Building the Frame
Cut one end of each of the 14-foot four-by-fours at a 35-degree angle to meet up with the sides of the top crossbeam. Get some help to lift the 16-foot, four-by-six crossbeam. Turn it so the long side is vertical and the short side is horizontal.
Drive a 4-inch nail into the angled end of the two legs to hold each leg to the vertical side of the crossbeam at the ends. This is a keeper nail till you bolt them in place. Stand the frame up.
Nail one 8-foot two-by-four cross brace across each end frame parallel to the ground about three to four feet up on the outside of the frame to complete the A shape. Use a 5-inch nail so it doesn't go all the way through. Cut off the excess two-by-four.
Drill a 1/2-inch hole horizontally through the leg, crossbeam and other leg. Use a 1/2-inch bit to countersink both ends of the hole so you can recess the head and nut of the 10 1/2-inch carriage bolt.
Insert the 10 1/2-inch galvanized carriage bolt through a washer and into the hole. Attach a washer and lock nut to the other end and tighten with the socket wrench. Cut off any excess bolt with a hack saw so it is level with the top of the hole. Repeat on the other end.
Drill both cross-brace two-by-fours with a 3/8-inch bit where the center of the cross braces meets each leg. Countersink the hole with a 1/2-inch bit as described above and bolt the legs and cross braces together with a 1/2-by-6-inch carriage bolt, washers and lock nut. Again, cut off any excess bolt even with the top of the hole.
Set the frame in place on the ground. Mark where the legs rest, and then move the frame so you can dig the post holes. Dig holes at the outer marks of where the legs rested, and then dig a second hole adjacent to the first to accommodate the slant of each leg. Dig the holes 2 feet, 2 inches deep.
Pour a 2-inch layer of pea gravel in the bottom of each hole and set the swing set frame legs into the holes. Pour another inch or so of pea gravel into the bottom of the hole to prevent the concrete from sealing the bottom of the hole.
Mix up enough ready-mix concrete to fill the holes to two inches below ground level. How much you use will depend on how wide you dig the holes. Buy four bags per leg and take any leftovers back to the store.
Scrape the grass and debris off the yard in a 28-foot-by-36-foot rectangle whose sides are at least 10 feet from the sides of the rectangle formed by the swing set frame. Use a garden tractor blade or light dozer, or dig up the grass with a shovel and rake.
Lay landscape timbers on the ground around the outside of the cleared space. Drill holes through the timbers and drive rebar through them to stake them to the ground with a sledge hammer.
Spread pea gravel or sand 4 inches deep in the fall zone marked by the landscape timbers around the swing set.
Drill four 1/2-inch holes vertically through the cross beam. The holes should be two feet apart, and each pair of holes should be four feet apart and four feet from the end of the crossbeam. Countersink the holes at the top to accommodate the washers and lock nut. Insert the eye bolt through a washer and then up through the beam. Screw on a washer and lock nut. Tighten with a socket wrench.
Attach S-hooks to each eye bolt and hang the swings. Adjust the chain length to give comfortable leg clearance for your kids while swinging. Close the S-hooks with channel lock pliers or an S-hook tool.