ISO 68-1 The Basics of the Metric Thread Form

In October 2023 the biggest revision of ISO 68-1 in 50 years was released. This standard defines the profile, or shape, of the metric thread. It is referenced by any metric thread fastener standard. Prior to the release of this latest standard, it defined just the basic thread profile, that is, the theoretical profile of the thread defined by dimensions and angles common to both internal and external threads. This profile is theoretical and forms the basis of the thread in that it is before any tolerances or radii are included. The basic profile of a metric (and Unified) thread is shown as the blue line below:

The thread is based upon an equilateral triangle of height H. The height of the parts of the thread are all based upon the H dimension. Longitudinal dimensions are based upon the pitch P of the thread. A key feature of the thread is the pitch diameter, the diameter at which the width of the thread and the width of the space between the threads are equal.

The big change the latest version introduces into this standard is the design profile. Unlike the basic profile, the design profiles for the internal and external threads are different. For the internal thread the profile is as the basic profile, that is, no radii at the thread root are specified, they are flat. For external threads, the root contour of the thread is rounded. Two types of rounding are defined, these are a fully rounded and a partially rounded root. These two types are illustrated in the image below.

The root radii are a proportion of the thread pitch P. For the fully rounded root, radii are 0.144P, and for the partially rounded root, the minimum radii R1 is 0.125P. The rounding of the root improves the fatigue strength and the life of the tooling used to make the thread. The crest contour of internal and external threads is flat, but they may be rounded.

The ISO inch thread series form, (the Unified thread) is defined in the standard ISO 68-2. Originally, the metric and Unified design profiles were identical. The minimum root radius of both threads being 0.108P. In 1975 it was proposed that for the metric thread the minimum root radius be increased to 0.125P, except for property classes less than 8.8. This was to improve the thread’s fatigue performance. Accordingly, the design forms of the inch and metric thread differ in regard to the minimum allowable root radius.

One change in ISO 68-1 is that it now states that unless specifically defined otherwise, the minimum radii R1 for the external thread shall be 0,125P. Prior to this, only property classes 8.8 and higher were required to have such a minimum radius. Accordingly, this minimum radius now applies to stainless steels and threads made from other alloys. If for whatever reason, the committee in charge of the specific standard for a particular fastener material decides that they didn’t want the minimum root radii requirement, it needs to be stated in the standard. Presently, no such exclusion requirement exists in these standards, for example, for stainless steel fasteners (covered in the standard ISO 3506-1). Hence, the minimum radius requirement currently applies to all metric fasteners.

The design profiles of the internal and external threads are shown mated together in the image below. With both threads on the design profile, there is no gap between the threads and so practically could not be mated together due to deviations in the form and pitch that occur in manufacturing processes. To enable the threads to mate together, the design forms require tolerances to be applied so that a gap exists between the threads. Tolerances are also needed so that threads can be economically manufactured. Such a tolerance system for metric threads is defined in the ISO 965 series of standards.

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