Editing Drill bit (section)

==Characteristics==
{{see also|Drill bit sizes}}

Drill geometry has several characteristics:
* The '''spiral''' (or rate of twist) in the drill bit controls the rate of [[swarf|chip]] removal. A fast spiral (high twist rate or "compact flute") drill bit is used in high feed rate applications under low spindle speeds, where removal of a large volume of chips is required.  Low spiral (low twist rate or "elongated flute") drill bits are used in cutting applications where high cutting speeds are traditionally used, and where the material has a tendency to [[galling|gall]] on the bit or otherwise clog the hole, such as [[aluminum]] or [[copper]].
* The '''point angle''', or the angle formed at the tip of the bit, is determined by the material the bit will be operating in. Harder materials require a larger point angle, and softer materials require a sharper angle. The correct point angle for the hardness of the material influences wandering, chatter, hole shape, and wear rate.
* The '''lip angle''' is the angle between the face of the cut material and the flank of the lip, and determines the amount of support provided to the cutting edge. A greater lip angle will cause the bit to cut more aggressively under the same amount of point pressure as a bit with a smaller lip angle. Both conditions can cause binding, wear, and eventual catastrophic failure of the tool. The proper amount of lip clearance is determined by the point angle. A very acute point angle has more web surface area presented to the work at any one time, requiring an aggressive lip angle, where a flat bit is extremely sensitive to small changes in lip angle due to the small surface area supporting the cutting edges.
* The '''functional length''' of a bit determines how deep a hole can be drilled, and also determines the stiffness of the bit and accuracy of the resultant hole.  While longer bits can drill deeper holes, they are more flexible meaning that the holes they drill may have an inaccurate location or wander from the intended axis. Twist drill bits are available in standard lengths, referred to as Stub-length or Screw-Machine-length (short), the extremely common [[Drill bit sizes#Jobber-length drill|Jobber-length]] (medium), and Taper-length or Long-Series (long).

Most drill bits for consumer use have straight shanks. For heavy duty drilling in industry, bits with [[Morse taper|tapered]] shanks are sometimes used. Other types of shank used include hex-shaped, and various proprietary quick release systems.

The diameter-to-length ratio of the drill bit is usually between 1:1 and 1:10. Much higher ratios are possible (e.g., "aircraft-length" twist bits, pressured-oil [[gun drill|gun drill bits]], etc.), but the higher the ratio, the greater the technical challenge of producing good work.

The best geometry to use depends upon the properties of the material being drilled. The following table lists geometries recommended for some commonly drilled materials.

{| class="wikitable" style="text-align:right;"
|+Tool geometry<ref>{{Citation|first1 = Robert H.|last1 = Todd|first2 = Dell K.|last2 = Allen|first3 = Leo|last3 = Alting|year = 1994|title = Manufacturing Processes Reference Guide|publisher = Industrial Press Inc.|pages = 43–48|url = https://books.google.com/books?id=6x1smAf_PAcC|isbn = 0-8311-3049-0|postscript =.}}</ref>
|-
! Workpiece material
! Point angle
! Helix angle
! Lip relief angle
|-
| {{left|Aluminum}}
| 90–135
| 32–48
| 12–26
|-
| {{left|[[Brass]]}}
| 90–118
| 0–20
| 12–26
|-
| {{left|[[Cast iron]]}}
| 90–118
| 24–32
| 7–20
|-
| {{left|[[Mild steel]]}}
| 118–135
| 24–32
| 7–24
|-
| {{left|[[Stainless steel]]}}
| 118–135
| 24–32
| 7–24
|-
| {{left|Plastics}}
| 60–90
| 0–20
| 12–26
|}

===Materials===
[[Image:Titanium nitride coating.jpg|thumb|right|75px|upright|[[Titanium nitride]] coated twist bit]]

Many different materials are used for or on drill bits, depending on the required application. Many hard materials, such as carbides, are much more brittle than steel, and are far more subject to breaking, particularly if the drill is not held at a very constant angle to the workpiece; e.g., when hand-held.

====Steels====
* Soft '''low-carbon [[steel]]''' bits are inexpensive, but do not hold an edge well and require frequent sharpening. They are used only for drilling wood; even working with [[hardwood]]s rather than [[softwood]]s can noticeably shorten their lifespan.
* Bits made from '''high-carbon steel''' are more durable than low-carbon steel bits due to the properties conferred by [[Heat treatment|hardening and tempering]] the material. If they are overheated (e.g., by frictional heating while drilling) they lose their [[tempering (metallurgy)|temper]], resulting in a soft cutting edge. These bits can be used on wood or metal.
* '''[[High-speed steel]]''' (HSS) is a form of [[tool steel]]; HSS bits are hard and much more resistant to heat than high-carbon steel. They can be used to drill metal, hardwood, and most other materials at greater cutting speeds than carbon-steel bits, and have largely replaced carbon steels.
* '''[[Cobalt]] steel''' [[alloy]]s are variations on high-speed steel that contain more cobalt.  They hold their hardness at much higher temperatures and 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.

====Others====
* '''[[Tungsten carbide]]''' and other [[carbide]]s are extremely hard and can drill virtually all materials, while holding an edge longer than other bits. The material is expensive and much more brittle than steels; consequently they are mainly used for drill-bit tips, small pieces of hard material fixed or [[brazing|brazed]] onto the tip of a bit made of less hard metal. However, it is becoming common in job shops to use solid carbide bits. In very small sizes it is difficult to fit carbide tips; in some industries, most notably [[printed circuit board]] manufacturing, requiring many holes with diameters less than 1&nbsp;mm, solid carbide bits are used.
* '''Polycrystalline diamond''' (PCD) is among the hardest of all tool materials and is therefore extremely resistant to wear.  It consists of a layer of diamond particles, typically about {{convert|0.5|mm|in|abbr=on}} thick, bonded as a [[sintered]] mass to a tungsten-carbide support. Bits are fabricated using this material by either brazing small segments to the tip of the tool to form the cutting edges or by sintering PCD into a vein in the tungsten-carbide "nib".  The nib can later be brazed to a carbide shaft; it can then be ground to complex geometries that would otherwise cause braze failure in the smaller "segments". PCD bits are typically used in the automotive, aerospace, and other industries to drill abrasive aluminum alloys, carbon-fiber reinforced plastics, and other abrasive materials, and in applications where machine downtime to replace or sharpen worn bits is exceptionally costly. PCD is not used on ferrous metals due to excess wear resulting from a reaction between the carbon in the PCD and the iron in the metal.

===Coatings===
[[Image:2mm diamond drill bits macro.jpg|thumb|Diamond-coated 2&nbsp;mm bits, used for drilling materials such as glass]]
* [[Black oxide]] is an inexpensive black coating. A black oxide coating provides heat resistance and lubricity, as well as corrosion resistance. The coating increases the life of high-speed steel bits.
* [[Titanium nitride]] (TiN) is a very hard metallic material that can be used to coat a high-speed steel bit (usually a twist bit), extending the cutting life by three or more times. Even after sharpening, the leading edge of coating still provides improved cutting and lifetime.
* [[Titanium aluminum nitride]] (TiAlN) is a similar coating that can extend tool life five or more times.
* Titanium carbon nitride (TiCN) is another coating also superior to TiN.
* Diamond powder is used as an abrasive, most often for cutting tile, stone, and other very hard materials. Large amounts of heat are generated by friction, and diamond-coated bits often have to be water-cooled to prevent damage to the bit or the workpiece.
* [[Zirconium nitride]] has been used as a drill-bit coating for some tools under the [[Craftsman (tools)|Craftsman]] brand name.
* Al-Chrome Silicon Nitride (AlCrSi/Ti)N is a multilayer coating made of alternating nanolayer, developed using [[chemical vapor deposition]] technique, is used in drilling [[carbon fiber reinforced polymer]] (CFRP) and CFRP-Ti stack. (AlCrSi/Ti)N is a superhard ceramic coating, which performs better than other coated and uncoated drill.<ref name="doi.org">[[doi:10.1115/1.4040916|Swan et al (September 7, 2018). "Tool Wear of Advanced Coated Tools in Drilling of CFRP." ASME. J. Manuf. Sci. Eng. November 2018; 140(11): 111018.]]</ref><ref>[[doi:10.1115/MSEC2019-2843|Nguyen, Dinh et al "Tool Wear of Superhard Ceramic Coated Tools in Drilling of CFRP/Ti stacks." Proceedings of the ASME 2019 14th International Manufacturing Science and Engineering Conference. Volume 2: Processes; Materials. Erie, Pennsylvania, USA. June 10–14, 2019. V002T03A089. ASME.]]</ref>
* BAM coating is [[Boron]]-Aluminum-[[Magnesium]] BAlMgB14 is a superhard ceramic coating also used in composite drilling.<ref name="doi.org"/><ref>[[doi:10.1115/MSEC2019-2843|Nguyen, Dinh et al "Tool Wear of Superhard Ceramic Coated Tools in Drilling of CFRP/Ti Stacks." Proceedings of the ASME 2019 14th International Manufacturing Science and Engineering Conference. Volume 2: Processes; Materials. Erie, Pennsylvania, USA. June 10–14, 2019. V002T03A089. ASME.]]</ref>