Here’s a follow-up to my earlier blog entry, Planing a Slab. In that post, I flatten one side of a slab using hand planes and winding sticks to guide the planing effort. Now it’s time to rip the slab in half and then join the halves with sliding dovetails. This joining technique requires no glue, is incredibly strong, keeps the two halves in perfect alignment, and can add extraordinary visual interest.
On the reverse, you can see the completed sliding dovetails. The position of each one corresponds to the support rails on the buffet sideboard base. Now, on to how to construct it.
Using a chalk line, snap a line along along the grain, butt end to butt end, of the slab. This line does not have bisect the slab equally, and it provides a visual reference for ripping the slab on the table saw.
Now make a sled out of 3/4″ plywood about 3″ wider than the slab and a foot longer. On this sled, snap another chalk line bisecting the sled along the long axis. This provides a reference line for lining up the chalk line on the slab. Align the slab on the sled and fasten wood blocks to the plywood to firmly hold the slab in place. If the slab is still a little loose, use wooden wedges to secure it snugly to the plywood sled. Adjust the table saw fence to cut along the chalk line. The plywood sled rides against the fence; the slab should be an inch or more away from the fence. Raise the saw blade to make the cut in one pass. With a 2″ slab riding on a 3/4″ plywood sled, the blade will be raised 3″ or more – a very dangerous set up! So, I do myself a favor – actually four favors: (1) use a very sharp blade; (2) use the blade guard; (3) use a long outfeed table; and (4) have someone assist me on the outfeed side. It’s much safer to use a bandsaw if your bandsaw has sufficient throat clearance.
Ten fingers, ten toes! All humor aside, here’s the slab with the completed rip cut.
One pass on the jointer will clean up the saw marks from the table saw, after which you can hand plane the edge smooth after cutting the dovetails.
This slab rides on a black walnut cantilevered base. Mark the locations for the dovetail cuts corresponding to the locations of the three supports.
Place the slab on the workbench and insert two spacer pieces thick enough to provide clear and safe access for the router bits.
Make wooden guides and true them up on the jointer.
Make a jig, in this case a simple wooden spacer, with a width equal to the radius of the router base.
On the cut guide lines, mark where to stop the cut. It’s better just to make a pencil line rather than to use tape, as tape can interfere with the travel of the router base.
Using two routers of the same make and model can take a lot of the guesswork out of this process. One will be fitted with a straight bit and the other with a dovetail bit. These are heavy duty router bits and very sharp.
Set the straight router bit to half the final depth (about 12mm or 1/2″), and make all six initial cuts. Then set the straight bit and dovetail bit to the finished depth and test this on a piece of scrap wood. Then rout out the waste, first with the straight bit, then with the dovetail bit to complete the cuts.
Here’s a view of the finished sliding dovetails.
Here’s a close-up view of the completed dovetail cut. A note on parallelism: No matter how careful one is, there will be some misalignment among the three cuts. All error is additive, and this turns out to be a good thing. As this joint is not glued, a one degree total parallel alignment error adds tension as the two halves are brought together, ensuring a strong and stable joint.
In another blog, I’ll cover how to make the other half of the sliding dovetail. In the meantime…
…this slab was air dried and is about 15 years old, so it has stopped moving. Still, the butterfly key provides a counterpoint to the unusual grain pattern in the other half of the slab.
Here’s the completed slab again.
My favorite time of year up here in Vermont, and thank you for visiting my blog!