Machinery's Handbook, 31st Edition
Metric Screw Threads M Profile 2017 Tolerance: The tolerance is defined by a series of numerical grades. Each grade provides numerical values for the various nominal sizes corresponding to the standard tolerance for that grade. In the schematic diagram the tolerance for the external thread is shown as negative. Thus the tolerance plus the fit define the lower deviation ( ei ). The tolerance for the mating internal thread is shown as positive. Thus the tolerance plus the fit defines the upper deviation ( ES ). Fits: Fits are determined by the fundamental deviations assigned to the mating parts and may be positive or negative. The selected fits can be clearance, transition, or interference. To illustrate the fits schematically, a zero line is drawn to represent the basic size as shown in Fig. 5. By convention, the external thread lies below the zero line and the internal thread lies above it (except for interference fits). This makes the fundamental deviation negative for the external thread and equal to its upper deviation ( es ). The fundamental deviation is positive for the internal thread and equal to its lower deviation ( EI ).
H 8
Internal threads
P
P 8
3 8 H
P 2
60 °
30 °
D , d
5 8
H
H
P 4
P 2
H 4
D 2 , d 2
D 1 , d 1
90 °
Axis of screw thread
External threads
P = 0.866025 P 0.125 H = 0.108253 P 0.250 H = 0.216506 P 0.375 H = 0.324760 P 0.625 H = 0.541266 P 3 2 H × =
Fig. 1. Basic M Thread Profile (ISO 68 Basic Profile)
H/8
3/8 H
5/8 H
Basic Profile
H/4
Fig. 2. Internal Thread Design M Profile with No Allowance (Fundamental Deviation) (Maximum Material Condition). For Dimensions see Table 3 D Min. Major Dia. D 2 Min. Pitch Dia. D 1 Min. Minor Dia.
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