Drilling 101: There’s More To Drilling Holes Than You Think

"An article about drilling holes?  Are you nuts?"  I don’t think so.  The truth is, if you’ve ever drilled a hole in something, bolted it down, then had the darned thing break or fall off a few days or miles later, you’re probably looking at an article of interest.  You see there are plenty of cars built out there (including plenty of “big buck” professional jobs) with pieces fastened incorrectly.  And a big part of the problem is the respective hole(s) drilled in a structure in order to fasten a given component.

A back yard-built car is one thing, but in the world of aviation, you simply can't take the chance that something will fall off (although aviation failures are big news today, remember those questions should be tempered against the sheer numbers of aircraft flights).  And one place where the FAA (Federal Aviation Administration) pays big attention is the area of drilled holes.  Now, before you shrug off the thought of aircraft tech in regard to simple old-fashioned home built hot rod, think about this:  When the high performance movement really picked up speed following WWII, plenty of those guys had aircraft maintenance and fabrication skills. Not everyone was simply a dirt bag grease monkey.  A lot of those early rodders and racers knew what they were doing, and we can copy them.  Here’s one piece of the puzzle:

Edge Margins -- How Close Can You Come?

If a hole is drilled too close to the edge of a given piece of metal, the small amount of metal between the hole and the edge of the metal might tear away while under periods of stress.  This distance between the edge of the material and the drilled hole is called an "edge margin".  Basically, a specified edge margin is the distance from the center of a hole to the nearest edge.

In aviation, the standard edge margin works out to twice the diameter of the hole.  As an example, if you have a 1/4-inch hole, the center point of that drilled hole should not be closer than 1/2 inch from the edge of the material (1/4-inch drilled hole X 2 = 1/2-inch).  So far so good, but what if you have to drill a series of holes in a structure?  How close can they become?  The aviation industry has an answer for that too:  The minimum separation between hole centers is three times the diameter.  This means that if you have a piece that requires a bunch of 1/4-inch holes drilled for fastening purposes, each hole center must be 3/4-inch apart (1/4-inch drilled hole X 3 = 3/4-inch).  If, for example, you have a piece of aluminum plate and it's mounted to a chassis tab by way of a series of holes, then each hole must maintain the edge margin and the series of holes must be a specific distance apart.

The following chart provides the 2X numbers for the edge margin along with the 3X numbers for hole separation, using the most common drill sizes:

Edge Margin & Hole Separation

(all figures in "inches")

Drilled Hole Diameter                             2X (Edge Margin)                                  3X (Separation)

______________________________________________________________________________

3/32                                                      3/16                                                      9/32

1/8                                                       1/4                                                       3/8

3/16                                                      3/8                                                       9/16

1/4                                                       1/2                                                       3/4

5/16                                                      5/8                                                       15/16

3/8                                                       3/4                                                       1-1/8

7/16                                                      7/8                                                       1-5/16

1/2                                                       1                                                          1-1/2

9/16                                                      1-1/8                                                    1-11/16

5/8                                                       1-1/4                                                    1-7/8

3/4                                                       1-1/2                                                    2-1/4

When you drill a hole, there's no question you want the hole in the exact spot you place the bit.  Typically, the drill bit will wander off that location.  In order to keep the drill bit in one spot, use a center punch to dimple the metal at the drill point.  Automatic center punches are available (I don't have one, but they operate with one hand) to make this job easier.

The Parts Of A Drill Bit…

A "twist drill" is a pretty simple device.  But what happens when bits get dull?  While there are several different drill sharpening devices available today, most of the printed information I’ve found from the aircraft industry goes something like this:  Find the nearest trash bin and dump the bit.  Drill bits aren't all that expensive and if accuracy counts, then you're better off replacing a dull bit.  You'll also find that the use of dull drill bits creates burrs around drilled holes.  More on burrs later.

There's more too:  Drilling a hole to a final size in one pass is sloppy.  Basically, the pieces of metal shavings rattle around during the drilling process and actually enlarge the hole.  To stop this problem, always drill the hole undersize so that only a slight amount of metal is actually removed during the final drilling.

How about the actual size of the drilled hole?  That's simple.  Right?  Maybe not.  First of all, check out the following chart that depicts drill sizes:

Drill Size vs. Diameter

Size                  Inch                  Size                  Inch                  Size                  Inch

_______________________________________________________________________

 

80                    .0135                32                     .1160                M                     .2950

79                    .0145                31                     .1200                19/64                .2969

1/64                  .0156                1/8                   .1250                N                      .3020

78                    .0160                30                    .1285                5/16                  .3125

77                    .0180                29                    .1360                O                      .3160

76                    .0200                28                    .1405                P                      .3230

75                    .0210                9/64                  .1406                21/64                .3281

74                    .0225                27                    .1440                Q                      .3320

73                    .0240                26                    .1470                R                      .3390

72                    .0250                25                     .1495                11/32                .3438

71                    .0260                24                     .1520                S                      .3480

70                    .0280                23                     .1540                T                      .3580

69                    .0292                5/32                  .1563                23/64                .3594

68                    .0310                22                     .1570                U                      .3680

1/32                  .0313                21                     .1590                3/8                   .3750

67                    .0320                20                     .1610                V                      .3770

66                    .0330                19                     .1660                W                     .3860

65                    .0350                18                     .1695                25/64                .3906

64                    .0360                11/64                .1719                X                      .3970

63                    .0370                17                     .1730                Y                      .4040

62                    .0380                16                     .1770                13/32                .4063

61                    .0390                15                     .1800                Z                      .4130

60                    .0400                14                     .1820                27/64                .4219

59                    .0410                13                     .1850                7/16                  .4375

58                    .0420                3/16                  .1875                29/64                .4531

57                    .0430                12                     .1890                15/32                .4688

56                    .0465                11                     .1910                31/64                .4844

3/64                  .0469                10                     .1935                1/2                    .5000

55                    .0520                9                      .1960                33/64                .5156

54                    .0550                8                      .1990                17/32                .5313

53                    .0595                7                      .2010                35/64                .5469

1/16                  .0625                13/64                .2031                9/16                              .5625

52                    .0635                6                      .2040                37/64                .5781

51                    .0670                5                      .2055                19/32                .5938

50                    .0700                4                      .2090                39/64                .6094

49                    .0730                3                      .2130                5/8                    .6250

48                    .0760                7/32                  .2188                41/64                .6406

5/64                  .0781                2                      .2210                21/32                .6563

47                    .0785                1                      .2280                43/64                .6719

46                    .0810                A                      .2340                11/16                .6875

45                    .0820                15/64                .2344                45/64                .7031

44                    .0860                B                      .2380                23/32                .7188

43                    .0890                C                      .2420                47/64                .7344

42                    .0935                D                      .2460                3/4                    .7500

3/32                  .0938                1/4                    .2500                49/64                .7656

41                    .0960                E                      .2500                25/32                .7813

40                    .0980                F                      .2570                51/64                .7969

39                    .0995                G                      .2610                13/16                .8125

38                    .1015                17/64                .2656                53/64                .8281

37                    .1040                H                      .2660                27/32                .8438

36                    .1065                I                       .2720                55/64                .8594

7/64                  .1094                J                       .2770                7/8                    .8750

35                    .1100                K                      .2810                57/64                .8906

34                    .1110                9/32                  .2813                29/32                .9063

33                    .1130                L                      .2900                59/64                .9219

15/16                .9375

61/64                .9531

31/32                .9688

63/64                .9844

 

As you can see, drill bits are available in fractional sizes, measured in increments of 1/64th of an inch along with "in-between" sizes.  The "in-between" sizes are based upon a number system that sizes the bit from 80 through 1 (80 being the smallest and 1 being the largest).  There’s more here too:  An alpha system exists, which begins with "A" and ends with "Z" ("A" being the smallest and "Z" being the largest).  As you can see from the chart, the alpha size system takes over once the numeric system ends (fractional system notwithstanding).

With the drill size out of the way, you're presented with another dilemma:  Bolts are usually undersize.  For example, a high quality 3/16-inch bolt (a good example is an AN3 fastener) is supposed to measure 3/16-inch or 0.1875-inch, but it doesn't.  Typically, it measures 0.185-inch.   This means a #13 drill bit is the exact size, but of course, you might have trouble getting the bolt in the drilled hole if everything is sized identically.

Whenever a hole is drilled, you'll almost always find that a burr remains around each hole.  It's a good idea to eliminate these burrs, simply because they're a place for stress risers to begin.  Some people will deburr the drilled hole by chattering a drill bit around the circumference.  While it removes the ridge, it usually destroys the surface around the drilled hole, and pretty much enlarges the drilled hole.  The recommended aircraft method words like this:  A deburring bit or countersink can be used on each side of the hole (using your hands as power rather than an electric or air tool) to remove the burr.  Basically, this process bevels the edge and removes the ridge left by the drill bit.  Finally, it's a good idea to use a piece of emery cloth over each deburred, drilled hole.

As you can see, there's a bit more to drilling a hole than first meets the eye.  Take some of this aircraft advice and apply it to your project.  It just might make it more reliable. And besides, think of your pals as they follow you down the road:  They don’t have to worry about dodging stray pieces when they fall off your car.

Drill bits are, for the most part inexpensive. Most aircraft sources do not recommend sharpening bits. Instead, they simply advise you to toss a dull bit and replace it.

Edge margins are pretty much straightforward. From the edge of the material, go two times the drill bit size from the edge before you drill. For separation between holes, go three times the drill bit size. See the text for more info.

It's next to impossible to drill a hole in an exact location without first using a center punch to dimple the metal at the drill point. This is the easiest way to stop drill bit wander. Automatic center punches are available (I don't have one, but they operate with one hand) to make this job easier.

When drilling a hole in a piece of metal, always begin with a drill size smaller than the intended hole size.

As mentioned in the text, pieces of metal shavings rattle around during the drilling process and actually enlarge the hole. To stop this problem, always drill the hole undersize so that only a slight amount of metal is actually removed during the final drilling.