#TruePosition, or just Position as the ASME Y14.5 standard calls it, is defined as the total permissible variation that a feature can have from its "true" position. The "True Position" is the exact coordinate, or location defined by basic dimensions or other means that represents the nominal value. In other words, the Geometric Dimensioning and Tolerancing "Position" tolerance is how far your feature's location can vary from its "True Position".
Position is one of the most useful and most complex of all the symbols in GD&T. The two methods of using Position discussed on this page will be RFS or Regardless of Feature Size and under a material condition (Maximum Material Condition or Least Material Condition). However, since this is such a useful symbol, we will continue to add content and examples for other uses of this nifty little symbol in the coming months.
Application:
Although incorrect, we title this page and sometimes refer to the symbol as "True Position" since this is typically the term people are referencing when they are looking for the position tolerance. However, it is correctly termed in the ASME Y14.5 standard as simply "Position".
Position may be applied to any feature of size (Feature with physical dimensions like a hole, slot, boss, tab, or sphere) and control the central elements of these size features. See the above central elements of a hole, slot and sphere. Locating surfaces is to be controlled via Profile. Position can be used with Max Material Condition (MMC), Least Material Condition (LMC), projected tolerances, and tangent planes.
In the image below you can see how a hole is called out using the position tolerance symbol. However, this can also be applied to any feature of size in need of a location tolerance, such as a pin, a boss, or even a keyway. When you have a hole in a part such as a bolt pattern, true position is usually called out. It can be used almost anywhere to represent any feature of size.
Position in terms of the axis, point, or plane defines how much variation a feature can have from a specified exact true location. Again, True Position is the exact perfect location of the feature, located and oriented to the datum reference frame using Basic Dimensions.
Position Tolerance Zone
The tolerance is a 3-Dimensional tolerance zone that surrounds the true position. When specifying a position tolerance, datum features are referenced within the feature control frame. This means that you will have an exact point where the position should be with respect to a datum reference frame and your tolerance specifies how far from this you can be. The position is most often referenced with two or three datum features to exactly locate and orient the true position. If the feature of size is a cylindrical feature such as a hole in a part, the size value in the feature control frame is preceded by a diametric symbol to represent a circular or cylindrical tolerance zone.
The cylindrical tolerance zone would extend though the thickness of the part if this is a hole. The entire features axis, midplane or point would need to be located within this tolerance zone.
To measure or inspect the feature regarding the position control, X and Y measurements can be taken of the feature with respect to the datum features called out in the feature control frame. These measurements can quickly be converted to a total diametric deviation. If your diametric deviation is less than the specified tolerance, the feature passes. Remember, in order to capture orientation deviations, multiple measurements will need to be taken along the "depth" of the hole or feature.