm into the chuck. SDS/SDS-Plus bits run up to 1-1/8-inch diameter.

SDS-Max: an 18 mm shank with three open grooves and locking segments rather than balls. It is designed for hammers over 11lbs (5 kg). The wedges grip an area of 389 mm(squared) and the shank is inserted 90 mm. SDS-Max drive bits go up to 2-inch diameter.

Spline drives are not as common in the marketplace but have a following with contractors. The design of the spline drive provides more surface-grab for high-torque use. Spline drive bits go up to 2-inch diameter.

Some additional common features you might look for:

Variable Speed Selection - Allows the user to adjust speeds on the tool. High speeds are used for small holes while slow speeds are used for large holes.

2 or 3 mode - Allows the user to select actions (hammer and drill, hammer only, drill only).

Reverse - Allows the user to back a bit out of the hole.

Depth Rod - A devise mounted on the top or side of the drill that lets the user set a desired depth for the hole.

Side Handle - Allows the user an additional hand to hold the tool while drilling, giving better control and more force . Most can be removed for use in a tight spot.