True Position Tolerance Zones - 4: Inventing a New Blueprint Note
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WARNING: Machine tools present a safety hazard. Improper operation can result in severe injury. These topics are for non-laboratory study only and are not to be used in conjunction with the operation of any tool or machine described herein. Never use a machine tool without the supervision of a qualified instructor.

Starting in the early 1940s engineers and tool makers abandoned square tolerance zones and began using a kind of shorthand to describe round tolerance zones. First of all, they dropped the +/- tolerance in the dimension and instead substituted the words "true position" which was later shortened to just TP or (TP). This told the machinist that the tolerance for the location of the center of the hole was not to be found in the location dimensions... so look somewhere else.

That somewhere else was in the note describing the hole feature itself. Like the blueprint shown below the call out was for a .020 diameter tolerance zone. By using the word “diameter” (or just dia) the tolerance zone {TZ or (TZ)} -- was known to be round or circular.

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label: true position, geometric tolerancing, ansi y14.5, tolernce zones