27.Apr.2012 Threaded Fasteners : Part 1

How we hold things together matters.  While final fastener choice is usually an engineering and manufacturing decision, a few basic rules about fasteners and understanding how they work helps us to create plausible designs and communicate effectively with other disciplines.  Being able to create logical designs on our own is also essential when working on small projects with startups and on tight deadlines where the bulk of initial design engineering tends to land on the designer who creates the initial digital models.

Thread Diagram

Basic Thread Vocabulary:

External Thread: A thread that protrudes outwards from a part.  Examples include bolts and the ends of bottles.

Internal Thread: A thread that protrudes inward towards the center of a part.  Examples include nuts and bottle caps.

Major Diameter: The distance between the outer most extents of a thread.

On external threads this is be measured between crests.
On internal threads this is measured between roots.

Minor Diameter: The distance between the inner most regions of a thread.

On External Threads this is measured between roots.
On internal threads this is measured between crests.

Pitch: The distance between parallel points on a thread.

When inch fasteners are used, pitch frequently referenced in terms of threads per inch.  To get the actual measurement of thread pitch in these cases, divide by 1.

When using metric threads, the actual pitch is stated after the major diameter of the fastener.

Angle of Thread: The angle between threads measured along a plane that runs through the center of the threads .

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