Metric and imperial thread translation and related

Have you ever confused of the translation between metric and imperial thread?

Do you know the formula of the thread hole diameter for tapping?

Have you ever hesitated on the proper hole diameter when using self-tapping screw?

Today we’d like to give you a summary on the thread issues? Here we go:

1. Imperial and metric thread translation

There are two types of imperial thread:  type A (1/4,1/2,3/8,5/16,etc.) and type B(5-40,2-56,3-48,4-40,6-32,etc.)

For type A: Inch value*25.4= Metric value

For type B: (0.013”* NO.+0.06”)*25.4= Metric value, NO. is the digit before the hyphen, say 5 is the NO. of 5-40.

2. Formula of the machining hole diameter(d1) under different thread value

2.1 Metric triangle threads (M2, M3, M4, etc.):

d1 is the value we should use.   d1=d-1.08t (t = pitch).

eg.: For M4*1,  d1= 4-1=3mm.

metric triangle threads

2.2 Imperial triangle threads (6-32, 1/4-30 etc.)

t=1/n(inch)=25.4/n (mm);    d1=d-1.28t

e.g. 6-32:     d=(0.013*6+0.06)*25.4=3.5mm;    t=25.4/32=0.794;   so: d1=3.5-1.28*0.794=2.5mm

e.g. 1/4-30: d=1/4*25.4=6.35;    t=25.4/30=0.847;     so: d1=6.35-1.28*0.847=5.3mm

imperial triangle threads

3. Hole diameter for plastic self-tapping thread

In most electronic product design, φ1.2, φ1.5, φ1.7,φ2.0 and φ2.3 are frequently used unless your part are very big. Dome diameter should be a little smaller for the engagement when working, typically 0.3mm to 0.5mm can be used. Smaller screw such as φ1.2, φ1.5 can be φ0.9 and φ1.2mm, φ1.9 dome for φ2.3 screw. So the same with imperial self-tapping screws!

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