CALCULATING THREAD DIMENSIONS Machinery's Handbook, 31st Edition
2001
Example 1, Rounding Inch Series:
n 1
=
. . 28 0 0357142857 003571429
threads per inch
=
28 = 1
n
P
= =
calculated and truncated to 10 places rounded to 8 places ^ ^ h
P P
h
Table 1. Number of Decimal Places Used in Calculations
Units
Pitch
Constants
Intermediate
Final
Inch Metric
8
8 7
6 5
4
as designated 3 3) For inch screw thread dimensions, four decimal places are required for the final values of pitch diameter, major diameter, and minor diameter with the exception of Class 1B and 2B internal thread minor diameters for thread sizes 0.138 and larger. The final values for the allowances and tolerances applied to thread elements are expressed to four decimal places except for external thread pitch diameter tolerance, Td 2 , which is expressed to six decimal places. Minor Diameter Exceptions for Internal Threads : Minimum Minor Diameter: All classes are calculated and then rounded off to the nearest 0.001 inch and expressed in three decimal places for sizes 0.138 inch and larger. For Class 3B, a zero is added to yield four decimal places. Maximum Minor Diameter: All classes are calculated before rounding, then rounded for Classes 1B and 2B to the nearest 0.001 in. for sizes 0.138 in. and larger. Class 3B values are rounded to four decimal places. 4) Metric screw threads are dimensioned in millimeters. The final values of pitch diameter, major diameter, minor diameter, allowance and thread element tolerances are expressed to three decimal places. 5) Values containing multiple trailing zeros out to the required number of decimal places can be expressed by displaying only two of them beyond the last significant digit. Example: 20 threads per inch has a pitch equal to 0.05000000 and can be expressed as 0.0500. Examples Inch Screw Threads.— The formulas in the examples for inch screw threads are based on those listed in ANSI/ASME B1.1, Unified Inch Screw Threads . Table 3 and Table 4 are based on a size that when converted from a fraction to a decimal will result in a num - ber that has only four decimal places. Table 5 and Table 6 are based on a size that when converted will result in a number with infinite numbers of digits after the decimal point. Fig. 1 is provided for reference. Metric Screw Threads.— The formulas for metric screw threads are based on those listed in ANSI/ASME B1.13M, Metric Screw Threads . The calculation of size limits for stan - dard diameter/pitch combinations listed in both ISO 261 and ANSI/ASME B1.13M use the tabulated values for allowances and tolerances (in accordance with ISO 965-1). The constant values differ from those used for inch screw threads, in accordance with the policy of rounding of this standard, because metric limits of size are expressed to only three decimal places rather than four. Thread Form Constants.— For thread form data see Table 2 . The number of decimal places and the manner in which they are listed should be consistent. Thread form con - stants printed in older thread standards are based on a function of thread height ( H ) or pitch ( P ). The equivalent of the corresponding function is also listed. There are some constants that would require these values to 8 or 7 decimal places before they would
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