There are a lot of ideas out there for buying or making mortising jigs. One time when I was down for a long time due to a bad back, I did a lot of looking around for what interested me most. I settled on a set of plans I found in Shop Notes magazine, issue #112. These plans are making a loose tenon jig, however it is still a wonderful jig for starting the mortise of a squared off solid tenon and mortise.
I liked this idea the most because it is simple to build and it is so versatile for any size of board you want to put into the jig. You build the jig to match the base diameter of your plunge router. To use the jig, you simply set the router on the jig. No special attaching is necessary. You’ll notice I made my own knobs to cut costs. Look at my blog post “Wooden Knobs” to make these.
My first project using this jig was for a flower box that was over four feet long and eleven inches wide. To route the ends of the four foot board, you simply elevate the jig up as high as necessary to accommodate the long board. Remove the universal clamp assembly and clamp the board directly to the jig face.
For the eleven inch wide board you use the backplate of the universal clamp to wedge the board up against the top. Then a couple of clamps out on the end of the jig holds the board up tight. One thing to note is that you will want to sandwich a piece of waste wood on top of your board so that the router bit won’t plunge into the backplate of the universal clamp.
When routing mortises you will want to use a spiral upcut bit which pulls the wood shavings up out of the mortise while you are routing. Another option is to use an end mill bit which is less expensive and is just as effective.