Screw jacks are a class of jack that work by applying tension to a leadscrew. A screw jack is typically self-locking, unlike a hydraulic jack, which requires constant pressure to stay in a locked position. There are many different applications for screw jacks, including as automotive jacks and in the construction industry. Small screw jacks are relatively simple to build, and the materials used are usually inexpensive.
Cut three square pieces from the steel plate using a cutting torch. Each square should be four-inches across. Weld the pipe onto one of the squares. This will serve as the base plate of the screw jack and prevent it from tipping. Drill a hole slightly larger than the diameter of the bolt into the second steel square. This will form the cap plate.
Thread the nut onto the bolt. Insert the bolt into the open end of the pipe. Slide the cap plate over the head of the bolt. Weld the cap plate to the pipe. Make sure that the nut and bolt assembly remain free. The nut should rest on the cap plate, but must remain free to turn.
Weld the third steel square onto the head of the bolt. Use a wrench to turn the nut, and the top plate of the screw jack will rise. The threads on the bolt will lock it into place. Turn the wrench in the other direction to lower the jack again.
Welding is required for this project. Do not attempt to weld unless you have experience. Be extremely careful when using any jack. An improperly balanced load can slip, causing injury or property damage.