The term double drill can be used to refer to a number of different tools. The two-headed drill press, a lifting mechanism, and a hand held power tool are examples. The term double drill may also refer to a type of drill bit.
The two-headed drill press, similar in appearance to a milling machine, is employed in the manufacturing industry. It differs from the conventional drill press in that it has a larger work table. Some models offer the option of independently angled power heads. This allows for either parallel or opposing angles for the holes being drilled. It can be either horizontal and vertical with the heads placed as desired.
Operations involving the movement of large machinery or other items may use a type of jack also called a double drill. The name comes from the use of two spindles with a deeply cut, coarse helical thread connected by a horizontal bar. The mechanism operates the same way as a C-clamp does to provide pressure. As the spindles are rotated, guides follow the thread and raise or lower the bar. This tool is most commonly used for well drilling.
A hand-held power tool with two separate operating chucks is also referred to as a double drill. The double drill makes it possible to drill a hole and then drive in a screw single-handedly. This eliminates the need to carry a second drill, and saves time as well. The tool uses a rotating platform which moves the desired bit to the driving position. The user clicks a lever with his or her thumb which rotates the chucks.
John Whitehead of New South Wales invented this tool in 1987. It took him 18 years to develop a working prototype. It was designed to allow the user to alternate between two different drill or driver bits. He was concerned about safety and efficiency, inspired by his own experience of climbing a ladder repeatedly in order to change drill bits.
Double drill is also used to describe a double-ended drill bit. This bit has cutting surfaces on both ends, and allows longer periods of use in between sharpening. This drill bit is often confused with a double margin drill which is a drill bit that is designed to cut the high precision holes needed in aircraft and motor manufacturing.
A tool that is less well known is the double pinion hand drill. This tool is sometimes classed as an antique hand drill. At first glance it is constructed in much the same way as a manual egg beater. The chuck is driven by a manually operated gear and pinion arrangement mounted on a straight handle. It is preferred by some craftsmen because of the amount of control it offers while drilling.