The set up uses the bearing from a flush trim router bit to precisely guide the workpiece and form a perfect radius.
I drilled out a hole on top of the router bit to show you the mechanics of everything.
Once the radius cut is complete, the setup functions like a jointer to straighten the adjacent edge.
Between adjusting the guide arm and the end stop, the setup can be configured to cut any radius. The elegance is in how simple everything sets up, while guaranteeing precision by design. To demonstrate I’ll take everything away, and show the set up from scratch.
I already have the parts cut out. See the description for a link to the plans, and a cut plan with dimensions.
To make the guide arm, line up the edges of parts C and D. The overlap should be about 2-inches. Secure them together.
With this assembly complete, load up a 1/2-inch flush trim bit in your router, and push this corner up to the bearing. Clamp this in place. I use a custom made MFT router table. As a side effect, it has lots of clamping holes that are ideal for situations like this. If you don’t have a suitable alternative, you can screw all these materials down to a sacrificial surface, and use it as an additional tabletop for your router table. That way, there’s only one large piece to secure.
Next I’m taking part A, remember to cut this chamfer here for chip clearance.
Butt this up against the left side of the guide arm, and use a straight edge to line it up perfectly flush with the bearing.
No special alignment is needed for the right side, just sandwich part E with part F, and clamp that down. That’s the setup!
Now remove the bearing from the flush trim bit, it’s not needed there anymore.
The bearing needs to be attached to your template stock with the distance from one edge to the center of the bearing being your desired template radius.