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The Le5 Mortice & Tenon Jig

Devised and Assembled by Jean Lefebvre

The Mortise & Tenon (M&T)is a skill that has intimidated me. I have studied various jigs meant to aid in the execution of this joint and have found them to either be too expensive or to provide only a part of the solution. I could have attempted to cut the mortices by hand, but I was not confident enough with my hand tool skills to attempt that.

Then it occurred to me that a mortise and an exactly corresponding tenon could be cut using a router, a 1/4" spiral up cut bit, and an inlay guide and bushing. The guide and Bushing are available at Lee Valley. ( I have seen, at All In One Wood Tools, an inlay guide and bushing that is used with a 1/8" spiral up cut bit at which would allow you to make smaller M & T)

Armed with the seed of an idea, I researched what plans were available for M&T jigs. However, while there were many plans for mortising jigs and many for tenonning jigs, I could not find one for cutting both a mortise and an exactly corresponding tennon.

I was having lunch with a friend (Gerry H.) and discovered that we had both been looking for an easier way to cut M&T joints. After a few discussions and some trials and errors, we came up with our own plan.

I started by building a box 10" high x 12" deep x 13" wide, with the top and bottom closed and both ends open. I routed two dadoes in the front face of the box to accommodate the "T" Track that will house the hold downs (see photo above right). I recomend using at least 6 #8 wood screws to fasten the "T" track. The track is under considerable pressure and the #6 screws I initially used pulled out.

figure 1: Jig table, top view

I then built a table using 1/2" and 1/4" baltic birch plywood. The completed table dimensions were 25" wide X 20.25" deep (see drawing above and below).

figure 2: Jig table, top view

Once 'A', 'B', and 'C' (in figure 2) were glued, I fastened two reinforcing beams (20.25" x 1" x 3/4" each) to the right and left sides on top of the jig table to prevent sagging. I fastened them to the top so that they wouldn't interfere with the work piece to be fastened on the bottom of the jig table. I chose to put a 1/2" deep X 3/8" wide dadoe along the length of the beams and glue them to the table sides with the bulk of the beam protruding above the table.

figure 3: Jig table, front view

See below for details on table inserts. Table inserts are made of 1/4" baltic birtch ply, they are cut to fit item 'B' (minus the dado overlaps) in figure 2.

figure 4: Jig table insert, top view

I cut a ~11" X ~3.5" piece of 1/4" baltic ply (cut width for a snug fit on part 'B' in figure 2). I drew a perpendicular line across the width of the piece of baltic ply at 9.5" and drew and marked the centre point of the resulting line.

Using a 1 1/16" Forstner bit to drill a hole 9.5" from one end of the table insert and centered on the width of the table insert as previously drawn (the size of the bit will vary with the desired tennon thickness). Then I cut the table insert on the line bisecting the hole, make sure the kerf is on the waste side of the cut, resulting in a perfect semi-circle on the end of a 9.5" X 3.5"piece of 1/4" baltic plywood. (The other half of the hole is not be usable because the width of the kerf). Use a template bit (copy bit) to make a exact copy of the first half of the table insert. NOTE: my initial 1 1/16" Forstner hole was not quite centered on the Jig Table Insert, I made due by cutting the Table Insert Spacer accordingly.

Length of table insert spacer = desired tennon Length - {(Forstner Bit size) - 11/16"} : this example "Length of table insert spacer" = 2 3/8" - {(1 1/16") - 11/16"} = 2"

Width of table insert spacer = {(width of table insert) - (size of Forstner bit used)} / 2 : this example "Width of table insert spacer" = {(3.5") - (1 1/16")} / 2 = 1 7/32"

You can make several jig table inserts, one set for each M&T thickness you wish to produce. The thinnest tennon you can make will be dictated by the size of the bushing you use. I have used the Template guide and bushing for a 1/4" router bit, but you can buy a Template guide and bushing for a 1/8" router bit which would allow you to make a considerably smaller M&T.

To attach the jig table to the jig box I used a 1.5" 1/4-20 stove bolt through a hole in the top of the box. I routed a corresponding 5/16" slot in the table. this allows me to slide the table back and forth to allow a various offset when cutting M & T joints. (see details on the next page)

Page 2 (of Mortice & Tennon Jig)

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