Round-headed screws rest on top of the material and are easiest to remove. Screw types include the wood screw for when stronger joining than a nail is needed, or for when other materials must be fastened to wood. This screw is tapered to help draw the wood together as the screw is inserted. A sheet metal screw can also be used to fasten metal to wood, as well as metal to metal, plastic, or other materials. Sheet metal screws are threaded completely from the point to the head, and the threads are sharper than those of wood screws.
Machine screws are for joining metal parts, such as hinges to metal door jambs.
Lag screws , or square-headed bolts with screw heads, are for heavy holding and are driven in with a wrench rather than a screwdriver. Consider as well moisture conditions and the makeup of the materials being fastened, to avoid corrosion. Use galvanized or other rust-resistant screws where rust could be a problem. Tips on Screwdriving Lubricate screws with soap or wax for easier installation. Whenever possible, hold the work in a vise or clamp when inserting a screw.