Machinery's Handbook, 31st Edition
2082 BUTTRESS THREADS Buttress Thread Tolerances.— Tolerances from basic size on external threads are applied in a minus direction and on internal threads in a plus direction. Pitch Diameter Tolerances: The following formula is used for determining the pitch diameter product tolerance for Class 2 (standard grade) external or internal threads: . . . PD D L p 0002 000278 000854 tolerance e 3 = + + . . . PD D L p 0002 000278 000854 tolerance e 3 = + + where D = basic major diameter of external thread (assuming no allowance) L e = length of engagement p = pitch of thread When the length of engagement is taken as 10 p , the formula reduces to . . D p 0002 00173 3 + . . D p 0002 00173 3 + It is to be noted that this formula relates specifically to Class 2 (standard grade) PD toler ances. Class 3 (precision grade) PD tolerances are two-thirds of Class 2 PD tolerances. Pitch diameter tolerances based on this latter formula, for various diameter pitch combinations, are given in Table 4. Functional Size: Deviations in lead and flank angle of product threads increase the func tional size of an external thread and decrease the functional size of an internal thread by the cumulative effect of the diameter equivalents of these deviations. The functional size of all buttress product threads shall not exceed the maximum-material limit. Tolerances on Major Diameter of External Thread and Minor Diameter of Internal Thread: Unless otherwise specified, these tolerances should be the same as the pitch diameter tolerance for the class used. Tolerances on Minor Diameter of External Thread and Major Diameter of Internal Thread: It will be sufficient in most instances to state only the maximum minor diameter of the external thread and the minimum major diameter of the internal thread without any tol erance. However, the root truncation from a sharp V should not be greater than 0.0826 p nor less than 0.0413 p . Lead and Flank Angle Deviations for Class 2: The deviations in lead and flank angles may consume the entire tolerance zone between maximum and minimum material product limits given in Table 4. Diameter Equivalents for Variations in Lead and Flank Angles for Class 3: The combined diameter equivalents of variations in lead (including helix deviations), and flank angle for Class 3, shall not exceed 50 percent of the Class 2 pitch diameter tolerances given in Table 4. Tolerances on Taper and Roundness: There are no requirements for taper and roundness for Class 2 buttress screw threads. The major and minor diameters of Class 3 buttress threads shall not taper nor be out of round to the extent that specified limits for major and minor diameter are exceeded. The taper and out-of-roundness of the pitch diameter for Class 3 buttress threads shall not exceed 50 percent of the pitch-diameter tolerances. Allowances for Easy Assembly.— An allowance (clearance) should be provided on all ex- ternal threads to secure easy assembly of parts. The amount of the allowance is deducted from the nominal major, pitch, and minor diameters of the external thread when the maxi mum material condition of the external thread is to be determined. The minimum internal thread is basic. The amount of the allowance is the same for both classes and is equal to the Class 3 pitch- diameter tolerance as calculated by the formulas previously given. The allowances for various diameter-pitch combinations are given in Table 5.
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