Drill bit: Difference between revisions

Center drill bits are used in [[metalworking]] to provide a starting hole for a larger sized drill bit, or a conical indentation in the end of a workpiece to mount a [[lathe center]]. These centers are used when turning or grinding workpieces. A workpiece machined ''between centers'' can be safely removed from one process (perhaps turning in a lathe) and set up in a later process (perhaps a [[grinding]] operation) without losing any concentricity.

While the above is common, it is incorrect practice.  Centre drills are meant to create a centre for lathe work only. The correct tool to start a hole is a [[spotting drill]], because the included angle of the spotting drill is the same as a conventional drill bit so the drill bit will then start without chatter. Centre drills wander as easily as anything else in hand-held power drills - for such operations, a [[centre punch]] should be used to spot the planned hole centre prior to drilling a [[pilot hole]]. That said, a centre drill works nearly as well as a spotting drill for most rigidly-clamped drilling operations, especially in softer metals such as aluminium and its [[alloys]].

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.

The hinge sinker bit is an example of a custom drill design for a specific application. Many European kitchen cabinets are made from [[particle board]] or [[medium-density fibreboard]] (MDF) with a laminated plastic veneer. Those types of [[pressed wood]] boards are not very strong, and the screws of butt [[hinge]]s tend to pull out. A specialist hinge has been developed which uses the walls of a 30 mm diameter hole, bored in the particle board, for support. This is a very common and relatively successful construction method.

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.

soft '''low carbon steel''' bits are used only in wood, as they do not hold an edge well, and require frequent sharpening. Working with [[hardwood]]s can cause a noticeable reduction in lifespan. They are, however, inexpensive.