Router Table Circle Jig Modification

[dgbreggin]

The last time I needed to cut a circle on my router table, I used a nail as the pivot and clamped a board to the router table. Crude, but it got the job done.

I planned to create a Thien Baffle for my woodworking dust collection system, and I realized that I really needed a better circle jig. I searched online and found a number of designs of varying complexity. The one I settled on is remarkably simple. I found a couple YouTube videos (cir. 2018) describing it, but neither claimed the design as original. I did a bit more research and discovered that it was originally designed by J. Phil Thien (inventor of the Thien Baffle) in 2007 (link below).

I decided to make a few modifications to better suit my application. Specifically, I wanted to be able to adjust the depth of the pivot pins.


Here are some pictures of the parts of the jig:



I created the slide from a piece of aluminum bar stock. I threaded a hole in it for the knob used to secure it in position (set the radius of the circle). I drilled and reamed two holes, one for 1/8" pivot and one for 1/4" pivot. I made the two pivot pins from steel drill rod and rounded the ends a bit. I drilled and threaded a cross-hole and use an aluminum screw to secure the depth of whichever pin in installed. I made a spacer for the knob so that a) I didn't have to shorten the threaded part, and b) so that the knob would easily clear whatever part of the pin might be sticking up though the slot. Spacer was made from 3/4" diameter aluminum round bar.


Here are some pictures of the slide with the pins installed:






I did not want to use a steel screw because it would likely mark / gouge the pins, making them difficult to install / remove. (I used an aluminum screw because I couldn't find my plastic screws.)

What I would have done differently: I might have reversed the positions of the two pins, placing the small pin near the end of the slide. This would allow a slightly smaller circle to be turned using the smaller pin.

I did use this jig for my Thien Baffle. Both the jig and the baffle work great.


References:

J. Phil Thien's original design page:
J. Phil Thien's Router Table Circle Jig
There is a lot of good information on this site.

I make no claim of having invented this jig. If you want to see more about how this jig is made and used, here are a couple videos that I found helpful: