ISO 956-1, “ISO general purpose metric screw threads – Tolerances – Part 1: Principles and basic data,” has undergone a technical revision. The new ISO 956-1 4th Edition dated 9/15/2013 is available now from Document Center. It cancels and replaces the 3rd Edition from 1998 as well as it’s Technical Corrigendum (correction sheet) from 2009.
The ISO 965 series defines metric screw thread system in much the same way that the FED-STD-H28 series and the ASME B1 Series define inch-based screw threads. These standards are the reason that you can go to the hardware store and get uniform screws and nuts in all those bins! The definitions are pervasive across industry, used in every sector and in every jurisdiction.
The 26-page Part 1 is the only one of the ISO 965 series that is impacted by changes at this time. The balance of the 5 series set is not under revision at this time.
Your scope paragraph says that “This part of ISO 965 specifies a tolerance system for general purpose metric screw thread (M) according to ISO 261.” This means that the diameter and pitch will be defined using ISO 261, “ISO General-Purpose Metric Screw Threads – General Plan.” Then you use ISO 68-1, “ISO General Purpose Screw Threads – Basic Profile – Part 1: Metric Screw Threads,” to define the profile. Both standards are from 1998.
The ISO 965-1 then fits in to define the tolerance grades and tolerance positions using a Letter/Number scheme. The tolerance positions are expressed by using a letter — the tolerance positions by using a number. The result are class designations like 6H and 8g.
You’ll use the tables to help you figure this all out. They are:
- Table 1 – Fundamental deviations for internal threads and external threads
- Table 2 – Minor diameter tolerances of internal thread (TD1)
- Table 3 – Major diameter tolerances of external thread (Td)
- Table 4 – Pitch diameter tolerances of internal thread (TD2)
- Table 5 – Pitch diameter tolerances of external thread (Td2)
- Table 6 – Length groups of thread engagement
- Table 7 – Recommended tolerance classes for internal threads
- Table 8 – Recommended tolerance classes for external threads
- Table 9 – Factors for multiple-start threads
- Table 10 – Td tolerances for the other grades
- Table 11 – TD1 tolerances for the other grades
- Table 12 – Td2 tolerances for the other grades
- Table 13 – TD2 tolerances
- Table 14 – Minimum root radii
There are also 6 figures which again help you visualize and calculate the requirements of the standard.
In writing this blog, I notice that you can find detail information from the document at some other sources. Because the standard is covered by a copyright from ISO, we don’t provide you with any of the actual information in the tables in this blog. And of course there is always the problem with providing details from a standard that may change over time — You provide information to users that has the potential to become obsolete, making your blog erroneous. So please be careful and always go to the source document when actually developing and manufacturing product.
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