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 GD “Position” Tolerance is how far your features location can vary from its “True Position”. Position is probably the most widely used symbol in GD.

True position is the deviation between the theoretical position on a drawing and the actual position, measured as the centerline, on the final product. True position can be calculated using the following formula: true position = 2 x (dx^2 + dy^2)^1/2.

Using a Coordinate Measuring Machine (CMM) Set the reference plane and put the stylus on the measurement point on the target. The measurement result is instantly displayed on the screen. Cartesian coordinates can also be measured, and composite true position can be output with a single measurement.

RE: How to measure positional tolerance without CMM?

1. Put magnet into a collet, holding on the datum feature diameter.
2. Find the largest gage pin that will fit into the hole; put it through the hole.
3. Using a deflection probe, master your indicator vertically at the axis of the collet.

Diametrical Actual Tolerance = 2 X under root (0.15) square + (0.00) square. Therefore actual GD Position Tolerance measured against 0.25 is 0.30. The part is rejected. Now calculate bonus tolerance = ( LMC Diameter – Actual Diameter ) = (4.20 – 4.10) = 0.10.

Parts of the Feature Control Frame

1. Leader Arrow. This points to the feature that the geometric control is placed on.
2. Geometric Characteristic Symbol.
3. Diameter Control / Cylindrical Tolerance Zone (if required)
4. Tolerance.
5. Modifier for the Tolerance.
6. Primary Datum (if required)
7. Secondary Datum (if required)

Geometric Dimensioning and Tolerancing (GD) is a structured language of symbols, rules and definitions that allows the geometrical features of mechanical parts to be defined according to functional limits of imperfection.

unequally disposed profile

Geometric tolerances are specified using symbols on a drawing. Currently, we have 16 symbols for geometric tolerances, which are categorized according to the tolerance they specify.

Total Runout is how much one entire feature or surface varies with respect to a datum when the part is rotated 360° around the datum axis. Total runout controls both the amount of variation in the surface as the part is rotated, but the amount of variation in the axial dimension.

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Concentricity is considered the circular form of GD symmetry. While symmetry measured the true midpoint plane of a feature to a datum plane or axis, concentricity measures the derived midpoint axis to a datum axis. Both are notoriously difficult to measure. Runout is a combination of concentricity and circularity.

Using a Dial Gauge Secure the target in place on the surface plate. Move the target or height gauge straight forward to perform measurement. The difference between the largest measured value (highest height) and the smallest measured value (lowest height) is the parallelism value.

The GD Flatness symbol references how flat a surface is. It is not dependent on any other datums or features. Flatness is a form control, because it is used to control the form or shape of a feature. It controls how much a surface on a part may deviate from an ideal plane.

The flatness of the optically flat surfaces are measured by techniques using the CMM stylus. The stylus can be operated or programmed to take specific, accurate measurements along the surface of the object and compare the data automatically to produce a report on the flatness of the object.

Flatness Gauge A flatness gage is a dial indicator type instrument used to measure lap plate flatness. It consists of a gauge body with two contact feet at one end and one vertically adjustable foot at the opposite end.

Surface Plate A measuring tool that provides an ideal plane as a reference for measurement. The type of surface plate with the highest precision is the precision surface plate, which is still used as the basic reference instrument for measurement and inspection.

So you can specify a flatness that EXCEEDS the parallelism, if required, but if the parallelism is equal or more-constraining than the required flatness, then the flatness is redundant.