A Primer on Screws and Screw Driving A Primer on Screws and Screw Driving
Screw heads are usually flat, oval or round, and each type has a specific purpose for final seating and appearance.
Flat heads are always countersunk or rest flush with the surface.
Oval heads permit countersinking, but the head protrudes somewhat.
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.
Machine screws are inserted into tapped (pre-threaded) holes and are sometimes used with washers and nuts.
Lag screws, or square-headed bolts with screw heads, are for heavy holding and are driven in with a wrench rather than a screwdriver.
Remember, when choosing screw length, that the screw should penetrate 2/3 of the combined thickness of the materials being joined. 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 Screw Driving
- Lubricate screws with soap or wax for easier installation.
- Whenever possible, hold the work in a vise or clamp when inserting a screw. If this is not possible, keep your hands and other parts of the body away from the tip of the driver.
- To remove a screw with a damaged slot another slot can be cut with a hacksaw blade if the head is exposed enough.
- A pilot hole (usually 2 sizes smaller than the shank of the screw) should always be made before driving a screw. This is especially crucial in hardwoods or when driving a screw near the end of the board. When working with screws of larger diameter, a pilot hole of the same diameter as the shank of the screw should be drilled into the wood to a depth of 1/3 the length of the screw.
- Always keep the screwdriver shank in line with the screw shank. This will avoid damaging the screw slot and pushing the screw out of line.