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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:

  1. 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.
  2. 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).

tl;dr
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!

EDIT:
This is the piece (copy-ring) that I'm talking about:
enter image description here

  • 2
    I'm not sure what you mean by "copy ring". Is that what I know as a guide bushing? – keshlam Sep 13 '15 at 0:41
  • 1
    Suppose that pictures would not hurt anyone here then – Matt Sep 13 '15 at 3:00
  • Second on the pictures. I'm having a hard time figuring out what you're talking about. – grfrazee Sep 13 '15 at 4:04
  • My favorite online tool merchant seems to think "copying ring" is the right translation: copying ring. Makita seems to call the same thing "template guide" – Damon Sep 13 '15 at 10:08
  • Festool calls it "copying ring", too: festool.com/Products/Accessories/Pages/… – Damon Sep 13 '15 at 15:59
2

Don't get stuck on creating a round hole.
As few as 3 points are enough to hold the copy ring.

Below is an image on how to create "holes" on a row with good accuracy; without the "holes" being round. The black is wood (or aluminium) and the grey is your copy ring.
With some thinking one can even make it take different copy ring sizes.

enter image description here

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2

So I found two practical solutions that actually work, exactly as desired.

The first one is a "Duh!" solution: Search long enough, and find a professional-grade tool trader, pick up the phone and ask them for help. Guess what, you can buy a 17mm router bit if that's what you absolutely want.
These are normally used for plow-routing C-tracks, they're a slightly (2-3€) more expensive than an average HW tool although they're only HSS. But for the intended purpose of creating a couple of templates, they're by all means good enough.

The second solution would be the template-template trick, which I've actually tried (before finding the 17mm bit). Although it adds the tool's clearance twice and thus is technically an inferior solution, it works pretty well in practice.
Having both versions side by side now, I really can't say one is better or worse than the other (looks the same, works the same, outcome looks the same, and if you hold the two templates on top of each other, you can't feel a step with your finger).

I've also figured (not tried, but should work) an easier combination which -- unlike the solution with the somewhat rare 14mm bit -- uses only very standard sizes:
Drilling a 25mm hole with a readily available method and using the 24mm ring will give a 1mm clearance, thus effectively making the router bit 1mm larger in diameter. If the router bit is a standard 16mm, that'll give 17mm on the second template.

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1

Plastic and heat or mould:
Heat the copying ring into a piece of plastic. After a few tries you hopefully have a perfect mould. After you manage to figure out how to make the plastic not stick to the copying ring...

Some other moulding material might do the trick but I can't think of any right now. Epoxy maybe.

This plastic piece can then be inserted into a jig.

Lathe:
Mount a piece of wood to a lathe and sneak up on the size. Then mount this piece of wood in a jig.

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  • (still got no good solution either) I'm somewhat reluctant to the heating and using gooey plastic stuff idea. How would the lathe approach work? One could (ignoring the fact of not owning a very high quality lathe) certainly create a solid piece of round wood with the same diameter, but I need the opposite (hole). One can of course make a fitting hole on the lathe, too, but that would have the inner diameter, and in order to stick the copy ring in, I need a hole with the outer diameter. – Damon Sep 21 '15 at 13:07
  • I agree on your still-no-good-solution comment. Regarding creating a hole with the lathe: Mont a flat piece of wood on the lathe, like if you wanted to cut a disc. Turn a hole in it; sneak up on the diameter. Make more of them. Then mount these pieces of wood in a jig where you use your router. – LosManos Sep 21 '15 at 13:20

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