Question: How are the allowable variation in lead, thread ...



Question: How are the allowable variations in lead, flank angle, and gage tolerance applied to the Max, Pitch and Min. diameters to obtain a cumulative affect and arrive at the maximum – material functional limit of the Plug G0 Gage indicated in # 1 below?

1) ASME B 1.2 – Internal thread-GO gage 4.0 page 11 states”

4 TYPES OF GAGES FOR PRODUCT

4.1 GO Working Thread Plug Gages

(Table 2 - Gage 1 .I)

INTERNAL THREAD

4.1.1 Purpose and Use. The GO thread plug gage inspects the maximum-material GO functional limit, A1, of product internal thread. The GO thread gage represents the maximum-material GO functional limit of the product internal thread, and its purpose is to assure interchangeable assembly of maximum material mating parts. GO thread plug gages must enter and pass through the full-threaded length of the product freely. The GO thread plug gage is a cumulative check of all thread elements except the minor diameter.

2) Functional diameter is defined as:

Functional diameter- the pitch diameter of the enveloping thread of perfect pitch, lead and flank angles, having full depth of engagement but clear at the crests and of a specified length of engagement. Design pitch diameter of the enveloping thread is equal to that of the product maximum material limit. Functional diameter may be derived by adding to the pitch diameter in the case of an external thread, or subtracting from the pitch diameter in the case of an internal thread, the cumulative effects of variation from specified profile, including variations in lead and flank angles over a specified length of engagement. The effects of taper, out of roundness and surface defects may be positive or negative on either external or internal threads. The term "functional diameter" refers to the "GO functional diameter". GO thread gages are used to inspect/evaluate "GO" functional diameters.

3) Table 10 of ASME B 1.2 Pg 55

This table lists the Maximum and Minimum Major, Pitch and Minor diameters for various classes of threads for the GO, NO-GO gages. A section of this table is reproduced below.

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4) ASME B 1.1 Table 4 Pg. 55.

This table lists the change in functional diameter due to allowable lead variation. A section of this table is reproduced below:

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5) ASME B 1.1 Table 5 Pg 76.

This table tabulates the allowable variation in 30º ½ angle of the external & internal threads. This table is reproduced below.

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6) ASME B 1.2 Table 7 Tolerance on the manufacturing of the thread gages.

A portion of this table has been reproduced below.

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