15 AcceptabilIty
Additional recommended methods for deterrmning the acceptability of miniature screw threads will be included as further experience with this thread standard $ reported. Until such time, agreements must be reached between purchaser and vendor regarding the basis for determining acceptance, since practices are likely to differ considerably. particularly for external thread,. Where a flee choice is possible, the procedures given below. which are being used with considerable success by some producers. are suggested.
1.5.1 External Threads. The major diameter of the external thread is measured by either contact gaging. optical projection, or laser inspection equipment. All other dimensions, such as pitch diameter, lead, thread form, and minor diameter may be inspected by optical projection methods, with a magnification of 100X recommended. A suggested chart for this method is shown in Appendix A. 11w thread plug gages and the lap are inspected in a similar manner to externally threaded parts Contact gaging such as the use of ‘GO and NOT G0 ring gages, measuring wires, and set plug gages may be used for sizes 0.70 UNM and above.
1.5.2 internal Threads. The minor diameter of the
internal thread is gaged with “GO and “NOT G0
plain cylindrical plug gages. All other elements are
checked only for assemble-ability limits by means of a G0 thread plug gage. taking extreme care not to damage the thread, For the minimum material limits of the internal threads, the accuracy and performance of the tap is relied upon. This implies that the major and pitch diameters of the tap do not exceed the maximum internal thread limits for these elements and disregards over cutting, which is rarely incurred because of the flethility of these small taps and the manner in which they arc generally fluted.
it is recommended that the minor diameter of the internal thread be gaged with one insertion of the NOT Go” plain cylindrical plug gage first. The “NOT GO plain cylindrical plug gage shall be inserted only until it meets some resistance. The minor diameter is then gaged with only one insertion of the plain cylindrical plug gage per thread, as the act of gaging can wear the thread oversize. At this point, the minor diameter is considered to he acceptable. The thread Is gaged with only one insertion of the G0’ thread plug gage. The thread is accepted or rejected on the basis of this one insertion Repeat insertions can wear out/damage the thread.
1.5.3 lefemnee Temperature. The reference temperature is 20CC fur the dimensions defined by this systent
1.5.4 UnIts of Measure. All dimensions in this Standard, including all tables, axe in millimeters unless otherwise specified. Inch conversions may be found in Nonrnandatory Appendices B, C, and 0.
1.5.5 FederaL Goven,ment Us.. When this Standard is approved by the Departmcnt of Defense (1)00) and federal agencies, and is incorporated into FED-STDH28/5. Screw-Thread Standards for Federal Services, SectionS, the use of this Standard by federal government will be subject to all requirements and liiTütations of FED-STD-H28/5.