Co-operating organizations

The Mechanical Engineering Industry Standards Committee, under whose supervision this British Standard was prepared, consists of representatives from the following Government departments and scientific and industrial organizations:

The Government departments and scientific and industrial organizations marked with an asterisk in the above list, together with the following, were directly represented on the Committee entrusted with the preparation of this standard:

This British Standard, having been approved by the Mechanical Engineering Industry Standards Committee and endorsed by the Chairman of the Engineering Divisional Council, was published under the authority of the General Council on 31 January 1966

The following BSI references relate to the work on this standard:

Committee reference MEE/148 Draft for comment D65/1299

Foreword

This standard makes reference to the following British Standards:

BS 919, Screw gauge limits and tolerances — Part 1: Gauges for screw threads of Unified form.

BS 949, Screwing taps — Part 2: Taps for Unified threads.

BS 1044, Gauge blanks — Part 1: Plug, ring and calliper gauges.

BS 1580, Unified screw threads — Part 3: Diameters below ¼ in.

When BS 3409 was issued in 1961 it made provision for Unified tapped holes in sizes ¼ in and over as recognition of Unified threads below ¼ in was restricted to five selected sizes.

Since that time, not only has the use of what are commonly known as the “numbered sizes” increased considerably, but the complete series with UNC and UNF threads has been incorporated in ISO Recommendation 263 — where it forms part of the international system of screw threads in inch units — and is now covered by a British Standard, namely, BS 1580, Part 3.

It therefore seemed desirable to extend the range of BS 3409 to provide tapped holes for wire thread inserts in these sizes and Part 2 has accordingly been prepared. As in the earlier standard, now known as Part 1, this part does not standardize the inserts themselves, but specifies limits of sizes for tapped holes which, when assembled with an insert, will give the required nominal size and class of tolerance of the internal thread in question.

Unlike Part 1, this standard provides only for Class 2B holes as there are no Class 1B tolerances for Unified threads in sizes below ¼ in. It should also be noted that, in the case of the minor diameter, the committee responsible for this specification have felt obliged to depart from the principle of adhering strictly to the Unified tolerance formulae. This has been necessary because the formula in question contains a constant which results in a large tolerance and consequent risk of too great a reduction in the possible depth of engagement on the smaller sizes. Tolerances which are two thirds of the values obtained by the standard formula have therefore been adopted for this element; it should be noted that they are based on the minimum major diameter of the hole for the insert and not on the nominal major diameter of the thread in question.

A British Standard does not purport to include all the necessary provisions of a contract. Users of British Standards are responsible for their correct application.

Compliance with a British Standard does not of itself confer immunity from legal obligations.

Summary of pages

This document comprises a front cover, an inside front cover, pages i and ii, pages 1 to 9 and a back cover.

This standard has been updated (see copyright date) and may have had amendments incorporated. This will be indicated in the amendment table on the inside front cover.

Section 1. Tapped holes

1 Scope

This British Standard comprises three sections:

Section 1, which specifies limits of size for tapped holes to accommodate wire thread inserts to provide thread sizes from No. 2 to No. 12 UNC and UNF;

Section 2, which recommends screwing taps intended to produce holes to the limits of size specified in Section 1;

Section 3, which gives a recommended gauging system for checking the tapped holes.

2 Form of thread and limits of size

3 Identification

Section 2. Screwing taps

4 General dimensions

5 Marking

6 Form of thread

7 Limits of sizes for cutting diameter

Section 3. Gauging system

8 General

9 Design and general dimensions

10 Tolerances and limits of size

BS 3409-2:1966

Specification for the tapping of holes to receive wire thread inserts (UNC and UNF threads) - Diameters below ¼ in

Current

Published:

31 Jan 1966