The Use Datums option in the Position dialog box (Insert | Dimension | Position) enables you to perform the analysis in three ways.
From the active alignment. See Option #1 below
From a mathematical Virtual Hard Gage simulation (Fit to Datums: ON). See Option #2 below
From a Datum Reference Frame simulation (Fit to Datums: OFF). See Option #3 below
These are discussed in turn. All examples reference the illustration below (Features, Datums, and Alignment):
Application: Use this method when there are no modifier(s) (MMC or LMC) on the datum(s) and when checking the Position of one or multiple features (single feature or a pattern) from a Datum Reference Frame (DRF). Bonus tolerance is only available on the feature(s).
Results: The Position of the selected feature(s) is evaluated in the active alignment. Therefore, the active alignment must be set up to reflect the specified DRF before creating the Position dimension(s).
Example of Position "Use Datums" Off:
Option #2: Use Datums: ON and Fit to Datums: ON
Application: Use this method when there are modifier(s) (MMC or LMC) on the datum(s) and when checking the Position of a single feature (multiple features are not supported in legacy dimension mode).
Position of multiple features and MMC/LMC on the Datum feature(s) is supported when using the newer Position Feature Control Frame method of dimensioning.
Results: A "Virtual Hard Gage" simulation is performed mathematically by translating and rotating based on the amount of bonus tolerance from the datum(s). This results in the measured values for the feature changing by the amount of "datum shift" which simulates the jiggling of the gage. In many cases, the measured values may be the same as the nominal values when there is sufficient bonus from the datum features. When the feature is out of tolerance, no fitting occurs, and the measured values and deviations reflect the actual location of the feature such that process adjustments can be made or an engineering analysis of the nonconformance can be performed.
The results are representing "Pass/Fail" analysis just as a functional gage does; therefore, it is not possible to monitor for process variation or perform statistical studies.
Bonus Column: The bonus column in the report shows the calculated amount of bonus of the feature (DF) and the calculated amount of bonus of each datum feature of size (D1 primary, D2 secondary, D3 tertiary). The total bonus value is determined based on the following conditions:
Condition #1A: In-Tolerance
When sufficient bonus tolerance from the datum(s) allows datum shift such that there is no deviation from the nominal values without utilizing up to 100% of the allowable bonus from the datum(s), the total bonus amount is the sum of the bonus from the feature and the unused amount of bonus from the datum(s).
Example of Condition #1A In-Tolerance:
The measured values changed based on the allowable datum shift derived from the bonus of datums D2 and D3 (simulates a functional gage). In this case the measured values check nominal resulting in zero deviation, and the total bonus is the sum of the bonus from the feature (0.006) and the unused bonus from the datums (0.002) for a total of 0.008 bonus tolerance.
Two degrees of freedom from bonus on D2 (0.008 bonus): translation in X and Y axis
One degree of freedom from bonus on D3 (0.008 bonus): rotation about Z axis
Condition #1B: In-Tolerance
When 100% of the bonus from the datum(s) is used then the total bonus amount is only the bonus from the feature. Typically this case would show some amount of deviation from the nominal values.
Example of Condition #1B In-Tolerance:
The tolerance for D2 and D3 were changed to illustrate using 100% of the bonus from the datums where there is deviation from nominal yet still an In-Tolerance condition.
Condition #2: Out of Tolerance
When there is insufficient bonus from the datum(s) to allow datum shift such that the feature is In-Tolerance, no fitting is performed, the measured values are not altered and the total bonus is only the bonus from the feature.
Example of Condition #2 Out of Tolerance:
The tolerance for the DF, D2, D3 and the TP were changed to illustrate the out of tolerance condition.
Option #3: Use Datums: ON and Fit to Datums: OFF
Application: Use this method when there are modifier(s) (MMC or LMC) on the Datum(s) and you do not want the measured data fitted (no datum shift). This option was added based on customer request to provide the ability to incorporate bonus from a Datum and still monitor for process variation (the measured values are not altered because no "datum shift" is applied).
This method is not compliant with Y14.5M 1994.
Results: A "Datum Reference Frame" simulation is performed mathematically by Translating and Rotating based on the degrees of freedom constrained by the selected Datum(s). The X, Y, or Z measured values and deviations are from the active alignment and are not altered based on the bonus from the datum features (no datum shift).
Bonus Column: The bonus column
in the report shows the calculated amount of bonus of the feature (DF)
and the calculated amount of bonus of each datum feature of size (D1 primary,
D2 secondary, D3 tertiary). The total bonus value is the sum of the bonus
from the feature and the datum with the smallest amount of bonus amongst
the selected datums.
Example of Condition #1 In-Tolerance:
Example of Condition #2 Out of Tolerance:
The tolerance for the DF, D2, D3, and TP were changed to illustrate the out of tolerance condition.
