The drill bits
come in a wide range of types and diameters. The most common are the
high-speed steel ones. Their diameter is usually embossed on the drill bit shank.
More often than not, the embossing is weak, poorly readable and requiring bright light. In many
cases the wear on the shank obscures the numbers entirely. Finding the desired diameter then
becomes a time-consuming (not much in absolute values but annoying when it happens) affair.
A color-coding scheme was chosen as a marking easily visible from any direction and in a wide
range of ambient light intensities. The code was borrowed from electronic color code, with
fixed-decimal point scheme; first one or two bars determine the diameter in millimeters, the third
bar is the first decimal point digit. For example, a 10.2 mm drill bit has brown, black and red bar,
while a 6.5 mm one has a blue and green bar.
The bars were chosen to be made as a paint in a groove; the groove provides mechanical protection against abrasion with the chucks and confines the paint in a neat place with straight edges. The placement of the grooves was chosen towards the end of the shank, as the main forces in the chuck are concentrated towards the front end of the chuck and therefore the groove does not act as a site of mechanical weakening and material fatigue inviting stress concentration.
The grooves at the placement of the bars are turned on a lathe (it is easy with a carbide bit). A model color is then carefully applied into the grooves with e.g. a tip of a toothpick.
The first version had both bars of identical width, and all colors were used bare, except orange which had to be mixed from red and yellow. (Note the 12 and 13 mm drill bit shanks being cut to shorter lengths, to conserve vertical space in the drill press and to allow being fit to a short (cheap) lathe.)
 Drill bits with grooves |  Color codes, early version |
After brief field tests the bits were reworked and a new batch added. Now the whole-millimeter bars are wider than the decimal-point one. The green, blue and brown colors were also mixed to lighter shade, to be better recognizable in low light, e.g. when the drill bit is in a shadow. More yellow was added to the orange as well, to make it better recognizable against red.
 Drill bits, color-coding |  Drill bits, color-coding, shank detail |  Drill bits, color-coding, shank detail |
There are also drill bits with 1/4" hex shanks, usable in electric screwdrivers - a very useful
thing for drilling holes into plastic boxes. (A step drill bit
with a hex-shank would be very useful for drilling larger-diameter holes in plastic boxes, e.g.
for switches and connectors; it is absent on the market, but it can be made by
modifying a conventional one
.)
The hex bits have a convenient groove already machined on them, by which some kinds of hex chucks can hold them in place. As this groove is intrinsically protected against wear, and the lock mechanisms of the chucks don't cause significant abrasion there, it was chosen as a natural site for the marking. As with the round bits, the wider bar marks the integer part of the value and the narrower bar is the decimal place.
 Hex-shank drill bits |  Hex-shank drill bits, color-coded |
The modification was quick and easy; the grooves can be machined quickly, and the paint application to the whole batch pictured above took only one movie on TV.
The modified drill bits are a huge timesaver; the values are immediately visible and the drill bit can be found immediately, without having to pick and read multiple bits before finding the desired one.