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]].

Core drill bits are similar in appearance to [[reamer]]s as they have no cutting point or means of starting a hole. They have 3 or 4 flutes which enhances the finish of the hole and ensures the bit cuts evenly. Core drill bits differ from reamers in the amount of material they are intended to remove. A reamer is only intended to enlarge a hole a slight amount which, depending on the reamers size, may be anything from 0.1 millimeter to perhaps a millimeter. A core drill bit may be used to double the size of a hole.  

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.

[[Printed circuit board]]s are usually made of [[fiberglass]], which due to being highly abrasive, would quickly ruin a normal drill bit, especially given the many hundreds or thousands of holes on most circuit boards. To solve this problem, solid [[tungsten carbide]] twist bits are almost always used, which drill quickly through the board while providing a moderately long life. Carbide PCB bits are estimated to outlast high speed steel bits by a factor of ten or more.

Installer bits are a type of twist drill bit for use with a hand-portable power tool. They are also  known as '''bell-hanger''' bits or '''fishing''' bits. The key distinguishing feature of an installer bit is a transverse hole drilled through the web of the bit near the tip. Once the bit has penetrated a wall, a wire can be threaded through this transverse hole, and the bit pulled back through the drilled hole. The wire can then be used to pull a cable or pipe back through the wall. This is especially helpful where the wall has a large cavity, where threading a fishtape could be difficult. Some installer bits have a transverse hole drilled at the shank end as well. Once a hole has been drilled, the wire can be threaded through the shank end, the bit removed from the chuck, and all pulled forward through the drilled hole. Sinclair Smith of [[Brooklyn, New York]] was issued US patent 597750 for this invention on [[25 January]] [[1898]].

A variant of the installer bit has a very long flexible shaft, up to 72 inches long in the US, with a small twist bit at the end. The shaft is made of [[Martensite|spring steel]] steel instead of hardened steel, and can be flexed and bent while drilling. This unique design allows the bit to be curved inside walls, for example to drill through [[stud]]s from a [[light switch]] box without needing to remove any material from the wall. These bits usually come with a set of special tools to aim and flex the bit to reach the desired location and angle, although the problem of seeing where the operator is drilling still remains.

The original [[patent]] for the [[dual cone roller bit|rotary rock bit]] was issued to [[Howard Hughes Sr.]] for his [[dual cone roller bit]] in 1909. It consisted of two interlocking wheels. [[Walter Benona Sharp]] worked very closely with Hughes in developing the Rock Bit. The success of this bit led to the founding of the [[Sharp-Hughes Tool Company]].  

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.

'''high Carbon steel''' bits are made from high [[carbon steel]] and are an improvement on plain steel due to the [[Heat treatment|hardening and tempering]] capabilities of the material. These bits can be used on wood or metal, however they have a low tolerance to excessive heat which causes them to lose their [[Tempering|temper]], resulting in a soft cutting edge.

'''[[High speed steel]]''' (HSS) is a form of [[tool steel]] where the bits are much more resistant to the effect of heat. They can be used to drill in metal, hardwood, and most other materials at greater cutting speeds than carbon steel bits and have largely replaced them in commercial applications.

'''[[Cobalt]] steel''' alloys are variations on high speed steel which have more cobalt in them.  Their main advantage is that they hold their hardness at much higher temperatures, so they are used to drill [[stainless steel]] and other hard materials.  The main disadvantage of cobalt steels is that they are more brittle than standard HSS.