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Screws are used for two purposes:

  1. To clamp things together.
  2. To control motion.

1.  Nuts, bolts and screws used to clamp things together

Nuts, bolts and screws that are used for clamping have screw threads.

An external screw thread is a helical groove that has been cut around a shaft.  A shaft with a thread cut into it is called a screw.  A screw with a parallel shank and a hexagonal head is called a bolt. A screw with any other shaped head, such as a pan head, cheese head or a countersunk head is simply called a screw, e.g. a countersunk head screw.

An internal screw thread is a helical groove that has been cut into a cylindrical hole. An internal screw thread may be cut into any product part or it may be cut into a hole in a short length of hexagonal bar.  When the internal screw thread is cut into a short length of hexagonal bar, the part is called a nut.

Nuts and bolts

There are standard sizes for screw heads and for nuts.  The sizes vary slightly according to the standard used, but they are similar.  The most commonly used standard in Europe is the ISO Metric screw thread, defined in ISO 68-1.

ISO Metric bolt

Metric nuts and washers

Countersunk hd. screw and cap screw

ISO Metric machine screws

Assorted screws, nuts and washers

The most commonly used bolts, screws and washers are shown below.  They are all used to clamp product parts together.  The use of nuts, bolts and screws is a temporary fixing method because nuts, bolts and screws can be unscrewed and the components that they were holding can be removed from their previously fixed positions.

Slotted countersunk machine screw
Raised CSK head screw
Slotted pan head screw
Coach screw
One way security screw
Slotted cheese head screw
Coach bolt
Socket cap screw
Pozi round head screw
Connecting nut
Flanged nut
Lock nut with nylon insert
Dome nut
Wing nut
Flat washers
Cup washer
Spring washer


Next: go to cutting screw threads

Click here to view the PDF version of this screws resource; it includes a description of screws, cutting screw threads, common thread forms, motion control, ball screws and roller screws. Screws PDF Screws PDF
Copyright Laszlo Lipot.