I made this a few years ago, and use it often. As my eyes age, this gauge makes selecting a drill from an assorted collection of odd sizes easy.
Step 1: A 12 Inch Steel Ruler.
Get a good 12 inch steel ruler with markings in inches and tenths.
Cut the ruler in half, at about the 6.3 inch mark. This assumes there is 15 mm from the 12 inch mark to the edge of the hole to hang the ruler on.
Cut 4 pieces of 2 mm thick aluminium (or better, 1.5 mm stainless steel), 50 x 10 mm.
File off any rough burrs, and clean with solvent (acetone is good), and dry by warming.
Step 2: Glue the First Pieces Together.
Using epoxy (Araldite), glue one piece to the "zero" end of the cut ruler, another to the "6" end, at right angles to the ruler. Alternatively, you could glue all 4 pieces to the 2 ruler sections at the same time but assembling with glue still wet can be messy.
Make sure that the scale inside the wedge is in inches and tenths, not inches and eighths!
When set, assemble the other parts, holding the aluminium ends with Bulldog (Binder) clips and pulling the edges of the rulers (to touch the drill bits) with elastic bands.
Step 3: Last Adjustments and Cleaning Before the Glue Sets.
Set the spacing by putting a 1 mm drill bit at the 1 inch mark, and a 5 mm bit at the 5 inch mark.
(Measure the drill bits first -- often they are made slightly undersized.)
(Yes, I know the photo has a 1.5 mm drill bit. I mislaid the 1mm bit.)
Two elastic bands ensure the ruler wedge holds the drill bits in their places.
Using a small paintbrush or Kleenex and methylated spirit, wipe off any bits of unset glue which would interfere with the measurements. Don't use acetone for this step as it reacts with the unset epoxy.
To finish, glue a paper label over the "7 to 12" ruler section, and sand-paper any sharp bits. Hang on your shadow-board for easy access.
You may say that the gauge does not measure the true diameter because of the angle of the wedge.
Yes, there is a correction to be made because the drill bits are not enclosed by parallel lines, but all that is corrected by correct placement of the (known) drill bit sizes.
It would not be useful to make the gauge start at zero, firstly because residual glue or damage at the ruler's end will make the reading inaccurate, and also that when checking drill bits below 1 mm, a finer taper would be needed for accuracy.
And lastly, a larger version of this gauge covering 1 mm to 12 mm could be made using two 12 inch rulers.