Leigh FMT/Super FMT

Frame Mortise & Tenon Jig

Production Routing & Smaller Joints

When you're engaged in a high-volume production job, you can speed up the process considerably by using the largest bit possible to cut tenons smoothly and quickly in one pass. Using a tenon bit larger than the joint guide size will make tenons that are smaller than the guide, so you'll have to use a bit smaller than the guide to cut matching mortises. The principle is simple: Any two bit diameters which add up to twice a given joint guide size can be used to make joints the size of the smaller bit. (See drawing below).

In the extreme, this technique lets you cut joints much smaller than the FMT/Super FMT's smallest 1/4" joint guides. Delicate furniture parts such as small cabinet door frames, doll house architecture and furniture, and very accurate miniatures all call for mortises and tenons as narrow as 1/8" or even less. The charts below show how different bit combinations can crank out such extraordinary joints just as quickly and precisely as larger ones.

Tons More Joint Sizes!

Note: Guides and stands are available individually. In North America contact Leigh, or internationally contact your local distributor.

Leigh's FMT/Super FMT can easily produce joints smaller than the standard 21 (24 Metric) joint guide sizes. Each joint guide produces the mortise and tenon in the same size as the selected guide. For example, a 1/4" bit used with a 1/4" (6mm) guide makes 1/4" (6mm) joints.

You can more than triple the number of joint sizes by using two bits of different diameters with any of the standard guides, except 1/2" (12mm) guides. A total of 47 (55 metric) two bit optional joint sizes are shown in the grey shaded boxes in the selection charts (inch chart above, metric chart below). Mortise sizes in the column headings and mortise bit sizes in the chart are highlighted in red.

Tenon bit sizes are under the tenon bit headings. The colored box at the right of each row shows which joint guide to use with each pair of bits.

For example, if you want to cut a 1/8" x 1/2" (3mm x 12mm) joint, find that size in the 1/8" (3mm) joint size column. Look to the left of the mortise bit size for the tenon bit size of 3/8" (9mm), and look to the far right of the row for the colored standard joint guide 1/4" x 5/8" (6mm x 15mm).

Further, by using combinations of small machine tool bits you can rout an infinite number of small and miniature joints.

Miniature Joints

Incredible Miniature Joints are Easy
The FMT/Super FMT can rout joints not just smaller than 1/4" but so tiny they're almost beyond belief — all with the same easy setup and adjustable fit you expect in larger joints. These examples of miniature joints show the extreme versatility of the FMT/Super FMT.

The 7-1/2" ebony and holly ladder leaning against a 1/2"X 5" tenon has ebony wedged tenons 1/16"X 1/8".

The matches have progressively smaller tenons mortised through other tenons, featuring joints .040" X .095", .023" X .068" and .013" X .053" respectively. The smallest mortise and tenon was routed with .010" mortise and 31/64" tenon machine tool bits. These matchstick joints actually creak when assembling!

And if you're wondering how pieces such as the matches, miniature table parts and some of the curved workpieces are held in place, here's how: there are twelve through-holes in the clamp plate with which shop-made auxiliary plywood plates, blocks, and work-holding fixtures can be attached and used together with the Leigh cam clamps. It's all in the easy-to-follow Leigh User Guide.

Learn More!
Note that the following links may feature earlier model jigs,
but the information still applies to the latest models.