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ГОСУДАРСТВЕННЫЙ СТАНДАРТ СОЮЗА ССР

ТРУБЫ ОБСАДНЫЕ И МУФТЫ К НИМ

ТЕХНИЧЕСКИЕ УСЛОВИЯ

ГОСТ 632-80

2.13.5. Натяг резьбы трубы АТ (расстояние от измерительной плоскости калибра-кольца до торца трубы) по резьбовому калибру-кольцу должен соответствовать величинам, указанным на черт. 7 и в табл. 13 и 14. Предельные отклонения ± 3,2 мм

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Note added at 1 hr (2007-07-07 16:34:06 GMT)
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Кажется, Георгий все же окончательно прав:
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The NPTF system begins with the use of the L-1 gage having a 1 turn in or out limit (2 turns total) the same as the NPT system. The purpose of this gage is to measure the pitch diameter size of the part. It is necessary to refine the ****standoff (distance from the step to the part)*** to a more accurate measurement because the 2-turn total tolerance limit must be divided into 3 equal zones. These three zones are known as the minimum, basic, and maximum zones, and the part is referred to as a minimum part, basic part, or maximum part. The basic zone is the linear area within 1/3 turn from the face of the part, in or out. If the plug gage is standing out from the face (or datum point) of the internal part morethan 1/3 of a turn you have a minimum part because the step on the gage stops within the minimum zone. Likewise, if the plug stands in more than 1/3 of a turn you have a maximum part. The larger the hole is, the deeper the plug will enter into it. A smaller hole, but within tolerance, is a minimum part. A larger hole, but within tolerance, is a maximum part. A hole close to the target size (within 1/3 turn) is a basic part