We've received a lot of calls and letters wondering why we use mortise and tenon, splines, or other "exotic" joints instead of dowels. To some extent it's a matter of personal preference. But there are some reasons why we tend to shy away from using dowel joints.
edge-to-edge gluing. When boards are being glued edge to edge (as on the table top of the Trestle Table shown in the last issue), it's customary to use dowels to "strengthen" the joint, and to align the boards as they're glued together.
However, straight edge-to-edge gluing and clamping provides more than enough strength for this type of joint. In fact, whatever else is added, whether it be dowels or splines, usually only serves to weaken the joint.
The real purpose of dowels or splines is to align the boards to eliminate a lot of extra handplaning later on.
The problem with using dowels to align the boards is really a problem of trying to align the dowel holes in the first place. Each set of opposing holes (for the dowels) must be exactly opposite each other for the length of the boards. Doweling jigs and dowel centers are designed to help with this problem. But they never seem to be able to pull it off, especially when you're working long boards (as on a table top).
Even the slightest misalignment can create major headaches during assembly. And even if the dowels can be persuaded to fit, they'll be creating quite a bit of stress on the glue line, which can cause the boards to warp, or the glue line to fail.
Using splines, on the other hand, eliminates a lot of these problems. All you have to do is cut a slot in the boards, and slip the spline in the slot. Cutting an accurately positioned slot is relatively easy to do: just keep the face of each board against the fence of the table saw with feather boards. (You can also use a router and a slot cutter if the boards are really warped.) This positions the kerf an equal distance from the face along the entire length of the board.
Shop Note: One trick we've found that makes using splines a whole lot easier, is to make the splines out of Vs" Masonite. It not only has no grain direction (which eliminates orientating the grain across the joint line), it also fits the kerf of our carbide tipped saw blade to a tee.
Another advantage of using splines is that they "feed" themselves into the groove as the boards are pulled tight. So once the splines are started in the groove, it's just a matter of drawing the two boards together.
end grain to long grain. Another common application of dowels is to join two boards end grain to long grain (as on the frame for a door). Usually dowels are used in this case to speed things along, or to avoid using one of the "exotic joints", such as a mortise and tenon.
Unfortunately, dowels are almost the worse choice for this type of joint ... for several reasons. First, the amount of the long-grain surface of the dowel that contacts long grain of the hole is miniscule. Since this long-grain to long-grain contact is where the gluing strength is, there's very little strength right from the start.
And to make matters worse, dowels have a tendency to deform to an oval shape as they first absorb moisture from the glue, and then release it. The result is that the dowel begins to break free from the sides of the hole as it takes on an oval shape. This not only further reduces the gluing strength of the joint, but it also severely reduces the physical bond between the dowel and the hole. It's usually just a matter of time before the entire joint works itself loose.
By using a mortise and tenon, the long-grain to long-grain gluing surface is (by design) quite large. Since both faces of a tenon are long grain and both surfaces (cheeks) of the mortise are also long grain, the bond between the mortise and tenon provides substantial gluing strength.
The only problem with a mortise and tenon is its reputation: it's an "exotic" joint that only a master craftsman can cut. Although this adds to the mystique, it also exaggerates the difficulty, (and skill required) to cut it. Granted, it may take a few practice shots, but that's part of the fun.
And while I'll admit that cutting a mortise and tenon does take a little longer to complete than drilling a couple of holes for dowels, the results more than compensate the effort. (For more information on mortise and tenon joinery, see pages 12 and 18 of this issue, and also Woodsmith issues No. 8, 13, and 18.)
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