Both screws and bolts are threaded into place with a spiral ramp that swivels around the stem of the piece. Both screws and bolts employ the science of the simple machine also called a screw. The difference in screws and bolts is the heading on top that helps the screw or bolt be put into place. Screws have heads with slots allowing screwdriver bits to be placed inside to unscrew or tighten the screw. Bolts are made with polygonal heads that are tightened or loosened with wrenches. Bolts also differ from screws in that they are used with fastening nuts.
Slot Head Screw
The slot head screw has a single slot in the head, in one direction. The slot is used to sink the screw into place in a piece of wood or device. The slot head screw is often used in woodworking because the head type allows this screw to be countersunk into a piece of wood. Countersunk heads are more easily covered because the screw heads are flush with the surface material or sunk down into the material. Flathead screwdrivers are used to remove this type of screw. The one-way screw is a variation of this type of screw, with a special slot requiring special tools in order to remove the screw.
The Phillips head screw is the second-most common type of screw. The Phillips head screw has a cross-shaped slot on the head. This type of screw can be removed with a flat head screwdriver but is more easily removed using a screwdriver of the same name. The Phillips head screw prevents slipping and minimises the amount of wear to the screw head from slipping during attempted removal. Phillips head screws are both bulbous and flat, allowing them to either stick out from the surface or be countersunk. The Torx head screw is a variation of this type of screw, with a six-point slot instead of a four-point slot.
Other Screw Types
Other types of screws require special screwdrivers or drill bits to remove or install. These other types of screws are used in electronic devices and not in woodworking. These screws include square head screws and hexagon head screws. The slots of these screws are shaped as either a square or a hexagon. The hexagonal slotted screws are also called Allen screws and require an Allen wrench to be removed.
Polygon Head Bolts
Bolts with square or hexagonal shaped heads require removal with a wrench. The wrench may be adjustable or sized the same as the head of the bolt. The bolts of this type sometimes have slotted heads for removal with screw drivers; this is often done for convenience. These types of bolts sometimes have pointed tips, like screws, instead of the common flat tip of most bolts.
Rounded Head Bolts
Some types of bolts have rounded heads. Rounded bolt heads typically stand out from a surface and are rounded to prevent danger to machine users. For example, rounded head bolts are used on playground equipment to prevent cuts and bruises from rough edges of the bolt heads. Some rounded head bolts have slots to easy removal.