Drill bit: Difference between revisions

Another type of left-hand bit is an extraction tool used expressly for removing broken or seized screws, other than by drilling. It has a highly tapered thread structure on it, and is inserted into a drilled hole (of the recommended size) in the damaged screw. If a left hand drill bit is used initially, and the act of drilling the hole does not release the screw, this tool may remove it. In use, the extractor is rotated and the action of the taper and spiral digs into the damaged material causing it to lock tightly and hopefully applies enough pressure to remove the screw. The tool has a tendency to continue winding in while being turned and this may cause the extractor to expand the screw in the hole causing it to bind further, leading to failure of the process. These bits are made of very hard, but brittle, steel, which means they can break off inside the screw if too much force is applied, making the removal much more difficult.  Because of this an alternative extractor has four parallel edges, which tends not to self-tighten.  Alternatively, the hole can be drilled with successively larger bits until it can be [[taps and dies|tapped]].

In metal, the lip and spur drill is confined to drilling only the thinnest and softest [[sheet metal]]s in a [[drill press]].  The drills are an extremely fast cutting tool geometry: no point angle and a large (considering the flat cutting edge) lip angle causes the edges to take a very aggressive cut with relatively little point pressure.  In metal, this means the drill tends to bind, or given a workpiece of sufficient thinness, the drills have a tendency to punch through and leave the drill's cross-sectional geometry behind.

An adjustable wood bit has a small center pilot bit with an adjustable, sliding cutting edge mounted above it, usually containing a single sharp point at the outside, with a [[set screw]] to lock the cutter in position. When the cutting edge is centered on the bit, the hole drilled will be small, and when the cutting edge is slid outwards, a larger hole is drilled. This allows a single drill bit to drill a wide variety of holes, and can take the place of a large, heavy set of different size bits, as well as providing uncommon bit sizes. A [[ruler]] or [[Vernier scale]] is usually provided to allow precise adjustment of the bit size.