Use galvanized nail-on plate or similar, to securely fasten all the frame members to each other.
The sizes listed above can be cut out of larger pieces with a pair of tin snips or other type of plate cutters.
See fig.1 and fig.2.
Fix the plates as shown in fig.3 with five 50mm (2") flathead galvanized nails into each member. That is, ten nails for each nail-on plate.
Making two galvanized U-shaped nail-on plates
The top of each "A" frame requires a U-shaped nail-on plate as shown in fig.6.
Each U-shaped plate can be made by clamping a 150x50 (2x6) nail-on plate between two 100x100 (4x4) pieces of lumber (fig.4)
and then by hammering down the parts that jut out, as shown in fig.5.
Each U-shaped plate can then be fixed to the top of the two "A" frames (as shown in fig.6) with
50mm (2") flathead galvanized nails.
Use 12mm (1/2") galvanized bolts 220mm (8 3/4") long to fasten the horizontal braces to the legs.
Use 100mm (4") galvanized nails to secure members together where needed.
Use two 10mm (3/8") galvanized bolts 150mm (6") long to hang the swing seat chain off.