Hybrid Sliding Dovetail Joint to Stretch a Table

On a recent build of a small dining table, a parameter changed after the rough cutting had been done. The table, originally to measure 30 x 40 inches now needed to be 30 x 46-48. No Bueno!

The stock I had chosen was a beautiful board of curly maple, 5/4 x 11 x 130 inches. I had already rough cut it into 3 planks of equal length and done 2 rounds of milling, glued up the top, and had shown the curl to my significant other; The Client. That board would have never been chosen for a longer table, but here I was. What to do?

Breadboard Math

The easiest way to add about 6-8” to a table and have it look intentional is by adding breadboard ends. For a standard breadboard end, you need a tenon about half the width of the breadboard to provide enough strength to be able to lift the table by its ends as is typical when moving it around (or for someone to lean or sit on the end); easy stuff.

The numbers became a problem. With a 40” blank needing 4” breadboards, I would need 2” tenons. To cut 2” tenons would reduce my blank to 36”; now the breadboards need to be 6”, requiring a 3” tenon which reduces the blank to 34”… the math spiral ended at 7” wide the breadboards that were visually ridiculous. A regular breadboard with a traditional pinned mortise & tenon setup was not going to work. I asked some friends how they would stretch a table and the consensus was a trip to the lumberyard to start over.

How about sliding dovetails?

I love the look of sliding dovetails for breadboard ends, however with an approximate 1” finished thickness there was a concern for the strength of the ends as before. Doing some sketching I came up with a hybrid joint incorporating sliding dovetails, a full width tongue & groove, and 3 beefy floating tenons. Would it work?

The joint in my head was unusual. I had a vision of 2” sliding dovetails at either side of the top, with a full-width T&G connecting them. In the center would be three ½” thick x 3” wide X 5” long floating tenons glued into the top, with the center one glued and pinned to the breadboard and the outer two pinned through elongated slots to allow for wood movement. I would have the look of the sliding dovetails, the full-width alignment of the T&G, and proper strength from the floating tenons. Time to test.

First: test it

I made a test in clear pine milled to 1” thick. The dovetails were laid out 15/16” high and 2” long using a 1:8 ratio and leaving ¼” of thickness at the outside of the widest point of the sockets. I went through two full tests to get a cutting sequence established before moving on to the real deal. The tests were a success so the plan was a go.

Breadboard material

Having zero leftover stock from the table blank I had to improvise a bit. I looked through the lumber stash and found an absolutely gorgeous plank of curly maple… even more figured than the top. It was marked with a sticker from the big orange box as 1x6x10. It was actually just shy of ¾” thick. 5 ½” wide, and 121” long. The big issue was the thickness. (**Every time I am in a home center I take a minute to look at the hardwood racks. When lumber is bought by the train load it is not sorted as carefully and nice things can slip through. That same plank at a real yard would have sold at a premium.)

121” broke down into 4 pieces just over 30”. Luckily, they were all still dead flat. I chose my outside faces, then ran the inside faces and one side of the “leftovers” over the jointer. The leftovers went through the planer to get two clean faces for the lamination.

Using epoxy to avoid introducing water I sandwiched the best plank of the leftovers between the jointed faces of the breadboards. When the sandwich cured I resawed the blank and planed them to match the thickness of the top. I had always planned to put an underbevel on the table and now the thickness of the added lamination would dictate where the bevel landed. The underbevel turned out slightly less than ¼” high. The last plank became stock for the floating tenons.

Game Day

I put the parts in the vise and marked & cut the pins and tails together. The dovetails were marked out using Tailspin collinear marking tools so no transfer was necessary on these awkward parts.

I had run the test blank across the tablesaw to establish the shoulder of the T&G just inside the narrowest point of the tails. This was a little unwieldly for the tabletop so I broke out the tracksaw instead. On the tabletop I plunge-cut the track saw down the waste line for the tongue and cut the tails to length, freeing the waste.

The mortise locations for the loose tenons were marked with the top and the ends clamped together. The tabletop surfaces were marked and used as reference faces.

Using a shop-made router jig and a ½” spiral upcut bit mortises were cut into the breadboard and the top to the maximum depth the router would reach, about 2 ¼”. The breadboard side socket for the tenons were wider by 1 ¾” in one direction to allow for the sliding action to seat the dovetails.

The breadboards had one dovetail socket at each end, to accept the single pin on either edge of the tabletop. I completed the chopping of the sockets and took the breadboards to the router table to cut the groove side of the T&G. I plunged in and out just inside the dovetail sockets using marks on the router table. Running the grooves in multiple passes on each face kept them centered in the stock.

I chopped off most of the tongue waste with a wide chisel and cleaned it up with a router plane, sneaking up on a snug fit and checking often with calipers, then I finished cutting the tails free and cleaned up the shoulders, finishing by chopping the interior mortise for the sliding dovetail that would be sliding from “inside” during assembly.

Fit up, glueup

I milled the tenon stock to size and planed the corners to allow for squeeze out. I used epoxy to set the tenons into the top and provide solid support as I fettled the dovetails. The two outer tenons had to be clipped a bit on one side to allow for movement in the top (the other side would expand or contract into the void made for the sliding action of final assembly). Fitting the two sliding dovetails in opposite directions was an interesting process. One side got more socket attention and the other more tail work. The goal of a clean fit on both sides, but approached from opposite directions was tweaky. Luckily the initial fit off the saw was pretty close so it went quickly. I repeated the process on the other end, not surprisingly it went smoother than the first.

With a leftover bit of tenon stock inserted to avoid blowout, holes were drilled for the pins, two in the center tenon and one each in the others. The center pins were drawbored slightly and the outer ones pinned through elongated slots. Bamboo barbecue skewers were used for the pins.

The glueup was straightforward and went smoothly. Glue went onto the center tenon & socket only, and onto the pins as they were driven home. After full cure they were flush cut through an index card and planed to grade. The top required some slight work with a smoother at the breadboard joint, followed by planing the underbevel to the breadboards’ lamination line. Viewed from above the glueline is invisible.

I ran through the grades with the ROS to 220, and used a block plane to put a slight chamfer on the top surface to break the sharp edge. To finish I applied a coat of oil, let it cure, sanded it off & reapplied twice to get a little more pop from the curl. The topcoat is General Finishes water-based poly in satin.