A round shank bit is subject to bit slippage, which could slow down productivity when the operator stops to tighten the bit. Another caveat: The chuck is held in place by a screw at its base. During drilling into concrete, the bit eventually gets pushed to the base of the chuck and over time could destroy the screw, making chuck replacement more difficult. Another consideration is that hammer drills produce a higher-pitched and louder noise than rotary drills.
Feature-rich hammer drills have two speed ranges and a gearbox for switching speed ranges. Typically, an electrician uses the lower speed for concrete drilling and the higher one for steel and wood. A two-speed gearbox offers more control than the one-speed variety, because the user is relying on more than just the trigger finger to control speed. Too fast an rpm on concrete can dull or glaze the bits and ruin them.
Other attractive features to look for in a hammer drill include: a variable speed trigger, a lock-on button, a front handle that can rotate 360 degrees, contoured and cushioned grips to minimize operator fatigue, and a depth gauge.
While tool choice is always an operator’s decision, a rule of thumb suggests use of a hammer drill when drilling up to 3/16 inch diameter holes in concrete; use of a rotary hammer when drilling holes larger than 3/16 inch for better productivity.