Frank Hagan
09-29-2008, 2:10 AM
I was looking for a router circle jig, and was shocked at the prices. Having a bit of hardboard laying around, I thought perhaps I could copy a design I found on the Internet, Kurt's Circle Jig (http://members.aol.com/pogo9959/Router.htm). Kurt used plexiglass for the "slider" portion, with a rabbet cut on each side. I went for the easy way out and just used three layers of 1/4" tempered hardboard I had in the shop.
The top layer has the base dimension of the router cut out.
The middle layer has just a "hole" sized the same as in the bottom of the router, so it supports the router and allows the bit to protrude through it. It also has a 1" wide slot cut from the long edge to the center of where the bit goes through.
The bottom layer has the same "hole" cut in it, and a 1/2" slot centered on the center of the bit. The slider is a piece of hardboard 15/16" wide and 18" long glued to a piece 7/16" wide and the same length. Glued together, this slider forms the trammel arm, and can slide right up next to the router bit or, reversed, slide away from the bit at least 18". A small hole drilled in it fits over a pin inserted in the middle of the stock to be cut. The picture of Kurt's jig, done much more professionally than mine, shows the concept.
A couple of t-nuts you can see in the pictures are inserted on the bottom of the top layer, so when it is all glued together you can tighten thumbscrews down to hold the trammel bar in place.
The jig works great, and because you can position the trammel arm right next to the bit, you can cut very small circles with it. Cutting a 2 1/4" circle in 3/4" plywood was no problem yesterday, although it comes close to maxing out the depth of my current router. Kurt's version, with the trammel arm at 1/4" total depth and rabbeted, saves another 1/4".
The top layer has the base dimension of the router cut out.
The middle layer has just a "hole" sized the same as in the bottom of the router, so it supports the router and allows the bit to protrude through it. It also has a 1" wide slot cut from the long edge to the center of where the bit goes through.
The bottom layer has the same "hole" cut in it, and a 1/2" slot centered on the center of the bit. The slider is a piece of hardboard 15/16" wide and 18" long glued to a piece 7/16" wide and the same length. Glued together, this slider forms the trammel arm, and can slide right up next to the router bit or, reversed, slide away from the bit at least 18". A small hole drilled in it fits over a pin inserted in the middle of the stock to be cut. The picture of Kurt's jig, done much more professionally than mine, shows the concept.
A couple of t-nuts you can see in the pictures are inserted on the bottom of the top layer, so when it is all glued together you can tighten thumbscrews down to hold the trammel bar in place.
The jig works great, and because you can position the trammel arm right next to the bit, you can cut very small circles with it. Cutting a 2 1/4" circle in 3/4" plywood was no problem yesterday, although it comes close to maxing out the depth of my current router. Kurt's version, with the trammel arm at 1/4" total depth and rabbeted, saves another 1/4".