Threaded Fasteners - Screw Threads - Bolts - Nuts

Threaded Fasteners & Screw Threads:

"n" = pitch. Edit: threads per inch. For metric it would be 4*n instead of 4/n.

Thread Depth:

Thread depth (C & A) is 2x nominal diameter (d) for Aluminium and 1.5x nominal diameter for Steel.

Thread Clearance:

Thread Clearance (B) is at least 2x thread pitch.  This is because you assume that the last two threads of a tap will not result in a full thread of engagement, so you need to "overshoot" the tap in order to get the actual full-thread depth.  This Wikipedia page has a good picture of a typical tap, and you can see the chamfer that will not create full threads.

https://en.wikipedia.org/wiki/List_of_drill_and_tap_sizes

Example: 

I'm using a .250-20 UNC.  There are 20 threads per inch, or .05 inches per thread.

.05 x 2 = .1 inch drill depth minimum beyond your full-thread depth.

Drill Depth:

The drill depth (E) is the same as the clearance above.  In order to get the full-thread depth per the requirement, the chamfered tip of the tap must have room to extend past the threaded portion.

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Imperial:

Metric:

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