Router template: How do I create template that mates to a circle?

[Tom Hyde]

I'm trying to figure out how to create matching router templates for 8" wide stock with a concave end that mates perfectly to a 10" circle. I've got the circle. Here's a photo of the idea.

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The mating piece on the right is an off cut from when I made the circle and doesn't match since the radius was on the outside of the bit (a bigger circle).

I was thinking that I could use my circle cutting jig for my router and reduce the 5" radius I used for the circle by the width of the bit (3/8") to cut a matching piece ... somehow. But I'm not sure I can get that accurate with the circle cutting jig. Maybe. I can't think of any other way to do this short of drawing it out, rough cutting it on the bandsaw and sanding to the line but it seems less likely that I can get the perfect fit I'm seeking using that method.

Ideas?

[Dan Hahr]

Mark centerline of board. Set axis of circle cutter. Set radius to 5" from outer edge of bit. Secure both sides and rout away.

Dan

[pat warner]

Whatever the cutter diameter was (that created the circle), use a collar and cutter whose offset = that cutter diameter.
Collar offset = (collar D - cutter D)/2.
That is, make a new circle with a collar and cutter whose offset = the original cutter diameter.

[Bryan Cramer]

I use bondo to make a exact copy. Appy wax or packaging tape to the part you're trying to match. Then rough cut the mating template and apply bondo to the edge. Lay both parts on a flat surface like a piece of melamine and press the two parts together. The bondo will conform to the mating surface and make a perfect match once dry. It won't stick to the protected edge you are copying. Then with a router and a flush trim bit make a new template following the bondo edge. The bondo is only good for one router operation. You might have to make multiple applicatons to get a good match.

[mreza Salav]

I've done this many times. If you want something that mates the circle you have perfectly:
1 - make a copy of the circle if you need the original (rough cut one a bit larger and then use flush trim router).
2 - Assuming you use a 1/2" flush trim router in step 4, use a rabbeting bit and cut away 1/2" rabbet all around the new circle.
3- Use a flush trim to trim the lip of the rabbet, so essentially you get a perfect circle whose radius is 1/2" smaller.
4- Clamp the board you want to create the mating piece to this new circle and use a1/2" patter bit to trace the circle on the board cutting 1/4" deep only.
5- Cut away at the center of the groove you have created using a bandsaw and then use a flush trim to trim it. It will mate perfectly the original circle.

[glenn bradley]

I would approach it as Dan H suggests for a one-off. As Mreza explains for multiples.

[Tom Hyde]

Brilliant! Thanks for all the replies. After getting the template right, it's working out perfectly on the project. Thanks again, this place is awesome!

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