Router table fence and router table insert with accurate radius corners

The last projects for my new Sawstop tablesaw were to install a router lift and build a detachable fence. I previously built an outfeed table and added adjustable legs, documented in a previous post.

The fence was a design that’s seen often. I made mine with 1/2” Russian plywood, added an enclosed opening with a dust port for my shop vac, T-track for feather boards, and a split fence made from black Melamine to accommodate different size bits.

The router lift I decided to buy was the Jessem Mast-R-Lift. It’s well rated and will accommodate different sized routers if I decide to buy a larger router in the future. The challenge for the installation was the 3/4” radius of the corners, since I wanted a very clean installation. Some plates have a 1/4” radius, so the usual technique is to attach pieces of wood around the plate to build a frame and use a 1/2” diameter flush trim bit to rout the recess. If I were to use this technique, I would need to buy an expensive, seldom used 1 1/2” flush trim bit to make the radius accurate.

I found an old forum post that referenced using guide bushings and making 2 templates, but the details were a bit vague. I liked the concept, so after some thought came up with the correct formulas to make a perfect template. I’m sure others have done this, I just haven’t seen it explained before. Here’s what I did:

A first template was made by attaching the plate with double stick tape to the template material (I used 1/2” mdf) and routing around the perimeter with a 1/4” straight bit and a 1/2” bushing. This leaves an opening that is 3/8” larger on all sides. You might have a few spots that need filling with wood filler and sanded smooth.

A second template is made by attaching the first template to another piece of mdf with double stick tape and routing the interior with the same 1/4” bit, but switching to a 1” bushing. This produces a perfect match to the router plate. It may sound counter-intuitive, but the formulas explain the math.
Template 1 (routing outside the perimeter): 1/2 bushing OD + 1/2 bit diameter = offset (3/8”)
Template 2 (routing inside the perimeter): 1/2 bushing OD - 1/2 bit diameter = offset (3/8”)

You can use different size bits and bushings, just size them accordingly. Once the final template was made, I attached it with tape to the extension wing of my saw and used a flush trim bit set slightly deeper than the plate thickness. Size of the bit doesn’t matter as long as it’s smaller in radius than the corner. I then added some spacers inside the template and used a straight bit to cut the opening for the router. Final steps were to strengthen the particle board with super glue, and add a solid wood frame for reinforcement under the table.

I hope that someone else finds this useful. I enjoy the problem solving aspects of woodworking, and I think I can use this knowledge for future projects.
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