What is a Thread?

By STG Machine July 23, 2020 No Comment

A thread is a helical form designed and engineered for the purpose of fastening and retaining components. There are many different types of threads, and knowing some basic terms in thread geometry is essential to understand the differences between them.   

A preliminary distinction to note between threads is internal and external. An Internal Thread a thread on a cylindrical or conical internal surface e.g. nuts and holes. External Thread–a thread on a cylindrical or conical external surface e.g. screws–fit together. Internal and external threads must be of the exact same size and type to work properly. However, this is a distinction without a true difference. The true differences in threads are geometric and explained in the key terminology below. Read through the definitions and look at the image immediately below to help conceptualize what is being identified and discussed.

Thread Definitions

Class of Thread

The class of a thread is an alphanumeric designation to indicate the standard grade of tolerance and allowance specified for a thread (Oberg & Jones 2016, 1861).

Crest

The Crest is that surface of a thread which joins the flanks of the thread and is farthest from the cylinder or cone from which the thread projects (Ibid.).

Depth of Thread Engagement

The Depth (or height) of Thread Engagement between two coaxially assembled mating threads is the radial distance by which their thread forms overlap each other (Ibid.).

Flank

The Flank of a thread is the surface connecting the crest with the root (Oberg & Jones 2016, 1861). The flank surface intersection with an axial plane is theoretically a straight line (Ibid.).

Helix Angle

The Helix Angle on a straight thread is the angle made by the helix of the thread and its relation to the thread axis (Oberg & Jones 2016, 1862). On a taper thread, the helix angle at a given axial position is the angle made by the conical spiral of the thread with the axis of the thread (Ibid.). The helix angle is the complement of the lead angle (Ibid.).

Included Angle

The Included Angle, also known as the Thread Angle is the angle between the flanks of a thread measured in an axial plane (Oberg & Jones 2016, 1862) . 

Lead

The Lead is the axial distance between two consecutive points of intersection of a helix by a line parallel to the axis of the cylinder on which it lies i.e. the axial movement of a threaded part (e.g. nut) rotated one turn in its mating thread–the nut moves parallel to the screw axis when the nut is given one turn. For single threads the lead is the same as the pitch (Budyanas & Nisbett 2016, 410).

Lead Angle

The Lead Angle in a straight thread is the angle made by the helix of the thread at the pitch line with a plane perpendicular to the axis (Oberg & Jones 2016, 1862). On a taper thread, the lead angle at a given axial position is the angle made by the conical spiral of the thread perpendicular to the axis at the pitch line (Ibid.). 

Major Diameter

The Major Diameter is the largest diameter of a screw thread (Budyanas & Nisbett 2011, 410).  On a straight thread the major diameter is that of the major cylinder (Oberg & Jones 2016, 1862). On a taper thread the major diameter at a given position on the thread axis is that of the major cone at that position (Oberg & Jones 2016, 1862).

Minor Diameter

The Minor Diameter or Root Diameter is the smallest diameter of a screw thread (Budyanas & Nisbett 410). On a straight thread the minor diameter is that of the minor cylinder(Oberg & Jones 2016, 1863); on a taper thread the minor diameter at a given position on the thread axis is that of the minor cone at that position (Ibid.).

Multiple Start Thread

A Multiple Start thread is a product having two or more threads cut beside each other (Budyanas & Nisbett 2016, 410). Standardized products such as screws, bolts, and nuts all have single threads (Ibid.). A double threaded screw has a lead equal to twice the pitch; a triple threaded screw has a lead equal to three times the pitch and so on. (Ibid.).

Thread Pitch

The Pitch is the distance between adjacent thread forms measured in parallel to the thread axis (Budyanas & Nisbett 410).  Pitch is equal to the lead divided by the number of thread starts (Oberg & Jones 2016. 1863).

Root

The Root is the surface of the thread which joins the flanks of adjacent thread forms and is immediately adjacent to the cylinder or cone from which the thread projects (Oberg & Jones 2016, 1863).

In conclusion, these key terms represent the main aspects of thread geometry that distinguish different types of threads and help determine their performance characteristics. Furthermore, each of these geometric differences has an applied advantage, and further posts will explore

For additional information please check out our blog posts on cut taps and roll taps, and how to chase a tapped hole. In conclusion, a helpful resource to use for further explanations can be found within a machinist handbook.

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