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 mounting an electrical box to a wall would be able to use the same hammer drill to drill into either wood studs (hammer disabled) or masonry walls (hammer enabled). History Ancient China's principal drilling technique, percussive drilling, was invented during the Han dynasty. The process involved two to six men jumping on a level at rhythmic intervals to raise a heavy iron bit attached to long bamboo cables from a bamboo derrick. Utilizing cast iron bits and tools constructed of bamboo, the early Chinese were able to use percussion drilling to drill holes to a depth of . The construction of large wells took more than two to three generations of workers to complete. The cable tool drilling machines developed by the early Chinese involved raising and dropping a heavy string of drilling tools to crush through rocks into diminutive fragments. In addition, the Chinese also used a cutting head secured to bamboo rods to drill to depths of . The raising and dropping of the bamboo drill strings allowed the drilling machine to penetrate less dense and unconsolidated rock formations. The origin of the first hammer drill is a matter of contention. German company Fein patented a ("drill with electro-pneumatic striking mechanism") in 1914. German company Bosch produced the first "Bosch-Hammer" around 1932 in mass production. The US company Milwaukee Electric Tool Corporation states that in 1935, it was selling a lightweight electric hammer drill (cam-action). Hand-cranked percussion drills were made in the UK in the mid-twentieth century. See also Drill Rotary hammer References External links NIOSH Sound Power and Vibrations Database New York City Quiet Vendor Guidelines Power tools Hand-held power tools