A set of wooden boards or metal plates, called slides, spans beneath table leaves to provide support under the joint between the separate pieces. Both wooden and metal slides are prone to bending or breaking, which inevitably causes your table to sag toward its center. Replacement slides are available in both wood and metal and they're often interchangeable; it's usually OK to replace your metal slides with wood, or wood with metal. However, your table won't function properly if you don't match the extension length of your replacement slides with the length of your originals.
Lay a furniture blanket on the floor in an open space. Flip the table over and place the table's top face down on the blanket. Use a screwdriver to loosen the screws that fasten the existing slides to the table's leaves. Remove the screws and the slides from the underside of the table.
Scoop a small portion of wood filler or wood repair epoxy from its canister with a putty knife. Force the filler or epoxy into the screw holes from the removed slides. Scrape excess filler or epoxy from the surface with the edge of the putty knife. If you're using two-part epoxy, mix the separate parts on a clean surface before application. Allow the filler or epoxy to cure according to the manufacturer's guidelines. Filling the holes allows you to drill custom holes for the screws that accompany your replacement slides.
Extend the separate arms of the replacement slides. Align the new slides in roughly the same position as the originals. Push a pencil tip through the slides' screw holes and mark the locations of the holes on the underside of the table. If your marking tool won't fit through the holes, measure the spacing between the holes with a tape measure and use a straightedge to lay out the holes in a straight line and at the proper spacing along the table's surface.
Select a twist bit roughly equal to the diameter of the replacement screws' shanks. Don't include the screws' threads in your measurement; only the smooth portion of the shank. Mount the bit in the power drill. Drill starter holes through each of the hole marks on the table. Starter hole depth should equal the length of the screws' shanks.
Align the replacement slides' screw holes with the starter holes. Select a driver bit that fits the screws' slots. Mount the bit to the power drill. Drive screws through the slides' screw holes and into the table to fasten the replacement slides to the table. Flip the table to an upright position. Unlock the table's leaf locks. Remove the leaves from the table. Push the separate sides of the table together to test the new slides.