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Introduction to Model Based Definition with Creo Parametric 7.0
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Setting Features with Asymmetric Tolerances to Mid-Spec

For manufacturing and inspection, it is best to set the nominal geometry to the center of the tolerance zone.

  • This is known as middle of the specification , or mid-spec .

  • Holes are most common.

  • Tolerance method: Bilateral

  • Unilateral

  • Driven dimension annotation tolerances are not linked to geometry.

  • Driving dimensions and tolerances are linked to geometry.

Figure

Figure 1 - Viewing Original Nominal Geometry and Dimensions

Figure

Figure 2 - Viewing Mid-Spec Geometry and Dimensions

Setting Features with Asymmetric Tolerances to Mid-Spec

Figure

Figure 1 - Viewing Original Nominal Geometry and Dimensions

Figure

Figure 2 - Viewing Mid-Spec Geometry and Dimensions For manufacturing and inspection, it is best to set the nominal geometry to the center of the tolerance zone. This is known as middle of the specification, or mid-spec.

Holes are the most common feature where it is best to set geometry to mid-spec.

There are two common types of tolerancing methods available:

  • Bilateral tolerancing — Also known as symmetric tolerancing, bilateral tolerancing is a method of tolerancing a dimension using equal plus and minus deviations from the nominal dimension.

  • Unilateral tolerancing — Also known as asymmetric tolerancing, unilateral tolerancing is a method of specifying a deviation in only one direction, either plus or minus, from the specified nominal dimension.

Driven or created dimension annotations have tolerance displays which are not linked to the geometry. Thus, the geometry can be changed to mid-spec, but the driving dimension tolerancing can be set to unilateral. Only driving dimensions are linked to the geometry, as are their tolerances. In Figure 1, the hole geometry is nominal. In Figure 2, the hole geometry was modified to mid-spec. Since the driving dimension tolerance display is not linked to the geometry, its value does not change. However, the driven dimension is linked to the geometry, and thus its dimension value updated to 2.002, which is the middle of the tolerance zone.

Setting Features with Asymmetric Tolerances to Mid-Spec

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Figure

Erase Not Displayed

Figure

Figure

MBD\Tolerance_Mid-Spec

Figure

SENSOR-MOUNT_MOD-DIMS5.PRT

Steps

  • Task 1. View and display a driving dimension asymmetric tolerance. Disable all Datum Display types.

  • Click File > Options .

  • In the Creo Parametric Options dialog box, select the Configuration Editor category and click Add .

  • In the Options dialog box, type tol_display as the Option name. Edit the Option value to yes .

  • Click OK > OK .

  • Click No , if necessary.

  • In the model tree, select feature Extrude 7 and click Edit Dimensions Figure from the mini toolbar. Figure

  • Select the 2.000 diameter dimension.

  • In the Tolerance group, select Plus-Minus Figure from the Tolerance types drop-down menu. Notice the asymmetrical tolerance.

Figure

  • Click twice in the background to de-select all geometry.

  • Task 2. Create a driven dimension annotation element. In the ribbon, select the Annotate tab. Select the 7D combined state tab.

  • Notice the 2.000 driving dimension.

Figure

  • Click ANGLE Figure from the Annotation Planes group.

  • Click Dimension Figure from the Annotations group.

  • In the Select Reference dialog box, select Select Entity Figure from the Select types drop-down menu.

  • Double-click the hole feature edge and middle-click to place the driven dimension. Figure

  • In the Precision group, select 0.123 from the 10.123 types drop-down list. Notice the dimension has updated to display three decimal places.

  • Click Cancel from the Select Reference dialog box. Figure

  • Task 3. Edit the geometry to mid-spec and verify the driven dimension update. In the ribbon, select the Analysis tab.

  • Select Dimension Boundaries Figure from the Tolerance Analysis types drop-down menu in the Design Study group.

  • In the menu manager, click Middle .

  • Select the 2.000 driving dimension. Figure

  • Click Done .

  • Notice that only the driven dimension value updated. Figure The geometry is now mid-spec, but the driving dimension callout for the geometry still retains its original value and asymmetric tolerance. The driven dimension updates based on the geometry change to mid-spec.