In recent years, router manufacturers have begun offering routers with height adjustments that can be accessed through the base, when the router is hanging upside down in a router table. This eliminates the cost and complexity of incorporating a router lift into the design to control the height of the bit. These new routers have that built-in.
Inspired by these, a number of writers have published designs for router tables using these new routers that aren't really tables, they're router table tops, meant to be attached to an existing workbench.
Bench-Mounted Router Table
Stow-and-Go Router Table
Stow-Away Router Table
When I was building my own Real Woodworker's Workbench, I had to buy a second router. I decided to buy one with an integrated lift, and to use it to build a router-table attachment for my bench, similar to those described above.
It took me longer to get around to it than I had planned, but a few weeks ago I finally got started.
I made it out of a 24"x48" piece of 3/4' Melamine-coated particle board, with a 3/4' backing of Medium Density Fiberboard (MDF), using a commercially purchased insert plate and fence. Melamine provides a slick, smooth, easily-cleaned surface, which makes it a pretty good choice for a top. 3/4' isn't enough to prevent bending, though, and the fiberboard core isn't as dense as MDF, hence the MDF backing.
Step 1: Laying out the mortise
I bought a commercial plate, and the manufacturer offered a template sized to match the plate. I didn't buy one. It required a 1/2" pattern cutting bit, which I didn't have, and didn't have any other immediate need for. So instead of shelling out $30 for the template and bit, I made up a template of my own that I could use with a guide bushing kit and the bits that I already had.
A guide bushing is used with a straight bit. It rides along the side of the template, keeping the bit a set distance away from the template. The critical factor is the distance between the outside of the bushing and the edge of the bit.
I constructed my template out of straight strips of 1/4" MDF, held in place with double-sided carpet tape. As I said, the critical dimension is the offset, the distance between the outside of the bushing and the outside of the bit. This equals the radius of the bushing minus the radius of the bit, or half of the difference between the diameter of the bushing and the diameter of the bit.
Fasten the insert plate to the top, in the position you want it to be, with double-sided tape. Tape straight strips of 1/4" MDF around the plate, separated from it by your calculated distance. Peel up the insert plate, and then tape down some pieces of 1/4" MDF in the center, to help provide stability for the router. The position of these inside pieces isn't critical, so long as the gap between them and the outside pieces is wide enough that you can route an area greater than the width of the lip you want to end up with.
I added some small triangular pieces into the corners, to keep the bit from cutting too sharp a radius.