3 Common Screw Types at a Glance – Machine, Sheet Metal, and Cap Screws

Machine ScrewsMachine Screws

  •  Fully threaded shank
  • Usually used with a nut

  • Typically smaller than the average screw

  • Designed to be fastened to an existing tapped hole

Machine screws, also sometimes referred to as machine bolts, are normally smaller than the average screw. They usually range in sizes up to ¾ of an inch (19.05 mm), but can still come in larger variations. Typically machine screws are designed to be fastened to an existing tapped hole on a metal surface, usually in conjunction with a corresponding nut. The main differentiating characteristics of machine screws are: overall size, shape of head, slot type, length, material, and thread type.

The two main drive types associated with machine screws are slotted (flat head) and Phillips. There are also a number of specialized drives that they can come in, these are typically associated with tamper-resistant screws. Some of these drive types include, torx – six pointed, spanner, and trident to name a few. The shape of the head will determine how the machine screw lies once it’s fastened in place. Round and Pan heads will protrude from a flat surface, while flat head machine screws are designed for holes that are countersunk so that they lay flush with the surface.

Machine screws are always threaded the entire length of its shank. Threading on a machine screw is very important because the corresponding holes that they are being fastened into are typically tapped for a specific size and type of screw thread. The two main characteristics of threads are size of the outer diameter of the threads and pitch, the distance between each thread. Machine screw can be made to have either clockwise (right-handed) or counter-clockwise (left-handed) thread.

Sheet Metal ScrewsSheet Metal Screws

  • Fully threaded shank

  • Very versatile fastener, can be used in metal, wood, and plastics

  • Most are self-tapping screws and only require a pre-drilled hole, some come with self-drilling tips

  • Specially hardened, sharp threads that allow it to cut into material and form its own internal thread

Sheet metal screws have a fully threaded shank with sharp threads and tip that allow them to cut through metal, wood, plastic, and various other materials. The size of sheet metal screws are commonly shown as a series of three numbers, these numbers represent the diameter, thread count, and length of fastener. A sheet metal screw listed as 4-32 x 1-½” has a diameter size of 4, 32 threads per inch, and a length of 1-½”.

There are two basic types of sheet metal screws, self-tapping screws and self-drilling screws. Self-tapping screws have a sharp tip that is designed to cut through metal, but the metal must be pre-drilled before these screws can be used. Self-drilling screws have a drill point tip that can easily cut through metal without a pre-drilled hole.

The head of sheet metal screws can come in a number of different styles. Pan or round head screw will have heads that extend above the surface of the material after being installed. Flat or oval countersunk screws will be flush with the top of the material after being installed. These screws can come with a Phillips, flat, or combo drive.

Sheet metal screws made out of carbon steel are the most common and typically the most economical. These screws are prone to rust and corrosion when exposed to moisture or chemicals, so they should typically only be used indoors. Galvanized or stainless steel screws are designed to resist rust and corrosion, but tend to cost more than the standard steel screws. Sheet metal screws may be coated with zinc or nickel to modify their appearance.

Cap ScrewsCap Screws

  • Normally used without a nut

  • Available in both English and metric sizes

  • Fasten machine parts – home appliances / consumer electronic devices

  • Large head, diameter is larger than threaded portion

Cap screws have a large head and a cylindrical shaft with external threads. The head has a larger diameter than the threaded portion, this provides a positive mechanical stop when tightening the screw. Cap screws are tightened directly into a threaded or tapped hole, usually without a nut. A cap screw can generate a high amount of clamping force when tightened.

Commonly used cap screw heads styles include: hex head, socket head, and button head. Cap screws are typically  manufactured out of carbon steel, stainless steel, and metal alloys. The choice of type and size of of cap screw for a particular application mostly depends on the forces required to adequately secure the mechanical connection.

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