How to make a matching curved piece?

[mreza Salav]

I hope someone can chime in with an idea as to how to do this:
I have a curved piece and I want to make a "matching" curved piece that fits into the original (as a male/female part). Say I have piece "A" below and want to make "B".

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Note that I want a glue line joint (has to be perfect).
Also note that I don't want to make an identical copy (so using a flush trim bit to follow "A" doesn't work).
I thought about putting piece "A" on a piece of wood and use a router insert with a plunge bit to cut "B" but the problem is that the radius of the curve "B" obtained is different from "A" by the thickness of the bit and so if you put them into each other I won't get a perfect fit.

Any ideas?:confused:

[Glen Butler]

Just gonna throw this out there. You can't just follow one to get the other because the thickness of the bit changes the radius. So reduce "A" by the thickness of the bit. Set the router fence behind a straight bit the same thickness as the bit. Don't cut all the way through "A" cause you need to leave material to keep butted against the fence properly. You cant get any closer to the bit than what the fence is set at. Make sure you take several passes to insure all the material is removed. Now lower the bit flip the piece over and do the same thing again, but you just need to remove enough material to expose proud the surface you just created. Now you have a peice that is reduced by the width of the bit that you can pattern "B" against with a bearing bit.

[mreza Salav]

Thanks Glen. Now that you said this I can see something similar (and perhaps easier).
- Make a copy of A (because I need A)
- Using a rabbeting bit, cut a rabbet say 3/8" deep and using that and a flush trim cut complete it (now the curve is the same as A minus 3/8").

Then make B using a bearing bit 3/8" or a router insert with that thickness.

[Andy Sowers]

You can do it with a router... google "routing complementary curves." I found a google book which gives an "ok" description.

Basically you use a router and straight bit along with guide bushings to make two separate templates for each piece.

Hope that helps...
Andy

[Glen Butler]

Yeah a rabbeting bit would make life easier. I just don't have one, so was finding a result using the tools I have. Clearly you get the idea I was trying to explain. Certainly if you can eliminate steps the better. There really is no need to do anything beyond the first rabbet if that works for your application.