The making of interchangeable external and internal threads depends on the selection of thread fit classes. The clearances and tolerances for thread fits are derived from the pitch diameter. The pitch diameter on a straight thread is the diameter of an imaginary cylinder that passes through the thread profiles at a point where the width of the groove and thread are equal. The mating surfaces are the flanks of the thread.The percentage of thread has little do to with fit but refers to the actual minor diameter of the internal thread.

The typical nut for machine screws has 75 percent threads, which are easier to tap than 100 percent threads and retain sufficient strength for most thread applications.

The lead or helix angle of a screw thread (Figure I-293) is larger for greater lead threads than for smaller leads; and the larger the diameter of the workpiece, the smaller the helix angle for the same lead. Helix angles should be taken

Also note that pitch and lead are the same for single lead screws. Helix or lead angles are given for Unified and other thread series in handbooks such as Machinery’s Handbook. A taper thread is made on the internal or external surface of a cone. An example of a 60-degree taper thread is the American National Standard pipe thread (Figure I-295). A line bisecting the 60-degree thread is perpendicular to the axis of the workpiece. On a taper thread the pitch diameter at a given position on the thread axis is the diameter of the pitch cone at that position.

The British Standard Whitworth thread (Figure I-296) has rounded crests and roots, and an included angle of 55 degrees. This thread form has largely been replaced by the into account when grinding tools for threading. The relief and helix angles must be ground on the leading or cutting edge of the tool (Figure I-294). A protractor may be used to check this angle.

Lead or helix angles may be determined by the following formula:

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