Head Screws : The Common Shape of Head Screws

(a) pan, A low disc with chamfered outer edge

(b) button, or dome head cylindrical with a rounded top

(c) round, Dome-shaped, commonly used for machine screws

(d) truss, Lower-profile dome designed to prevent tampering

(e) flat (countersunk), Conical, with flat outer face and tapering inner face allowing it to sink into the material

(f) oval Pan head, or raised head countersunk with a rounded top

Head Screws

Some varieties of screw are manufactured with a break-away head, which snaps off when adequate torque is applied. This prevents tampering and disassembly and also provides an easily-inspectable joint to guarantee proper assembly. An example of this is the shear bolts used on car steering columns, to secure the ignition switch.

Types of screw

• Screw heads are usually flat, oval or round, and each 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 used for joining metal parts, such as hinges to metal door jambs. These 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.

When choosing screw length, remember that the screw should penetrate 2/3 of the combined thickness of the materials being joined. Consider as well moisture conditions and the make-up of the materials being fastened, to avoid corrosion.

Use galvanized or other rust-resistant screws where rust could be a problem.

Posted by : Agus Mardiana