The Leigh FMT Mortise and Tenon Jig is super fast at milling mortise and tenon joints. A little sibling to the industrially machined aluminum Leigh Pro FMT, this less expensive model is made out of 10 and 12 gauge bent and bolted steel. Arriving standard with the jig is a 5/16” spiral upcut bit and 5 snap-in joint guides allowing for a decent range of 5/16” tenons. Additional bits and snap-in joint guides are available for an almost endless range of mortise and tenon joints as small as 1/16” x 1/8” all the way up to 1/2” x 5”!
The FMT consists of two distinct parts; a plate that is mounted to a router with a plunge base (in the background), and a bench mounted jig (in the foreground). The mounting plate is 12-gauge sheet steel with an array of holes bored to accommodate virtually any brand or model of plunge router. It looks pretty complicated, but the mounting isn’t that difficult (30 minutes for my Makita). Also integral to the function of the jig are two adjustable knurled brass posts that protrude through the base and function as guides for the operation of the router. Once the plate is mounted to the router, the jig is ‘clamp-and-go’. You can see the alignment plate in the center of the jig. It determines the centerline for both mortises and tenons.
For as technical as the jig appears, the operation is very simple. On the left is a interchangeable HDPE guide which determines both the mortise and tenon size. Depending on both the guide and the selected router bit, the FMT has a virtually endless selection of options. On the right is a slot that holds the router in alignment for accurate cutting. On top are knobs for adjusting to different thickness of materials. All of the parts came with perfect finish and precise angles for square cuts straight out of the box.
This tenon isn’t seated to the bottom of the mortise, but you can see the accuracy with which the FMT can cut this joint. I usually cut all of the tenons first, and before cutting mortises I set the depth of the plunge router 1/16” deeper than the length of the tenon to provide a bit of room for excess glue.
I’ll be honest, I’ve made a lot chairs where the seat frame connects to the back leg with a compound tenon and miter. I’ve never seen a jig or operation more efficient or accurate than the FMT. This cut on a tablesaw requires mind-bending math and spatial gymnastics! Make a simple angled tenon by clamping the workpiece at an angle (does it get simpler than that?) and make a compound tenon by angling the clamp face at the same time. Fixing stops to the table make production of many parts a breeze.
There are a number of additional inserts that enable the user to make unorthodox mortises, like the three horizontal mortises shown above. Every complex project seems to have one strange element of joinery and the FMT is up to the challenge.
Once stops are attached to the clamp face, the FMT truly excels at repetitive mortises and tenons, but that hardly precludes it from creative applications as well. I’ve never had the opportunity to utilize a four-tenoned apron like the one above, but that is because it would have been practically impossible to fabricate. Not any more! The FMT took my joint making to the next level!
The FMT truly excels at repetitive joints in a single sitting. It isn’t programmable, or micro-adjustable, and the stops and adjustments are decidedly analog. Like most other traditional joinery, it is only as accurate as its user. But, once an operation is set up, the cuts really fly. This jig is ideal for a small one or two person custom shop, where each project is the different, but quick, accurate joinery is essential.
My misgivings about the FMT are minor, but worth mentioning. The jig only comes with one 5/16” router bit and the related guides. To get the full breadth of the jig, one must invest in each increment and the related guides; a considerable additional cost. This goes for the horizontal mortise guide as well as an available louver guide, and even the vacuum box kit (I ran it without vacuum, and spent half of my time cleaning up after myself). In my opinion, once the mounting plate is attached to the plunge base, the plunge base becomes part of the jig. I wouldn’t like to fine tune the jig from scratch more than once as it is a bit time consuming.
Once assembled, the jig no longer fits in the box it came in. If all of the guides, plates and router bits are acquired, custom storage is essential. It would be really easy to mix up or lose the parts without making a box with drawers or special compartments. Having said this, the jig is really an excellent tool, and I am happy it is part of my shop.
Leigh Super FMT Mortise and Tenon Jig