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Accurate Formula To Compute The Pitch And Major/Minor Diameters For Before Plate Thread Plugs And Thread Rings*
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If no plating tolerance is given, then it is assumed that the maximum plating thickness will be nominal or minimum thickness given plus 50%. (see ASME B1.1-1989, pg. 16, 7.4.2; pg. 18, 7.5.2)   The diameters of the gaging for external threads will be smaller while the diameters of the gaging for internal threads will be larger, than standard.
THREAD PLUGS

For the Go plug pitch diameter, add the MAX. amount of plate. For the Not Go pitch diameter, add the MIN. amount of plate. For the majors of work plugs, increase the major diameter on Go work plug by using half the MAX. amount of plate. On Not Go work plugs, increase the major diameter by using half the MIN. amount of plate.

Example

Based on .0002 to .0003 allowance per side (multiply by 4)
.0002 X 4 = .0008 MIN.  
.0003 X 4 = .0012 MAX.
1/4-28 UNF 2B B/P
Basic GO P.D. .2268 + .0012 MAX. = .2280 Go P.D.
Basic NOT GO P.D. .2311 +.0008 MIN. = .2319 Not Go P.D.
Basic GO major .2500 + (.0012 / 2) = .2506 B/P Go major.
Basic NOT GO major .2466 + (.0008 / 2) = .2470 B/P Not Go major.

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RATIO OF PITCH DIAMETER CHANGE TO THICKNESS OF COATING (60 ONLY)

t = thickness of coating
a = pitch diameter change due to coating

0.25a = t and a = 4t or the pitch diam. of a 60 thread changes
by four times the thickness of the coating

BEFORE PLATING RATIOS
60 thread
= 4:1
29 Acme = 8:1
7-45 Buttress = 4.3138:1
14 30'-5 Buttress = 11.9311:1
10 Square thread = 23:1

THREAD RINGS

For the Go ring, subtract the MAX. amount of plate. For the Not Go ring, subtract the MIN. amount of plate. For the minors of thread rings, decrease the minor diameter on the Go thread ring by using half the MAX. amount of plate. On Not Go thread rings, decrease the minor diameter by using half the MIN. amount of plate.

Example

Based on .0002 to .0003 allowance per side (multiply by 4)
.0002 X 4 = .0008 MIN. .0003 X 4 = .0012 MAX.
1/4-28 UNF 2A B/P
Basic GO P.D. .2258 - .0012   MAX. = .2246 Go P.D.
Basic NOT GO P.D. .2225 - .0008   MIN. = .2217 Not Go P.D.

Basic GO minor .2103 - (.0012 / 2) = .2097 B/P Go minor.
Basic NOT GO minor .2148 - (.0008 / 2) = .2144 B/P Not Go minor.

   
* In accordance with ANSI/ASME B1.1-1989

 

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