[pianotech] Screw sizes and TPI

[chrisstor at aol.com]

"BTW, Wondering what the "#8" of an 8-32 tap/die refers to. Wire Size?" 

Joe, it's a Universal Thread Standard (UTS) controlled by ANSI

"The standard designation for a UTS thread is a number indicating the nominal (major) diameter of the thread, followed by the pitch measured in threads per inch. For diameters smaller than ¼ inch, the diameter is indicated by an integer number defined in the standard, for all other diameters, the inch figure is given.


Unified Screw Threads - UNC, UNF and UNEF [2]

Major diameter
[in (mm)]
Threads per inch (Pitch)
Tap drill size - Preferred sizes[3]

Coarse (UNC)
Fine (UNF)
Extra fine (UNEF)
Coarse
Fine
Extra fine

#0 = 0.0600 (1.5240)
–
80


3/64 in


#1 = 0.0730 (1.8542)
64
72

#53
#53


#2 = 0.0860 (2.1844)
56
64

#50
#50


#3 = 0.0990 (2.5146)
48
56

#47
#45


#4 = 0.1120 (2.8448)
40
48

#43
#42


#5 = 0.1250 (3.1750)
40
44

#38
#37


#6 = 0.1380 (3.5052)
32
40

#36
#33


#8 = 0.1640 (4.1656)
32
36

#29
#29


#10 = 0.1900 (4.8260)
24
32

#25
#21


#12 = 0.2160 (5.4864)
24
28
32
#16
#14


1/4 = 0.2500 (6.3500)
20
28
32
#7
#3


5/16 = 0.3125 (7.9375)
18
24
32
F
I


3/8 = 0.3750 (9.5250)
16
24
32
5/16 in
Q


7/16 = 0.4375 (11.1125)
14
20
28
U
25/64 in


1/2 = 0.5000 (12.7000)
13
20
28
27/64 in
29/64 in


9/16 = 0.5625 (14.2875)
12
18
24
31/64 in
33/64 in


5/8 = 0.6250 (15.8750)
11
18
24
17/32 in
37/64 in


3/4 = 0.7500 (19.0500)
10
16
20
21/32 in
11/16 in


7/8 = 0.8750 (22.2250)
9
14
20
49/64 in
13/16 in


1 = 1.0000 (25.4000)
8
12
20
7/8 in
59/64 in



The following formula is used to calculate the major diameter of a numbered screw greater than or equal to 0: Major diameter = Screw # × 0.013" + 0.060". For example, a number 10 calculates as: #10 × 0.013" + 0.060" = 0.190" major diameter. "

Your #8 screw calculates out to a major diameter of 0.164".  I'm assuming they use size numbers for the same reason that other sizing standards have arisen - to avoid using a lot of decimal points.

<G> Wikipedia is your friend!

http://en.wikipedia.org/wiki/Unified_Thread_Standard
http://www.engineersedge.com/screw_threads_chart.htm

C-
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