ISO 68-1 Metric Thread- Basic Profile and Dimensions

ISO 68-1 specifies the basic and design profiles for ISO general purpose metric screw threads (M). It defines the fundamental geometry of metric fastening threads and provides the reference profile for the design and manufacture of internal and external threads. The standard forms the basis for the dimensional and tolerance specifications used for ISO metric screw threads in fasteners and other threaded components.

Dimensions of basic profile:

Design profile

The design profiles for internal and external threads are different. They are shown as the thick lines in
Figures 2 and 3, respectively.

The dimensions of the design profiles are given in Table 2. They have been calculated by the followingformulae. The values of h 3 are rounded to the nearest sixth decimal place; the values of R are roundedto the nearest third decimal place. The values of R 1 min are decided per the below paragraph for the
partially rounded root type.

h3 = 17H/24 = 0,613 434 661 PR= H/6 = 0,144 34 P
R1< 0,144 34 P
The root contour of internal threads is flat. It may be fully or partially rounded, and cleared beyond the
P/8 flat width. No root radius is specified. The internal thread height, H 1, is listed in Table 1.

The root contour of external threads is fully or partially rounded with a continuous smoothly blendednon-reversing curve, and cleared beyond the P/4 flat width. The root bottom is located in the positionof H/6 truncation. The root form is one of the following types:

– A fully rounded root with a large circular arc, radius R, tangential to the straight flanks. See Figure 3
a).

– A partially rounded root with two small circular arcs, radi R 1, tangential to the straight flanks andthe flat on the bottom. See Figure 3 b). The radius R 1 values shall be given in the relevant threadedproduct document. Unless specifically defined otherwise, the minimum radii R 1 shall be 0,125P.

NOTE: The rounded root contour reduces the rate of threading tool crest wear and improves the fatigue
strength of external threads.The crest contour of internal and external threads is flat. Depending on the threading methods, it may be rounded.

Design profile of Internal Thread

The internal thread is formed on the inside surface of a hole. In fastening applications, internal threads are commonly used in hex nuts, flange nuts, lock nuts, and other internally threaded components. The internal thread is designed to engage with the corresponding external thread and provide a secure mechanical connection.

Design profile of External Thread

The external thread is formed on the outside surface of a cylindrical or conical part. In fastener applications, external threads are commonly found on hex bolts, socket head cap screws, heavy hex bolts, carriage bolts, flange bolts,stud bolts, U-bolts, Eye Bolts, anchor bolts, and T-bolts. The thread profile and basic dimensions are defined according to the applicable thread standard to ensure proper engagement with the corresponding internal thread.