# Understanding True Position

## Presentation on theme: "Understanding True Position"— Presentation transcript:

Understanding True Position
with MMC in Calypso

This presentation shows how MMC applied to features and datums effect the calculation and evaluation of True Position characteristics. For each case, the GD&T control frame for the True Position, Calypso characteristic window and Calypso custom printout to go with each True Position, and an explanation of each result is shown.

The Print: Ø 1.0 ± 0.1 typ 3 places
A True Position Tolerance is given to this hole with MMC applied to feature and datums. B 10.0

Feature Actuals and True Position Callout:
X=0 Y=10 D=1 C Ø 0.1 A B C Ø 1.0 ± 0.1 typ 3 places X=0 Y=0 D=1 X=10.1 Y=0 D=1 B 10.0

Calypso Characteristic:
Calypso Report:

Why the Answer?: The result for True Position can be thought of as the diameter of a circle, centered at the target (basic dimensions), that contains the actual center. -C- Center Y True Position Result = Ø 0.2 True Position Tolerance = Ø 0.1 Feature Target Center X -B- Center Feature Actual Center 10.0 10.1

Feature Actuals and True Position Callout:
X=0 Y=10 D=1 C Ø 0.1 M A B C Ø 1.0 ± 0.1 typ 3 places X=0 Y=0 D=1 X=10.1 Y=0 D=1 B 10.0

Calypso Characteristic:
Calypso Report:

Why the Answer?: When MMC is applied to the Feature, the Tolerance Band grows by the amount of the bonus. (bonus = actual Ø – smallest allowable Ø for a hole at MMC) -C- Center Y True Position Result = Ø 0.2 True Position Tolerance with Bonus = Ø 0.2 Feature Target Center -B- Center X Feature Actual Center 10.0 10.1

Feature Actuals and True Position Callout:
X=0 Y=10 D=1 C Ø 0.1 A B M C Ø 1.0 ± 0.1 typ 3 places X=0 Y=0 D=1 X=10.1 Y=0 D=1 B 10.0

Calypso Characteristic:
Calypso Report:

Why the Answer?: When MMC is applied to the Centering Datum, the coordinate system center can move within a circle centered at the actual datum center, with a diameter of the bonus. In this case, the Y axis must go through the exact center of the clocking datum, since no MMC is applied there. -C- Center X Y True Position Result = Ø “Bonus” on Center Ø 0.1 True Position Tolerance = Ø 0.1 Feature Target Center -B- Center 10.0 Feature Actual Center

Feature Actuals and True Position Callout:
X=0 Y=10 D=1 C Ø 0.1 A B C M Ø 1.0 ± 0.1 typ 3 places X=0 Y=0 D=1 X=10.1 Y=0 D=1 B 10.0

Calypso Characteristic:
Calypso Report:

Why the Answer?: When MMC is applied to the clocking datum and not the centering datum, the origin of the coordinate system cannot change. The rotation, however, can change, so the Y axis passes through the bonus circle around the clocking datum’s center. Since Rotation does not bring the feature center closer to the target center, the answer is the same as if no MMC is applied at all. “Bonus” on Clocking Ø 0.1 -C- Center Y True Position Result = Ø 0.2 True Position Tolerance = Ø 0.1 Feature Target Center -B- Center X Feature Actual Center 10.0 10.1

Feature Actuals and True Position Callout:
X=0 Y=10 D=1 C Ø 0.1 A B M C M Ø 1.0 ± 0.1 typ 3 places X=0 Y=0 D=1 X=10.1 Y=0 D=1 B 10.0

Calypso Characteristic:
Calypso Report:

Why the Answer?: When MMC is applied to the Centering Datum and the Clocking Datum, the Coordinate system is free to rotate and translate within the constraints of the bonus circles centered on both datums. The Coordinate system can move half of the datum bonus amount toward the feature center. “Bonus” on Clocking Ø 0.1 -C- Center Y True Position Result = Ø 0.1 “Bonus” on Center Ø 0.1 True Position Tolerance = Ø 0.1 Feature Target Center -B- Center X Feature Actual Center 10.0 10.05

Feature Actuals and True Position Callout:
X=0 Y=10 D=1 C Ø 0.1 M A B M C M Ø 1.0 ± 0.1 typ 3 places X=0 Y=0 D=1 X=10.1 Y=0 D=1 B 10.0

Calypso Characteristic:
Calypso Report:

Why the Answer?: When MMC is applied to the Feature and Datums, all mobility of the Coordinate system discussed in previous slides apply, plus there is a larger overall tolerance zone for the feature. Calypso will “bestfit” until either the true position result is within tolerance, or the result cannot be improved further. “Bonus” on Clocking Ø 0.1 -C- Center Y True Position Result = Ø “Bonus” on Center Ø 0.1 True Position Tolerance with Bonus = Ø 0.2 Feature Target Center -B- Center X Feature Actual Center 10.0 10.075