Datum Shift Simulators in Variation Analysis

Datum Shift refers to the available assembly clearance of a part when located in a fixture for gauging.

Edited 6-10-2015

When a datum in a datum reference frame (DRF) is specified at MMC/MMB, the datum feature simulators (e.g. gage pins) will be made as though the datum features are at their MMB (e.g. holes at their smallest size.)  Therefore, when a part with datum features not at MMB is placed on the gage, it can be adjusted so that the features being measured are closer to nominal.  Since the measured variation is less, the features can have more variation relative to each datum axis or center-plane.

Applying Datum Shift in 3DCS

In 3DCS, Datum Shift is applied by activating the floats in the internal feature moves used to control the features in the callouts back to their respective DRFs.

To test Datum Shift in 3DCS, with the new example model (...\example_models\DatumShift), first turn on Datum Shift for the model. 

The option to add Datum Shift to a model is located in the 3DCS Preferences -> MTM Defaults tab -> GD&T tab. The Datum Shift option is on by default.

Then, simulate the Worst Case of the model using the dcu_simu_fn.dll. Launch the DLL and apply 1,000,000 to the total runs field and pres Start>>. 

Example Drawings of Datum Shift:

Outputs to Investigate:         

DCS added a series of measurements to the test model, to confirm all tolerances and datum shift numbers affect the measurements correctly. Each available tolerance has a corresponding measure with direction specified at Y, Z and hole diameter size.

• Hole-5: Y, Z, & D
• Hole-6: Y, Z, DRF, & D

Example Models for Datum Shift:

After installing Version 7.3.2.0, the example model used in this article will be saved into the following directories for you to test with (these are default directories):

Windows 7 & 8: C:\Users\Public\Documents\DCS\3DCS_MC_7_3_2_0_win64

Windows 7 & 8: C:\Users\Public\Documents\DCS\3DCS_V5_7_3_2_0_win64

Windows 7 & 8: C:\Users\Public\Documents\DCS\3DCS_V6_7_3_2_0_win64

The model will be named Datum Shift, and can be used immediately (the measurement data has already been created) by replacing the parts in the assembly. 

• Datum Shift example using MMC: Two parts containing Pins and Tabs, or Holes and Slots
• Datum Shift example using LMC: Two parts containing Pins and Tabs, or Holes and Slots

If you have any questions, please post them in the DCS Community so we can share the answers and information with other users. 

http://community.3dcs.com/3DCS

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