True Position Calculator

True position defines the total permissible variation that a feature's center, axis, or plane can have from its exact theoretical location. First, the coordinate offsets are found by subtracting the true coordinates from the measured coordinates: Position_offset_x = Measured_x - True_x and Position_offset_y = Measured_y - True_y. The total position variation is then computed using the radial distance formula: Position_variation = 2 * sqrt(Position_offset_x^2 + Position_offset_y^2). Depending on whether your feature is evaluated at Maximum Material Condition (MMC) or Regardless of Feature Size (RFS), bonus tolerances are added to the specified position tolerance to account for departures from the MMC boundary.

Imagine you are inspecting a drilled hole with a drawing specification of 0.500 inches for size and a position tolerance of 0.010 inches at MMC. The allowable manufacturing size ranges from a minimum (MMC) of 0.495 inches to a maximum of 0.505 inches. During inspection, you measure the hole diameter at 0.502 inches, and its coordinate offsets yield a position variation of 0.008 inches. Because the measured size departs from the MMC size by 0.007 inches (0.502 - 0.495), you gain a bonus tolerance of 0.007 inches. This expands your total allowable position tolerance to 0.017 inches (0.010 base + 0.007 bonus). Since your position variation of 0.008 inches is well within the 0.017-inch total tolerance, the part passes inspection.

Always verify your datum reference framework before computing true position, as incorrect datum alignment invalidates coordinate measurements. Pay close attention to material condition modifiers (MMC or LMC) on your engineering drawings, since ignoring bonus tolerances can cause you to reject perfectly acceptable components. Lastly, ensure that your measuring equipment, such as a CMM or precision calipers, has been recently calibrated to eliminate systematic coordinate errors.