Limits and Fits

Following last blog about “Tolerancing”, “Limits and Fits” would be another fundamental topic related to mechanical design and quality. As the evolution into the series of geometrical product specification, current edition of ISO 286 focus on the elaboration of ISO code system for tolerances on linear sizes. There are some important norms and practices which should be outlined here.

A basic hole is a hole for which the lower limit deviation is zero.

A basic shaft is a shaft for which the upper limit deviation is zero.

The fundamental deviation is that limit deviation, which defines that limit of size which is the nearest to the nominal size. The fundamental deviation is identified by a letter (e.g. B, d).

The tolerance is also the difference between the upper limit deviation and the lower limit deviation.

Tolerance interval: variable values of the size between and including the tolerance limits.

Clearance: difference between the size of the hole and the size of the shaft when the diameter of the shaft is smaller than the diameter of the hole. In the calculation of clearance, the obtained values are positive (see B.2).

Interference: difference before mating between the size of the hole and the size of the shaft when the diameter of the shaft is larger than the diameter of the hole. In the calculation of an interference, the obtained values are negative (see B.2).

Clearance fit: fit that always provides a clearance between the hole and shaft when assembled.

Interference fit: fit that always provides an interference between the hole and the shaft when assembled.

Transition fit: fit which may provide either a clearance or an interference between the hole and the shaft when assembled.

Hole-basis fit system: fits where the fundamental deviation of the hole is zero, i.e. the lower limit deviation is zero.

Shaft-basis fit system: fits where the fundamental deviation of the shaft is zero, i.e. the upper limit deviation is zero.

1. Captial or small alphabet to decide the legend for external (hole) or internal (shaft) feature.
2. Alphabet characters represent different fundamental deviation (H: basic hole, h: basic shaft).
3. Need to check the table to know the exact EI, ES, ei, and es for different nominal sizes.
4. The fundamental deviation in micrometres is a function of the identifier (letter) and the nominal size of the toleranced feature.
5. For identifiers like J, j, K, k, etc. fundametnal deviation is also a function of tolerance interval which between IT3 to IT8 by including parameter delta.
6. Apend internal tolerance grade after alphabet characters to represent tolerance interval independently. This is called tolerance class.

From: ISO 286–1, for condition 1, 2, and 3 of K, see marks in below figure

From: ISO 286–1, Delta could be found in Table 3

The “hole-basis fit system” should be chosen for general use. This choice would avoid an unnecessary multiplicity of tools (e.g. reamers) and gauges. Therefore the choice of the system should be based on economic reasons.

Usually in fits system, tolerance grade of holes is no less than shafts.

An application example: ISO 965–1 ISO general purpose metrics screw threads-tolerances

the letter M followed by the value of the nominal diameter and of the pitch, expressed in millimetres and separated by the sign “x”

Each class designation consists of

— a figure indicating the tolerance grade;

— a letter indicating the tolerance position, capital for internal threads, small for external threads.

A fit between threaded parts is indicated by the internal thread tolerance class followed by the externalthread tolerance class separated by a stroke.

EXAMPLE:M6–6H/6g

M20 x 2–6H/5g6g

The absence of tolerance class designation means that tolerance quality “medium” with the following tolerance classes are specified:

Internal threads — 5H for threads up to and including M1,4;

— 6H for threads M1,6 and larger.

NOTE Except for threads with pitch P = 0,2 mm for which the tolerance grade 4 is defined only (see Table 3 and Table 5).

External threads — 6h for threads up to and including M1,4;

— 6g for threads M1,6 and larger

In each case, grade 6 shall be used for tolerance quality medium and normal length of threadengagement.

From ISO 965–1

From ISO 965–1

From ISO 965–1

Any of the recommended tolerance classes for internal threads can be combined with any of therecommended tolerance classes for external threads. However, in order to guarantee sufficient overlap, the finished components should preferably be made to form the fits H/g, H/h or G/h.

For thread sizes M1,4and smaller the combinations 5H/6h, 4H/6h or finer shall be chosen.

For coated threads, the tolerances apply to the parts before coating (electroplating), unless otherwise stated.

From ISO 965–1

Tolerance classes within broad frames are selected for commercial external and internal threads.

Tolerance classes in bold print are first choice.

Tolerance classes in ordinary print are second choice.

Tolerance classes in parentheses are third choice.

There is a practical blog for thread classes and setting plug gages which includes the UN thread gages as below:

Product and Technical Support Resources | Vermont Gage

The mission of GLInB is to bring most value by virtualization of your supply chain quality function to fit for challenges in today’s business environment.

Please visit us at http://www.glinb.com for more information.