True Position Tolerance Zones- 1: Square Tolerance Zones
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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.

Tolerance zones are imaginary bounding geometries that restrict where a machined feature may be located. Put another way: tolerance zones determine if a part is machine to the engineer’s (draftsman’s) specification.

In the above blueprint a .250 diameter hole is required to be placed 1.000 inch from the left side and 1.000 inch up from the bottom. Due to the +/- .010 tolerance each distance may be as short as 0.990 distant from the sides or as far as 1.010 distant from the sides. This creates a .020 square tolerance zone for the center of the hole.

The animation below shows how the center of the hole can be at any position within the red square (the tolerance zone's bounding geometry) and will satisfy the engineering requirements.

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