Purchase four extension springs with closed looped ends. Make sure the ends coil tightly because they'll take a lot of pressure. You don't want an end to unwrap during your workout and snap back. Lay the springs down one on top of the other with the closed loop on top and the holes lined up.
Insert the metal tube through all four loops on each side. The metal tube should be at least 1/2 inch longer than the springs on both sides.
Space the springs out so there's at least a 1/4-inch gap between each spring and use a pencil to mark the spot where the metal loops of the springs hit the metal tubes. Remove the springs and file or cut through the tube far enough to make a slot for the spring. Put the metal tube back through the spring opening and line the ends with the slots.
Get four flat pieces of 6-by-1 inch steel or other firm metal at your hardware store. Predrilled metal plates are best, but you can also drill them yourself. Drill one hole centered 1 inch from each end.
Slide a threaded rod through one of the tubes not attached to the springs. Put the metal pieces on each end and lock it down with nuts on both ends. Line one of the tubes with the springs attached to the bottom hole. Insert a metal rod through the hole, tube and exiting hole. Put nuts on both ends.
Start the other side in the same manner. First, thread a rod through an outside plate, tube and then the other outside plate. Attach the springs in the same manner. Tighten the nuts down firmly.
Wrap the outer tube with foam and tape it with grip tape to hold. Test the chest expander you just built. It should provide enough tension to work out your upper body without exhausting you before you perform the first rep.