There are many ways to cut mortises and tenons, and I think I've tried them all. I now use a router with an up cut spiral bit. I found that this produces the fastest and cleanest way of making mortises (a clean cut, but frequently chips everywhere). It works best to plunge cut the bulk of the mortise out and then move the router back and forth to clean out and smooth the side walls of the mortise whenever using a router to cut mortises. It reduces the side pull of the bit so you get better accuracy of the mortise dimensions. I never bother to square my mortises, unless they will be through mortises and the ends will be seen. I just leave the mortises and tenons with round ends and they are just as strong If making floating tenons i leave the mortise with round ends and make the tenon fit the length of the flat sides of the mortise and leave the ends square. The half round gap in each end of the mortise leaves a place for the excess glue. It's the fit of the flat sides of the mortise and the tenon that gives the joint the strength, so rounding the ends of the tenon doesn't gain anything.
For mortising accuracy I now use a Leigh FMT Pro because of it's accuracy and the fact that it lets me cut both the mortise and the matching tenon using the same setup, router, and bit. A template is used to guide the router. You follow the outside of the template to make the tenon and the inside of the template to make the matching mortise. Only the part clamping needs to change and there is a centering cross hair for getting the first piece located. Then all additional pieces can have the same mortise or tenon cut without needing alignment. I can fit the first tenon to the 23rd mortise and it will fit just as perfectly as the first. No hand tuning of each tenon to mortise is ever needed. I just cut them and then assemble them.
The jig has an adjustment for how loose or tight that you want the tenon to fit into the matching mortise. This feature doesn't seem to exist on any other M & T jig. I can also use the jig to make precise floating tenon joints with it too, by cutting matching mortises in both adjoining pieces, then using floating tenon stock made with my table saw and planer.
There is a vacuum port on the FMT that works quite well, except when cutting the front side of tenons. I Velcro attach a piece of clear Lexan to hang down the lower front of my FMT, which prevents these chips from hitting me directly. It also helps get some of the chips from the front side cutting of the tenon to be collected by the rear vacuum port, but most of them just fall to the floor. At least the Lexan keeps them from hitting me.
I like my mortise and tenon joints to "POP" when I dry fit them and then pull them apart. It's easy to get them to fit this accurately when making them with an FMT jig. Add glue and they are "forever" without all of the hand fitting that's usually involved when making them other ways.
Charley
