Anatomy of drill bits

Anatomy of a drill bit

With a large number of drill bit types and 1000s of sizes out there we’re going to explain the differences and help you decide on the right drill bit to suit your application.

Materials used to make drill bits:

  • Carbon Steel
  • Solid Carbide
  • High Speed Steel (HSS)
  • Tool Steel w/ Carbide Tips
  • Cobalt Steel

Coatings used on drill bits:

  • Black Oxide – One of the most cost-effective coating. Black oxide gives rust protection, will increase tempering and stress relieving of the bit, minimizes galling and chip welding, and supports retaining drilling lubricants. Black oxide is great for drilling iron and steel however is not appropriate for drilling magnesium, aluminum, or similar materials.
  • Bronze Oxide – Bronze oxide improves tempering and stress relieving of the drill bit and is typically used alone to visually recognize cobalt steel or with black oxide to acknowledge superior grades of high speed steel.
  • Titanium Nitride (TiN) – A much more expensive coating that boosts the hardness of the bit and offers a thermal barrier causing increased manufacturing rates and prolonged tool life in harder materials. TiN also offers precisely the same benefits as bronze and black oxide. TiN coated bits are compatible with drilling iron and steel along with magnesium, aluminum, etc.

Drill bits are generally classified as Twist Drill, Countersink, Counterbore, Flat Bottom Boring, and Specialty. On our website we’ve divided these into 2 categories:

  • Drill Bits – Used with portable drills or drill presses fitted with regular chucks. These machines work with a key to tighten up or loosen the clamping mechanism.
  • Boring Machine Bits – Usually found in automated machinery like drill banks or CNC machines fixed with tool holders or single size collets. These devices need tools to assemble and disassemble the clamping mechanism. A lot of boring machine bits can be found in left or right hand rotation.

*Be aware that most traditional drill bits work extremely well in automated machinery (using the proper adapter) and a lot of boring machine bits are available in a portable drill or drill press (presuming the chuck is large enough). We’ve only organized them in this way to make selection better.


Twist Drill ExampleTwist Drills

General Description: Twist drill bits are the commonest variety of drill bit and are also used for everyday drilling for all sorts of material. They’re also the most confusing due to the sheer number of size, tip, and material specifications.

Length Designations: The length of a twist drill has a lot to do with its solidity – a shorter bit will likely be stronger and much less to wander or break but might not have the reach necessary for all jobs. Twist drill bits for use in programmed machinery offer an actual length specification (e.g. 4-1/2″) while many (not all) twist drills to be used in portable drills are graduated length and use an identity to specify the length variety:

  • Jobber Length: These are the basic and common twist drills and are also a good compromise in between length and strength. Jobber drills differ in length in accordance with their diameter and frequently have a flute length of 9 to14 times the cutting diameter, i.e. a 1/2″ jobber drill bit has a flute length of 4-1/2″ (nine times the diameter) with smaller drills having a bigger ratio.
  • Mechanics Length: It is Usually shorter than jobber drills, mechanics length drills are named as such as they fit into tighter spaces and are also less likely to break while still allowing an acceptable flute length.
  • Screw Machine Length:Also known as “stubby length”, these are the smallest common drill bits. Initially designed for screw machines, a lot of people prefer these because of their high strength and extra working clearance.
  • Extra Length: These are extra long drill bits (around 18″) with flutes stretching the full length of the bit. Extra length drills are often very fragile and merely broken therefore it’s usually better to drill as deep as possible having a jobber or smaller bit before switching to the extra length drill bit.
  • Aircraft Extension: Very similar in length to extra length bits, aircraft extension drill bits emphasise reach over cutting depth and also have a shorter flute length (about the exact same as a jobber drill). This will make the bit much superior and less prone to bending or breaking.
  • Silver and Deming: Over a length specification, Silver or Deming drill bits are 6″ long having a 3″ flute length plus a 1/2″ diameter shank. All Silver or Deming bits are more than 1/2″ cutting diameter, starting from 33/64″ to 1-1/2″, and are primarily designed for use in a drill press.

Size Designations: Typical twist drills to use in portable drills, etc. come in fractional inch, letter sizes, wire sizes, and metric decimal millimeter. Twist drills for use in programmed machinery are merely found in fractional inch and decimal millimeter. Take a look at our Drill Bit Size Chart for the breakdown of letter, fractional, and wire sizes.

Tip Styles: Twist drill bits can be found with various tip styles to fit different applications as layed out below:

  • Standard Drill PointConventional Drill Point: It is the most popular tip style as seen on every day general purpose drills. The tip angle is often 118 degrees but can change from 90° to high angle “Plexi Point” to use in acrylics. Conventional drill point drills will be the most cost effective and are easily re-sharpened. Suited to non-ferrous metals, wood, and mild steel.
  • Split Drill PointSplit Drill Point: This is an advanced drill point that prevents walking and provides improved penetration with less effort. Available in 118 or 135 degree angles, split point drill bits are better for drilling in curved surfaces or in alloy steels. They are more expensive and more difficult to re-sharpen than standard drill points.
  • V-Point DrillV-Point: This is a special high angle tip used on drill bits for automated wood boring machines. V-point drills are used to create thru-holes in sheet stock for dowels or other assembly hardware.
  • Brad Point DrillBrad Point: Suitable for creating blind holes in timber and other tender materials for shelf pins, dowels, etc. Bradpoints are often used for thru-holes in CNC applications where the typical drill point would penetrate the table below the panel. Bradpoints have spurs at the outer edges to stop splintering and chipping of the surface material and also a center spur to prevent moving as the bit penetrates the exterior.
  • Fishtail DrillFishtail Point: These special drill points form a reverse “V” in the tip and are designed for drilling into a surface at an angle without walking. They are commonly used as center drills in counterbores for furniture assembly where panels must be joined at right angles.
  • Taper Point DrillTaper Point: These drill bits have a very large taper, extending far up the drill which creates a tapered hole. Primarily used for old style wood screws.

Flute Styles: Most twist drills have flutes to evacuate the chips at an unspecified angle, and are suitable for the majority of applications. Some specialty twist drills may be designated as “Fast Spiral”, “High Helix”  or “Slow Spiral”, “Low Helix” for specific applications requiring higher or lower spindle speeds or feed rates.

Shank Styles: Twist drill bits designed for use in automated machinery have fixed diameter (usually 1/2″ or 10mm) shanks, threaded shanks, or specialty shanks designed for certain machines. General purpose twist drills for use in portable drills have shanks the same diameter as the bit size (up to a certain diameter), larger diameter bits incorporate a reduced shank (either 1/4″, 3/8″ or 1/2″) to fit into a standard drill chuck. Some bits have 3 flats on the shank to prevent spinning under high torque loads. Others have 1/4″ hex shanks for use in a portable drill with a hex bit holder.

Materials: General purpose twist drills for use in portable drills are available in different grades of high speed steel as well as cobalt steel and solid carbide. Twist drill bits for automated machinery are available in high speed steel, carbon steel, solid carbide and carbide tipped.

Coatings: General purpose drill bits are available with bronze oxide, black oxide, a combination of bronze and black oxide, and TiN coatings. Twist drills for automated machinery on our site are primarily for use in wood or plastics and are not coated.

Counterbore Drill ExampleCounterbore Drill Bits

General Description: Counterbore drill bits create a flat bottom blind hole with a smaller diameter center hole that penetrates through the material. The purpose of a counterbore is usually to conceal the fastener head (by covering the hole) or provide a recess to prevent the fastener from protruding above the surface of the material being drilled. Counterbores on our site are designed for use in wood or plastics and are not intended for counterboring steel.

Tip Styles: The cutting tip of a counterbore drill consists of one or more flat blades extending from the center drill to the outer edge. Counterbores are available with or without spurs (teeth) at the outer diameter of the bit. Counterbores with spurs on the outer edge prevent chipping and splintering on wood or laminated surfaces.

Flute Styles: Some counterbores do not have flutes (except for the center drill) and simply shave away the material, others are designed similar to a twist drill with no tip angle and a replaceable center drill.

Shank Styles: Counterbores for use in hand-held drills typically clamp on to a standard twist drill and therefore have a straight shank the same diameter as the center drill. Counterbores for use in automated machinery have fixed diameter (usually 1/2″ or 10mm) shanks, threaded shanks, or specialty shanks designed for certain machines.

Materials: Counterbores are available in Carbon Steel, High Speed Steel, or Carbide Tipped

Coatings: Counterbores on our site do not contain special coatings.


Flat Bottom Boring Drill ExampleFlat Bottom Boring Bits

General Description: Flat bottom boring bits are like counterbores but don’t come with a center drill. These bits are meant to drill flat bottom blind loopholes for European style hinges, etc. Flat bottom boring drill bits are usually used for drilling sizeable diameter thru-holes with no plug that is typical when working with a hole saw. These thru-hole bits are used for door knobs, locks, wiring holes, etc.

Common types of Flat Bottom Boring Bits include:

  • Forstner Bits
  • Mortising Bits
  • Door Hinge Bits
  • Three Wing Drills
  • Spade Bits

Tip Styles: The cutting tip of any flat bottom boring drill features one or more smooth blades extending within the center to the outer edge. Flat bottom boring bits can be bought without or with spurs (teeth) with the outer diameter of the bit. Bits with spurs at the outer edge reduce chipping and splintering on timber or laminated surfaces. Quite a few flat bottom boring bits have a center spur to hold the bit from moving during the early cut – Spade bits make the perfect example for this.

Flute Styles: Large flat bottom boring bits do not have flutes, they consist of only the cutting surface and the chips remain in the hole until the bit is removed.

Shank Styles: Most large flat bottom boring bits have a fixed size hexagonal shank. The hex shank prevents the bit from spinning in the chuck under excessive loads.

Materials: Flat bottom boring bits are available in Carbon Steel, High Speed Steel, or Carbide Tipped.

Coatings: Flat bottom boring bits on our site do not contain special coatings.


Specialty Drill Bits

General Description: Specialty drill bits are any bit that doesn’t fall into one of the four categories above. These bits include:

  • Plug CutterPlug Cutters – Essentially a hole saw with no center drill, these bits can cut round plugs from wood or other stock that are would once fill a counterbore after fastening the parts. Once gluing the plug to the counterbore, the plug will likely be cut and sanded flush to hide the hole. Plug cutter bits are assessed by their inside dimension (the size of the plug that’ll be left) and aren’t supposed to drill completely through the stock. Rather, the stock is drilled to the depth slightly bigger than the hole to be filled as well as the plug is popped out by using a compact screwdriver or any other tool. Plug cutters are usually used in a handheld drill.
  • Glass and Tile DrillGlass and Tile Drills – Designed to drill holes (usually for fasteners) in tile, non-tempered glass, and similar materials. These bits feature carbide tips and straight shanks to use either in hand operated drills or automated machinery.
  • Masonry DrillMasonry Drills – Useful to drill holes in brick, concrete, etc. by using a special “hammer drill” which pounds the drill bit while it rotates. A threaded anchor is often installed if the hole shall be used to affix objects on the surface. Masonry drills have carbide tips and either standard or high helix flutes based on the intended use. They’re typically painted with black oxide to avoid corrosion and feature shanks that are possibly the same size as the bit or reduced diameter to match the portable handheld drill chuck.
  • Auger Drill BitAugers – Designed to bore fairly large holes far into wood or any other similar materials. Auger tips have a protruding tapered screw in order to to pull the drill bit into the wood with 1 or 2 cutting edges much like a counterbore and may even include outer spurs. The flutes of the auger drill bit are immense to pull the chips up and out of the hole. Augers tend to be used in hand-held drills and the shank could be round or hexagonal.
  • Annular CutterAnnular Cutters – Designed to drill larger thru-holes in metal (7/16″ and up), such bits need a special machine. Much like a hole saw, annular cutter bits cut the outer diameter of the hole by leaving a good center slug. This results much more effective drilling process with burr-free holes in sheet metal, tubing and iron bar stock.


Drill Bit Size Chart

This data chart shows decimal inch equivalents for fractional inch, letter size, wire size and metric drill bits up to 2 ” in diameter.

SizeDecimal EquivalentSizeDecimal EquivalentSizeDecimal EquivalentSizeDecimal Equivalent
970.00592.60 mm0.1024M0.29563/64″0.9844
960.0063370.1047.50 mm0.29531″1
950.00672.70 mm0.106319/64″0.296925.50 mm1.0039
940.0071360.10657.60 mm0.29921-1/64″1.0156
930.00752.75 mm0.1083N0.30226.00 mm1.0236
920.00797/64″0.10947.70 mm0.30311-1/32″1.0312
910.0083350.117.75 mm0.305126.50 mm1.0433
900.00872.80 mm0.11027.80 mm0.30711-3/64″1.0469
890.0091340.1117.90 mm0.3111-1/16″1.0625
880.0095330.1135/16″0.312527.00 mm1.063
.25 mm0.00982.90 mm0.11428.00 mm0.3151-5/64″1.0781
870.01320.116O0.31627.50 mm1.0827
860.01053.00 mm0.11818.10 mm0.31891-3/32″1.0938
850.011310.128.20 mm0.322828.00 mm1.1024
840.01153.10 mm0.122P0.3231-7/64″1.1094
.30 mm0.01181/8″0.1258.25 mm0.324828.50 mm1.122
830.0123.20 mm0.1268.30 mm0.32681-1/8″1.125
820.01253.25 mm0.12821/64″0.32811-9/64″1.1406
.32 mm0.0126300.12858.40 mm0.330729.00 mm1.1417
810.0133.30 mm0.1299Q0.3321-5/32″1.1562
800.01353.40 mm0.13398.50 mm0.334629.50 mm1.1614
.35 mm0.0138290.1368.60 mm0.33861-11/64″1.1719
.36 mm0.01423.50 mm0.1378R0.33930.00 mm1.1811
790.0145280.14058.70 mm0.34251-3/16″1.1875
.38 mm0.0159/64″0.140611/32″0.343830.50 mm1.2008
1/64″0.01563.60 mm0.14178.75 mm0.34451-13/64″1.2031
.40 mm0.0157270.1448.80 mm0.34651-7/32″1.2188
780.0163.70 mm0.1457S0.34831.00 mm1.2205
.42 mm0.0165260.1478.90 mm0.35041-15/64″1.2344
.44 mm0.01733.75 mm0.14769.00 mm0.354331.50 mm1.2402
.45 mm0.0177250.1495T0.3581-1/4″1.25
770.0183.80 mm0.14969.10 mm0.358332.00 mm1.2598
.46 mm0.0181240.15223/64″0.35941-17/64″1.2656
.48 mm0.01893.90 mm0.15359.20 mm0.362232.50 mm1.2795
.50 mm0.0197230.1549.25 mm0.36421-9/32″1.2812
760.025/32″0.15629.30 mm0.36611-19/64″1.2969
750.021220.157U0.36833.00 mm1.2992
.55 mm0.02174.00 mm0.15759.40 mm0.37011-5/16″1.3125
740.0225210.1599.50 mm0.37433.50 mm1.3189
.60 mm0.0236200.1613/8″0.3751-21/64″1.3281
730.0244.10 mm0.1614V0.37734.00 mm1.3386
720.0254.20 mm0.16549.60 mm0.3781-11/32″1.3438
.65 mm0.0256190.1669.70 mm0.381934.50 mm1.3583
710.0264.25 mm0.16739.75 mm0.38391-23/64″1.3594
.70 mm0.02764.30 mm0.16939.80 mm0.38581-3/8″1.375
700.028180.1695W0.38635.00 mm1.378
690.029211/64″0.17199.90 mm0.38981-25/64″1.3906
.75 mm0.0295170.17325/64″0.390635.50 mm1.3976
680.0314.40 mm0.173210.00 mm0.39371-13/32″1.4062
1/32″0.0312160.177X0.39736.00 mm1.4173
.80 mm0.03154.50 mm0.1772Y0.4041-27/64″1.4219
670.032150.1813/32″0.406236.50 mm1.437
660.0334.60 mm0.1811Z0.4131-7/16″1.4375
.85 mm0.0335140.18210.50 mm0.41341-29/64″1.4531
650.035130.18527/64″0.421937.00 mm1.4567
.90 mm0.03544.70 mm0.18511.00 mm0.43311-15/32″1.4688
640.0364.75 mm0.1877/16″0.437537.50 mm1.4764
630.0373/16″0.187511.50 mm0.45281-31/64″1.4844
.95 mm0.03744.80 mm0.18929/64″0.453138.00 mm1.4961
620.038120.18915/32″0.46881-1/2″1.5
610.039110.19112.00 mm0.47241-33/64″1.5156
1.00 mm0.03944.90 mm0.192931/64″0.484438.50 mm1.5157
600.04100.193512.50 mm0.49211-17/32″1.5312
590.04190.1961/2″0.539.00 mm1.5354
1.05 mm0.04135.00 mm0.196913.00 mm0.51181-35/64″1.5469
580.04280.19933/64″0.515639.50 mm1.5551
570.0435.10 mm0.200817/32″0.53121-9/16″1.5625
1.10 mm0.043370.20113.50 mm0.531540.00 mm1.5748
1.15 mm0.045313/64″0.203135/64″0.54691-37/64″1.5781
560.046560.20414.00 mm0.55121-19/32″1.5938
3/64″0.04695.20 mm0.20479/16″0.562540.50 mm1.5945
1.20 mm0.047250.205514.50 mm0.57091-39/64″1.6094
1.25 mm0.04925.25 mm0.206737/64″0.578141.00 mm1.6142
1.30 mm0.05125.30 mm0.208715.00 mm0.59061-5/8″1.625
550.05240.20919/32″0.593841.50 mm1.6339
1.35 mm0.05315.40 mm0.212639/64″0.60941-41/64″1.6406
540.05530.21315.50 mm0.610242.00 mm1.6535
1.40 mm0.05515.50 mm0.21655/8″0.6251-21/32″1.6562
1.45 mm0.05717/32″0.218816.00 mm0.62991-43/64″1.6719
1.50 mm0.05915.60 mm0.220541/64″0.640642.50 mm1.6732
530.059520.22116.50 mm0.64961-11/16″1.6875
1.55 mm0.0615.70 mm0.224421/32″0.656243.00 mm1.6929
1/16″0.06255.75 mm0.226417.00 mm0.66931-45/64″1.7031
1.60 mm0.06310.22843/64″0.671943.50 mm1.7126
520.06355.80 mm0.228311/16″0.68751-23/32″1.7188
1.65 mm0.0655.90 mm0.232317.50 mm0.68944.00 mm1.7323
1.70 mm0.0669A0.23445/64″0.70311-47/64″1.7344
510.06715/64″0.234418.00 mm0.70871-3/4″1.75
1.75 mm0.06896.00 mm0.236223/32″0.718844.50 mm1.752
500.07B0.23818.50 mm0.72831-49/64″1.7656
1.80 mm0.07096.10 mm0.240247/64″0.734445.00 mm1.7717
1.85 mm0.0728C0.24219.00 mm0.7481-25/32″1.7812
490.0736.20 mm0.24413/4″0.7545.50 mm1.7913
1.90 mm0.0748D0.24649/64″0.76561-51/64″1.7969
480.0766.25 mm0.246119.50 mm0.767746.00 mm1.811
1.95 mm0.07686.30 mm0.24825/32″0.78121-13/16″1.8125
5/64″0.07811/4″ E0.2520.00 mm0.78741-53/64″1.8281
470.07856.40 mm0.25251/64″0.796946.50 mm1.8307
2.00 mm0.07876.50 mm0.255920.50 mm0.80711-27/32″1.8438
2.05 mm0.0807F0.25713/16″0.812547.00 mm1.8504
460.0816.60 mm0.259821.00 mm0.82681-55/64″1.8594
450.082G0.26153/64″0.828147.50 mm1.8701
2.10 mm0.08276.70 mm0.263827/32″0.84381-7/8″1.875
2.15 mm0.084617/64″0.265621.50 mm0.846548.00 mm1.8898
440.0866.75 mm0.265755/64″0.85941-57/64″1.8906
2.20 mm0.0866H0.26622.00 mm0.86611-29/32″1.9062
2.25 mm0.08866.80 mm0.26777/8″0.87548.50 mm1.9094
430.0896.90 mm0.271722.50 mm0.88581-59/64″1.9219
2.30 mm0.0906I0.27257/64″0.890649.00 mm1.9291
2.35 mm0.09257.00 mm0.275623.00 mm0.90551-15/16″1.9375
420.0935J0.27729/32″0.906249.50 mm1.9488
3/32″0.09387.10 mm0.279559/64″0.92191-61/64″1.9531
2.40 mm0.0945K0.28123.50 mm0.925250.00 mm1.9685
410.0969/32″0.281215/16″0.93751-31/32″1.9688
2.45 mm0.09657.20 mm0.283524.00 mm0.94491-63/64″1.9844
400.0987.25 mm0.285461/64″0.953150.50 mm1.9882
2.50 mm0.09847.30 mm0.287424.50 mm0.96462″2
390.0995L0.2931/32″0.9688
380.10157.40 mm0.291325.00 mm0.9843


 
 

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