Perpendicularity is a GD&T orientation control symbol used to limit deviation between planes or lines. It helps control and limit the 90 degree angularity between two surfaces.
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Perpendicularity GD&T Symbol:
The GD&T symbol for perpendicular looks like this:
Examples of the callout referencing other datums looks like this:
In this example the perpendicular callout is placing a limit of .05 mm in reference to datum A. The far left segment of the part has the GD&T symbol attached to it. This indicates that this surface must fall within .05 mm of perpendicularity in reference to datum A. However, each drawing will be slightly different, so it is important to note what surfaces the perpendicularity callout is referring to.
How to Measure GD&T Perpendicularity
Perpendicularity can be measured by running a height gauge along a surface perpendicular to the gauge. The total run out of the measured surface will be the corresponding perpendicularity.
Square Ruler and Gauge:
Place the desired surface perpendicular to a square ruler or inspection block. Utilizing pins or gauge blocks check the gap between the part and the inspection block. The gap represents the perpendicularity of the surface, and the pins or gauge blocks will provide a numerical value for the perpendicularity.
A CMM is probably the most popular method of measuring perpendicularity. It can account for multiple datums and can be used to measure hard to reach surfaces.
Common GD&T Perpendicularity Callouts:
Perpendicularity callous will vary greatly depending on the application and size of the part. Due to the fact that standard machining tolerance is +/- .005’’, it is common to see a perpendicularity callout of .002’’ or less. However, when multiple datums start getting involved the tolerances can quickly stack up. Consequently, it can become increasingly more difficult when the referenced datums are large or in difficult places to reach.