It's not terribly complicated to build a garden swing frame. The structure consists of two upright posts and a heavy-duty crossmember. This design combines a rustic look with the strength of heavy timbers and steel bolts. Easy to assemble with basic tools, the frame shouldn't take more than a day to build, including the time it takes for the concrete to cure.
Dig one elongated post hole 30 cm (12 inches) long by 20 cm (8 inches) wide at the top, 60 cm (2 feet) deep and belled outward at the bottom. Dig an identical hole parallel with the first 2.4 m (8 feet) apart centre to centre.
Lay two posts side by side. Lay a 60 cm (2 foot) long 10 cm x 10 cm (4 x 4 inch) between the posts so that the top of the short 4-by-4 is 12.5 cm (5 inches) below what will be the top of the posts.
Drill two holes through all three 4-by-4s about 30 cm (1 foot) apart. Insert the 1.3 cm (1/2 inch) carriage bolts with washers on either end. You may need to tap the bolts in with the hammer. Bolt in place and tighten with the socket wrench. Repeat with the other two posts. Erect the pairs of bolted-together posts and set them in the holes. The openings between the top 12.5 cm (5 inches) of the post pairs should face each other so the crossbeam can sit in the notches atop the post pairs. Pour 2.5 cm (1 inch) of pea gravel in the bottom of the hole around the base of the post.
Mix the concrete with water and a shovel in the wheelbarrow. Pour three bags worth in each hole. Pour enough concrete to reach 5 cm (2 inches) below the surface and slope the top surface down toward the outside of the hole. Allow to set and cure and fill the rest of the hole with dirt.
Lift the 3 m (10 foot) long 10 cm x 25 cm (4 x 10 inch) crossbeam into the notches on top of the posts. You can often find beams of this size in old buildings being demolished or scrap lumber yards. You can also make one by nailing and bolting two 2-by-10s together. Once the beam is in place, adjust its position so it overhangs the support posts the same distance on both ends.
Drill holes for 1.3 cm (1/2 inch) bolts through the top of the support post pairs and the crossbeam. Put washers on both ends of the bolts, push or pound the bolts through the hole and tighten the nuts. Space the screw eyes underneath the beam so they are 1.8 m (6 feet) apart if you have a 1.8 m (6 foot) wide swing or adjust the eyes to match the width of whatever swing you use. Pre-drill the holes for the screw eyes and screw them into the crossbeam. Hang the swing.
Get help with lifting the crossbeam in place. The beam is very heavy and will require more than two people to raise safely. Lift one end at a time into the notches on top of the posts.