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The Anatomy Of A Screw Or Bolt: Vocabulary Basics

Anatomy of a screw or bolt

A screw or bolt is made up of a few standard sections and attributes, as shown in the graphic.

The Drive is the area of the screw in which a tool is used to turn the fastener, such as a screwdriver, bit, or wrench.  This geometric feature, on the fastener, DRIVES the screw or bolt to its final position.

The Head Is the top part of the fastener and which normally contains the drive.  The Head can come in different shapes that are optimized for different applications.  The Head style also determines how length of the fastener will be measured.

The Body is the section from the bottom of the head downwards to the bottom edge or point of the fastener.  The body includes the threaded, unthreaded portions and point of the fastener.  When referring to the Body, one is referring to the longitudinal sections of the fastener below its head.

The Shank refers to the radial and diametric features of the body.  It contains the peaks and valleys of the thread as well as the areas of the body in which there are no threads. The Shank the stem of the fastener.

The Threads can come in different variations and is the helical spline or splines which spiral down the shank of the fastener.  This is the part of the fastener which does the work involved in tightening and locking parts together. (Where "mechanical advantage" occurs.) The rotation of the threads causes linear motion of the screw, nut or bolt relative to a fixed part.  Different thread styles are designed and optimized for different applications. Depending on the unit of measurement (standard or metric) the threads are measured differently.

The Point is the bottom tip of the fastener in which the shank and body both end. The point is optimized, like the other attributes, based on applications. The point does not need to be “pointy” or even sharp. It can be blunt, sliced and even tapered.


Notice that these naming conventions are broad and high level. Each attribute can contain multiple variations, which needs to be further broken down in order to identify a specific bolt or screw. These attributes above also do not address the material and finish of the fastener, which are other properties that are optimized based in application nor the sizing and naming conventions. 


The Material is what the fastener is made of, specifically if you were to cut the screw or bolt open. (Steel and stainless steel are the most common materials) The material is what gives the fastener its varying levels “strength”, "flexibility" and "durability".  Different materials are stronger, weaker, more ductile, or more brittle, like comparing plastic to metal and rubber.  Different materials have different mechanical properties.  You can think of fasteners like an M&M candy.  The chocalate is the material and the candy shell is the Finish of the fastener. 

The Finish is the material coating, which coats and protects the entire fastener. A finish is typically used when the fastener will be exposed to harsh environmental surroundings, like moisture, or chemicals.  A plain steel fastener will rust over time.  If the steel is plated (coated) with a Finish, like zinc, the fastener will rust slower as the zinc layer protects the inside material from corrosion.  Just like an M&M, this coating will not last forever.  An M&M can melt, and a zinc plated fastener can and will rust over time.  The time it takes to rust depends on the environment the fastener is used and stored in. Zinc finished fasteners are recommended for indoor use as the zinc layer will not provide long lasting corrosion protection in outdoor wet environments. A list of finishes can be found here


Each of these attributes can have lots of different combinations and variation to their features.  Some of these combinations are documented and setup as standard.  As an example a machine screw, will always share the same style of attributes.  A machine screw will always have a BLUNT point, whereas a self-drilling screw will always have a drilling point. 


Thursday, March 18, 2021