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ThreadSpecs3 ™


This software does not replace the applicable standards. It only serves as a convenient tool to rapidly determine dimensions of screw threads and gages for certain profiles. The user is expected to have a working knowledge of screw threads and screw thread gaging.
ESP Web Site Profiles Gages Wires FAQ

General Screw Thread Concepts

Thread pitch is the distance, in an axial direction, between corresponding points on adjacent threads. For metric threads, the pitch, in millimeters, is part of the designation. For thread forms measured in inches, the pitch is typically indicated by its inverse, threads per inch.

Most threaded parts have a single helical groove. However, in some applications, the parts may have multiple grooves, indicated by the number of "starts". The lead is equal to the number of starts times the pitch.

Major Diameter
The major diameter, sometimes called the OD (for Outer Diameter), is the outermost diameter. For external threads, this is also the "crest" diameter. For internal threads, it is the "root" diameter. For some profiles and tolerance classes, this is the same as the "nominal" diameter.

Minor Diameter
The minor diameter is the innermost diameter of the thread. For external threads, this is also the "root" diameter. For internal threads, it is the "crest" diameter.

Pitch Diameter
For non-tapered threaded products, the pitch diameter is defined to be the diameter at which the width of the material thickness is equal to the width of an adjacent groove. These widths are taken in the direction parallel to the axis of the threaded item, and not at an end, where the thread may be partial. Sometimes the pitch diameter is halfway between the major and minor diameters, but this is not generally true.

Thread Height
The height of a thread is the radial distance from minor diameter to major diameter on one side, i.e. half the difference between major and minor diameters.

Basic Sizes and Tolerances
For each of the three diameters, external and internal, there is a "basic" diameter. For external threaded products, this is also the maximum size. If a tolerance is applicable, it is applied in the negative direction, toward a minimum size. For internal threaded products, the basic size is the minimum size. If a tolerance is applicable, it is applied in the positive direction, toward a maximum size. In both cases, the basic size is the one representing the maximum material.

For some dimensions, e.g. the external minor diameter of UN threads, only a basic dimension is specified, with no tolerance. It is taken to be a maximum or a minimum according to whether the thread is external or internal, respectively.

Generally, crests and roots are not sharp, but flattened. However, for some profiles, roots are rounded. A root radius may be specified.

In addition to tolerances on diameters, there may be allowances. An allowance is an adjustment to the basic size to allow for easier assembly of parts, or accomodate coatings.

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