My router comes with a 30mm copy ring, and you can optionally buy 27, 24, 17, 13.8, and 8.5.
Happens that spiral router bits come in even full-millimeter sizes from 6 to 20 as well as 12.7 (but of course not 13.8), and straight blades come in odd and even full-millimeter sizes from 3 to 30 except 17, 24, and 27.
Forstners usually come in 15, 20, and 25, but neither 17 nor 24. Some manufacturers sell 16 and 18 as well, but not 17 (or 24), and certainly not 13.8.
In one word, unless you are willing to go for 30mm (which is really a huge hole!), there is no single way to make any kind of hole that will fit any copy ring exactly.
Which leaves me with two options that I can think of:
- Drill a 14mm hole and wrap a layer of Tesa tape around the 13.8mm copy ring. That sounds like mucho WTF but might actually work.
- Make a template-template by drilling a 30mm hole and using a 27mm copy-ring (3mm gap) with a 14mm bit, producing the actual template. That is somewhat less WTF, but it adds the router's mechanical clearance twice, reducing precision (it will probably not matter, but it's a systematic error).
What is this actually needed for? I would for example like to drill dowel holes or domino slits with such a template.
Of course, drilling one hole and using those little metal spikes to mark the center works for dowels. And of course simply using a double parallel guide and making the hole a bit longer works for dominoes, too.
However, with a proper template, one should be able to reproduce exact holes at exact positions every time without trying or measuring.
As in, hold template against piece, stick copy-ring through hole... and go!