The purpose of this Standard is to establish uniform and specific practices for calculating and rounding the numeric values used for inch and metric screw thread drsign data dimensions only. No attempt is made to establish a policy of rounding actual thread charactenstics measured by the manufacturer or user of thread gages. Covered is the Standard Rounding Policy regarding the last figure or decimal place to be retained by a numeric value and the number of decimal places to be retained by values used in intermediate calculations of thread design data dimensions. Values calculated to this Standard for mdi and metric screw thread design data dimensions may vary slightly from values shown in existing issues of ASME 81 screw thread standards and are to take precedence in aLl new or future revisions of ASME 131 standards as applicable except as noted in para. 12
12 MetrIc Application
Allowances (tundamental deiations) and tolerances for metric M and MJ screw threads are based upon formulas, which appear in applicable standards. Values of allowances for standard tolerance positions and values of tolerances for standard tolerance grades are tabulated in these standards for a selection of pitches. Rounding rules specified below have not been applied to these values but have followed practices of the International Organization for Standardization (ISO), For pitches, which are not included in the tables, standard formulas and the rounding rules specified herein are applicable
hK7lt 1St) ruundsng practices. lot screw thread tolerances and allowances. use rounding to 11w nearvst values in the 1(40 woes of numbers in accoedance wIth 1503 In some cases, the rounded values have been adjusted to produce a smooth prugreILln. Since the ISO rounded values have been standardized internationally.
metric screw threa, It would lead to contusion it tolerances and allowances isvir recalculated using 111.30 rules. lot use in the USA- 81.30 roundIng rules are therefore. only applicable to special threads where tabulated values do not oust In ISO standse& Values ctkulattd using the ISO R40 series values may differ Iron those calculated using 11130 In sudi a case the specsa1 thread values generated using 111.30 take precedence.
(2) Maximum Minor Diamefrr. All classes are calcubled before rounding, then rounded for Classes lB and 28 to the nearest 0.001 in, for sires 0.138 in. and brger Class 3B values are rounded to four decimal places.
3.1.4 Metric screw threads are dimensioned in millimeters. The final values of pitch diameter, ma)or diameter, minor diameter, allowance and thread element tolerances are expressed to three decimal places.
3.1.5 Values containing multiple trailing zemc out to the required number of decimal places can be expressed by displaying only two of them beyond the last significant digit.
20 threads r Inch has a pach quaI to O050(XX)0 and can be ezpiesard as 00500.
3.2 Examples
The formulas in the examples for inch screw threads in Tables 2,3,4 and 5 are based on those listed in ASME 61.1, Unified Inch Screw Threads. Tables 2 and 3 are based on a size that when converted from a traction tø a decimal will result in a number that has only four decimal places. Tables 4 and 5 are based on a size that when converted will result in a number with infinite numbers of digits alter the decimal point.
The formulas in the examples for metric screw threads in Table,. 6,7,8 and 9 are based on those listed in ASME B1.13M, Metric Screw Threads. Tables 6 and 7 show the use of the tabulated values for allowances and tolerances (in accordance with ISO 965-1) for the cakiilation of size limits for standard diameter/pitch combinations listed in both ISO 261 and ASME B1.13M. 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.