Machinery's Handbook, 31st Edition
Interference-Fit Threads 2111 Lead and Angle Variations: The lead variation values tabulated in Table 6 are the maxi mum variations from specified lead between any two points not farther apart than the length of the standard GO thread gage. Flank angle variation values tabulated in Table 7 are maximum variations from the basic 30 ° angle between thread flanks and perpendicu lars to the thread axis. The application of these data in accordance with ANSI/ASME B1.3, the screw thread gaging system for dimensional acceptability, is given in the Standard. Lead variation does not change the volume of displaced metal, but it exerts a cumulative unilateral stress on the pressure side of the thread flank. Control of the difference between pitch diameter size and functional diameter size to within one-half the pitch diameter tolerance will hold lead and angle variables to within satisfactory limits. Both the vari ations may produce unacceptable torque and faulty assemblies. Table 6. Maximum Allowable Variations in Lead and Maximum Equivalent Change in Functional Diameter ANSI/ASME B1.12-1987 (R2018)
External and Internal Threads
Nominal Size 0.2500-20 0.3125-18 0.3750-16 0.4375-14 0.5000-13 0.5625-12 0.6250-11 0.7500-10 0.8750- 9 1.0000- 8 1.1250- 7 1.2500- 7 1.3750- 6 1.5000- 6
Allowable Variation in Axial Lead (Plus or Minus)
Max Equivalent Change in Functional Diam. (Plus for Ext, Minus for Int)
0.0008 0.0009 0.0009 0.0010 0.0011 0.0012 0.0012 0.0013 0.0014 0.0016 0.0017 0.0017 0.0020 0.0020
0.0013 0.0015 0.0016 0.0018 0.0018 0.0020 0.0021 0.0022 0.0024 0.0027 0.0030 0.0030 0.0036 0.0036
All dimensions are in inches. Note: The equivalent change in functional diameter applies to total effect of form errors. Maximum allowable variation in lead is permitted only when all other form variations are zero. For sizes not tabulated, maximum allowable variation in lead is equal to 0.57735 times one-half the pitch diameter tolerance. Table 7. Maximum Allowable Variation in 30 ° Basic Half-Angle of External and Internal Screw Threads ANSI/ASME B1.12-1987 (R2018)
Allowable Variation in Half-Angle of Thread (Plus or Minus)
Allowable Variation in Half-Angle of Thread (Plus or Minus)
Allowable Variation in Half-Angle of Thread (Plus or Minus)
TPI
TPI
TPI
14 13 12 11 10
8 7 6 5
32 28 27 24 20 18 16
0 ° 55 ′ 0 ° 55 ′ 0 ° 50 ′ 0 ° 50 ′ 0 ° 50 ′ 0 ° 50 ′ 0 ° 50 ′
0 ° 45 ′ 0 ° 45 ′ 0 ° 40 ′ 0 ° 40 ′ 0 ° 40 ′ 0 ° 40 ′
1 ° 30 ′ 1 ° 20 ′ 1 ° 20 ′ 1 ° 15 ′ 1 ° 10 ′ 1 ° 05 ′ 1 ° 00 ′
11 1 ∕ 2
4 1 ∕ 2
4
9
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