page_title: List of gear nomenclature
section_title: Form diameter
hierarchical_section_title: List of gear nomenclature / Form diameter
caption_attribution_description: English: Form diameter
context_page_description: This page lists the standard US nomenclature used in the description of mechanical gear construction and function, together with definitions of the terms. The terminology was established by the American Gear Manufacturers Association, under accreditation from the American National Standards Institute.
context_section_description: Form diameter is the diameter of a circle at which the trochoid (fillet curve) produced by the tooling intersects, or joins, the involute or specified profile.  Although these terms are not preferred, it is also known as the true involute form diameter (TIF), start of involute diameter (SOI), or when undercut exists, as the undercut diameter.  This diameter cannot be less than the base circle diameter.
caption: Form diameter
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