Slip Frame Joints
Slip joints are essentially open-ended mortise-and-tenon joints, and resemble simple
lap joints. Essentially, they are formed by creating a slot along the end of one
board, the same width as the tenon of the other board which is formed normally.
The mortise slot is relatively easy to make on a tablesaw using a mortising jig,
or even an appropriately-configured router. Traditional mortises require the use
of a drill-press or hand-chisel. One disadvantage of this joint is that the cheeks
must be clamped together during glueup, in addition to the regular clamping required
to keep the mortise against the shoulder of the tenon.
Tapered Slip Joint
When applying a veneer to the face of a frame, the joint line where the grain changes
direction may move at different rates, and may eventually cause damage that will
be visible in the veneer. To prevent this, the tenon is cut so it is almost full-thickness
at the shoulder, and slightly narrower than normal at its end. The slot is tapered
to match the tenon. The purpose of this construction is to reduce the amount of
wood movement at the transition point where the two grain-directions meet, and distribute
the stress over a wider area. This is usually sufficient to prevent any visible
damage to the veneer surface.
Mitered Slip Joint
A slip joint with the appearance of a mitered joint can be made by first mitering
the slot of the slip joint, then by cutting the tenon on an angle to match the miter
so the shoulders have a mitered appearance, but the tenon itself ends square. A
blind mitered slip joint can be constructed by stopping the mortise before it becomes
a complete slot, and trimming the tenon to match.
Bridle Joint
A bridle joint is formed by creating a groove on both sides of a board and inserting
it into the slotted end of an open-ended slip joint, giving the appearance of a
T-joint. This joint offers great load-carrying capacity, as the vertical member
can support the weight of the crossing board. To hold maximum weight, the slot should
be two-thirds of the board's thickness. For maximum twisting strength, the slot
should be one-half of the board's thickness to make the (total of the) slip's
walls and the middle tenon the same thickness. |