Left hand drill bits, often simply called “screw extractors” are used to remove a broken or damaged screw that cannot be taken out in the conventional manner using a wrench or screw driver. When the head of a bolt becomes rounded through repeated use or it breaks off altogether it is very difficult to remove from the machine. The same is true if you happen to overstress a drill bit and it breaks off in the material. A screw extractor is the tool used to remove the broken or damaged screw with minimal damage, if any, to the objects surface.
A screw extractor has a similar appearance to left hand drill bits, they are available in a wide variety of sizes and they are manufactured from high strength steel. Whereas a standard drill bit has a tapered cutting surface, a screw extractor is tapered to a point and the body of the device is threaded very much like a screw. The tools top has been machined into a square, this allows the tool to be used in a hand held device or an electric drill; some machinists have a preference for using a pair of pliers rather than a special device.
To remove the broken screw the first thing that the operator does is to drill a hole on the top of the broken screw using a conventional right hand drill bit. This hole is what guides the screw extractor. Once the hole has been drilled an extractor of the correct size is selected, the correct size extractor will minimize any potential damage to the surrounding surface. The correct size extractor is one which is as close as possible to the diameter of the broken screw.
Although an electric drill can be used to turn the extractor, it is often better to mount it in a handle for manual removal. The point of the extractor is inserted into the pilot hole that was drilled and the handle is turned clockwise, the same as if a bolt or screw was being installed. This motion causes the extractor to lock into the broken screw. As the extractors are similar to left hand drill bits, as they are turned clockwise, the screw turn counterclockwise and removes the screw.
