CALCULATING THREAD DIMENSIONS CALCULATING THREAD DIMENSIONS Introduction Machinery's Handbook, 31st Edition
1999
The purpose of the ASME B1.30 standard is to establish uniform and specific practices for calculating and rounding the numeric values used for inch and metric screw thread design data dimensions only. No attempt has been made to establish a policy of rounding actual thread characteristics measured by the manufacturer or user of thread gages. Cov ered is the Standard Rounding Policy * regarding the last figure or decimal place to be re - tained 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 ASME B1.30 Standard for inch and metric screw thread design data dimensions may vary slightly from values shown in existing issues of ASME B1 screw thread standards and are to take precedence in all new or future revisions of ASME B1 standards as applicable except as noted in following paragraph. Metric Application.— Allowances (fundamental deviations) and tolerances for metric M and MJ screw threads are based upon formulas which appear in applicable standards. Val ues 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 in ASME B1.30 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. ISO rounding practices, for screw thread tolerances and allowances, use rounding to the nearest values in the R40 series of numbers in accordance with ISO 3 (see page 689). In some cases, the rounded values have been adjusted to produce a smooth progression. Since the ISO rounded values have been standardized internationally for metric screw threads, it would lead to confusion if tolerances and allowances were recalculated using B1.30 rules for use in the United States. The B1.30 rounding rules are, therefore, only applicable to special threads where tabulated values do not exist in ISO standards. Values calculated using the ISO R40 series values may differ from those calculated using B1.30. In such a case the special thread values generated using B1.30 take precedence. Purpose.— Thread dimensions calculated from published formulas frequently may not yield the exact values published in the standards. The difference in most cases are due to rounding policy. The ASME B1.30 standard specifies that pitch, P, values shall be rounded to eight deci mal places. In Example 1 that follows on page 2001 , the pitch of 28 threads per inch, 0.03571429, is correct; using 1 ∕ 28 or 0.0357 or 0.0357142856 instead of 0.03571429 will not produce values that conform to values calculated according this standard. The rounding rules specified by the standard are not uniform, and vary by feature. Pitch is held to eight decimal places, maximum major diameter to four decimal places, and tolerances to six decimal places. In order to maintain the same screw dimensions, everybody has to follow the same rounding practice. The basic profile of UN and UNF screw threads are shown in Fig. 1. Examples given on the following pages, including the detailed calculations of UNEF and UNS external and internal threads, are intended to provide a basis for better understanding of the formulas, the calculations, and the rounding process. * It is recognized that ASME B1.30 is not in agreement with other published documents, e.g., ASME SI-9, “Guide for Metrication of Codes and Standards SI (Metric) Units,” and IEEE/ASTM SI-10, “Standard for Metric Practice.” The rounding practices used in the forenamed documents are designed to produce even distribution of numerical values. The purpose of this document is to define the most practical and commonly used method of rounding numerical thread form values. Application of this method is far more practical in the rounding of thread form values.
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