Butt frame joints are formed by simply butting the end of one board against the edge of another board. The contact is entirely end grain to long grain with no mechanical interlocking. This situation forms a relatively weak joint
In order to overcome some of its weaknesses, fasteners are frequently used to secure the joint.
Nails and Screws
Nails and screws are intended to have approximately two-thirds of their length embedded in the “second” material, with one-third of the material being supported by the nail’s head. Because of this, nails and screws only become an option if the frame members are relatively narrow, otherwise an alternate joint would be more suitable.
The end piece can be toenailed to the edge piece. This involves driving nails diagonally from edge-to-end through the end-board, and into the edge of the second board. The nail should start far enough from the end to prevent splitting the wood and provide enough material to hold the butt joint together, yet still embeds two-thirds of its length into the second material. It is recommended that a pilot hole be drilled through delicate woods to prevent splitting.
A nail or screw can also be driven through the opposite edge of the edge board, entirely through the edge board, and into the end grain of the end board. For nails, a sufficiently long nail can be used so that the nail can reamain flush.
For screws, the screw hole should first be drilled and counterbored to a sufficient depth to allow the screw’s shoulder to reach the far edge of the material, or the diameter of the screw hole should be large enough to prevent the screw from gaining purchase. The end piece should also receive a pilot hole, but its diameter must be appripriately sized for the screw shaft, in order to maximize grip. This technique allows the screw to apply maximum pulling force to keep the joint tight.
Doweled Butt Joint
A dowel can be used as a fastener in butt frame joints. The dowel forms a loose tenon that is glued into matching holes drilled into the end of one board and the edge of the other board. Doweled joints should have a minimum of two dowels to prevent the joint from twisting itself free. If accurately drilled, the dowel provides positive registration during assembly, however the strength of the joint is somewhat suspect: the surfaces are long grain to long grain in the side piece, but long grain (dowel) to end grain in the end piece. Therefore it is recommended that the dowel in the end piece be fixed in place by short brads driven into the dowel ends through the face or edge of the end piece.
The most difficult aspect of this joint is the accurate alignment of the holes, and this can be mitigated by the use of a special-purpose drilling machine or a suitably configured drilling jig.
Biscuited Butt Joint
Biscuits can be used as a loose tenon in butt frame joints. They have the distinct advantage of being easy to produce, and since the biscuit slot is slightly longer than the biscuit, the joint can be finely tuned during assembly to overcome any alignment errors. This joint is commonly seen in cabinetry to join the rails and stiles in place of traditional mortise and tenon joinery.