Timber frame joints are used for many types of timber structures. Barns, cabins, and other types of buildings commonly use timber frame joints. Wood joints for projects such as cigar boxes to barns all carry the same principals, and dovetail joints still tend to be the strongest. Cutting dovetail timber frame joints is not terribly complicated, however it does require precision with measurements and cuts. It is best to take plenty of time when cutting these types of joints.
Lay one timber flat. Draw a straight line across the timber, 4 inches away from the end, and parallel with the end. Measure 1 inch in from each end of this line, and draw two diagonal lines out to the top two corners of the timber. You should have the outlines of two 4-by-1-inch right triangles. The 4-inch sides of the triangles are made up by the sides of the timber, while the 1-inch base of the triangles rest on the 6-inch line. The triangles should mirror each other, with their bases 4 inches apart.
Cut out the triangular sections of wood using your hand saw. Cut along the hypotenuses, and the 1-inch sides. The end of the timber should still be 6-by-6-inches. This is the dovetail end of the joint process.
Lay your second timber flat and horizontal. Measure 15-inches in from one end to mark the centre. Use your combination square to draw two 4-inch lines perpendicularly across the side of the timber, so their ends are 2 inches away from the opposite side you are measuring from. Each line must be 3 inches on either side of the centre. Connect the top ends of these lines with a 6-inch line. Measure in 1 inch towards the centre from the bottom of each 4-inch line and draw diagonal lines out to the top of the 4-inch lines. You should have two triangles that precisely mimic the size and position of the triangles in the first step.
Lay the end of the cut timber over the outline on the second timber. This is a chance to make any corrections to your outline. When you cut out the wood on the second timber, the end of the first timber needs to fit tightly inside.
Cut along the hypotenuses with your hand saw. Drill six holes along the 6-inch line that connects the tops of the two 4-inch lines. The tangents of these holes should only touch your pencil markings, and be between the 4-inch line and the bottom of the triangles.
Use your hammer and chisel to knock out the piece of wood that resembles a trapezoid shape. Smooth the edges inside of the timber with your hammer and chisel so it is flat. This is the notch for your dovetail joint.
Place the end of the first timber inside the notch you created on the second timber. Use your hammer to tap it into place so the sides are flush. Screw through the notched timber using your four 6-inch screws into the end of the timber that rests inside of it.