True Position Tolerance Zones - 3: The Problem with Square Tolerance Zones
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So far we have used a fairly rectangular movement of the hole for inspection and the plug gage tells us we have machined a good part. However what happens if the center of the hole is moved .010 to the right and .010 downward at the same time? This diagonal movement would result in some interference. Look closely at the animation below. The center of the hole is within the red square tolerance zone and yet the plug gage will not accept the part because it has some overlap in the upper left hand corner.

Clearly there is an error in the process somewhere. The engineer said the part would be good if the hole was in the location shown on his blueprint. However the inspection apparatus will reject this part. What to do?

Below is the same set up with a "round" tolerance zone instead of a square one. Note that a round tolerance zone prohibits the long diagonal offset and maximizes hole tolerance to within a specified radial distance from absolute center. This was probably what the engineer "meant" but had no way to express it in his blueprint.

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