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BS ISO 2903:2016,ISO metric trapezoidal screw threads – Tolerances.
ISO 2903:2016 specifies a tolerance system for metric trapezoidal screw threads in accordance with ISO 2902. The tolerances refer to the design profile in accordance with ISO 2901.
The tolerance system does not apply to trapezoidal screw threads with special requirements on axial displacement, for example, machine tool lead and feed screws and nuts.
5 Tolerance system
The system is based on the tolerance system for ISO general-purpose metric screw threads of ISO 965-1.
6 Tolerance positions
The following tolerance positions are standardized for the pitch diameters.
For internal threads: H with zero fundamental deviation(E1).See Figure 1
For external threads: c and e with negative fundamental deviation(es).See Figure 2
The tolerance position for the minor diameter (D) and the major diameter of the internal threads is always H, i.e. with zero fundamental deviation.
The tolerance position for the major diameter(d)and minor diameter (d3) of the external threads is in all cases h, i. e with zero fundamental deviation (es),and it is independent of the tolerance positions of the pitch diameter
The fundamental deviations for the pitch diameters of internal and external threads are given in Table 1.
9 Recommended tolerance classes
In order to reduce the number of gauges and tools, the tolerance classes should preferably be chosen om Tables 8 and 9.
The tolerance class should be selected according to the tolerance quality (medium and coarse) and the
the length group of thread engagement (N and L)
The following general rules can be formulated for the choice of tolerance quality.
—Medium: for general use
—Coarse: for cases where manufacturing difficulties can arise.
If the actual length of thread engagement is unknown, group N is recommended.

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