Screws that join pieces of metal can become loose over time, especially if the joint is under any kind of constant movement or stress. The reason metal screws become loose is due to the coarse threads and how they "bite" into the metal. When you attempt to tighten the metal screws, sometimes they simply spin in the hole. It is possible to tighten metal screws even when you think they are stripped. Often the metal screw is good and the hole needs some adjustment.
Remove the metal screw with either a flat-head screwdriver or Phillips-head screwdriver depending upon the screw head. Removing the screw provides an opportunity to inspect the screw threads and the hole.
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Tap the top of the screw hole with a hammer to flatten the outside edges. Often a metal screw works loose because of the flange that develops around the outer edge of the screw hole.
Inspect the threads on the screw to ensure they are intact. Apply a small bead of thread locking compound around the screw threads. Most locking compound either have an applicator brush or are simply a squeeze tube.
Thread the screw into the hole and tighten it with the appropriate screwdriver. Only tighten until the screw is tight but do not over-tighten the screw.