• Plated screws are good for outdoor use.screws image by Eldin Muratovic from Fotolia.com
  
  Literally hundreds of different types of screws, ranging in size from tiny to giant, are used today in the assembly of many household items. Screws are generally used in assembly as fasteners to hold parts or layers of objects together. The two most common types of screw are slotted -- for use with a flat-head screwdriver -- and Phillips head, which has cross-shaped grooves in the head and requires a matching Phillips screwdriver.
  
  

Types of Screws

• There are six main types of screws. Slotted screws are the oldest and most common type, and have a single slot for the screwdriver. Phillips head screws are the second most common type and have a cross-shaped groove, which provides a larger contact surface between the screw and the screwdriver, minimizing wear and preventing slipping. In hex screws, the entire head is formed as a hexagon, and an adjustable wrench or wrench set is required for tightening hex screws. Square screws (or Robertson screws) have a square indentation in the head and require a special tool for tightening or loosening. One-way screws are a variant of the slotted screw, and can be tightened with a normal slotted screwdriver, but require a special tool for removal. Torx screws have six-pointed recesses to provide more surface area for driving, and are relatively uncommon.
  
  

Head Shape

• Screw heads are manufactured in seven main shapes, with numerous variations. The pan head is the standard screw head profile. The button head is similar to a pan head but with a slightly convex curve. The round head is even more convexly rounded than the button head. A flat head has a flat top tapered underneath intended to be driven into a countersunk hole. An oval head is tapered underneath like a flat head, but the top has a more oval shape. A truss head is broad head with a low profile. A fillister head has a thick profile and is rounded at the top.
  
  

Type of Thread

• There are five major types of threads on screws. Wood screw threads are usually relatively deep to help them bite into the wood. Machine screws have finer threads and are usually designed for use with a nut or a tapped hole. Sheet metal screw threads are medium coarse and deep to help them make good surface contact with the sheet metal. High-low threading involves a different top and bottom threading on the screw, and these screws are usually used with low-density materials. Self-tapping (or thread-forming) threads are designed to allow the screw to tap its own hole, and are only for use with softer materials.