A grub screw, also known as a set screw, is more often than not actually a type of bolt with threaded instead of tapering screw grooves. The part is also referred to as a headless grub screw, because grub screws are a type of fastener that have a head which is usually no larger in diameter than the body of the screw or bolt itself. They also often contain a recessed drive port which usually fits a hexagonal Allen wrench, square Torx wrench, or star shaft twisting tool. These types of screws are an important fastening feature in mechanical systems where metal surfaces rub together and cannot be obstructed by screw or bolt heads, such as in pulleys and gears. They are also often used where recessed screws are necessary for aesthetic reasons such as where they fasten wall joints together, or for security purposes so that they cannot be easily unscrewed, such as in public restroom dividers.
The use of grub screws in fastening parts together has several advantages over the use of traditional screws or bolts, as well as some disadvantages. One of the primary advantages is that usually the entire length of a grub screw is encased in the material that it is fastening, with no leading or trailing edge exposed. This makes it more corrosion-resistant than traditional screws and also allows for the screw to be tightened with more torque, as the surrounding material into which it is screwed reinforces its head in the process. Some grub screws have the standard slotted head for a flat head screwdriver, but they are uncommon because a hex- or Torx-shaped head allows for more torque to be applied before the metal slot in the head begins to soften and deform, reducing the ability to remove the screw later. A major weakness of using grub screws is that, if they do corrode in place or the head slots become badly deformed by screwdrivers, they can be nearly impossible to remove without drilling out the screw itself, which is often made of case-hardened steel.
Some grub screw designs are also made with a pin hex entry hole. This is typical hex slot with a pin in the center, which can only be turned by a special pin hex tool that is not widely available to consumers. This gives the pin hex grub screw added security since it is not easily removed, and the center pin adds strength to the head as well. Pin head grub screws are often found in lock boxes that contain money, such as in pay phone or vending machine designs, or in other applications where there is an added incentive towards disassembly.
Grub screw sizes and shapes also vary considerably depending on for what they are used. There are six different end shapes to a grub screw, including the flat end for general purpose use and which is often found in toy applications, such as in remote-controlled car parts, and the domed point end which is stronger than the flat end design and has a slightly pointed tip that can be used to apply pressure to a part opposite the head of the screw. The cone point tip is used to apply force to soft materials or to fit into a depression in metal facing the end of the grub screw, and the cup point does the same thing only with greater force. The knurled point is shaped the same as a cup point end except that the protruding cup of metal on the end of the screw is ridged with parallel serrations that can be gripped by other equipment or parts. The dog point grub screw has a rivet-like cylindrical head at the end which can be used as a pivot shaft for equipment such as metal or plastic parts that need to rotate around it.