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In most cases, datums are established from the extremities or high points of datum features, through the use of a datum feature simulator for each datum feature referenced in a feature control frame. The set of datums will constrain the datum reference frame relative to the corresponding set of datum feature simulators. Orientation and location controlling tolerance zones applied to features on the part are then constrained reative to the datum reference frame by basic dimensions. Consequently, datum features often do not get checked unless there is a tolerance that applies to the datum feature.
When we tolerance datum features, there will be a chain of sequential control, based on the order of datum feature precedence, reading left to right in a feature control frame.
On the first drawing below, the form (flatness) of datum feature A is controlled by the size tolerance of ±0.1mm per Rule #1. It would not be necessary to add a flatness control unless the design requirement for flatness was tighter than 0.2mm.
For the secondary datum feature B, the size and form are controlled by the size tolerance, the orientation of the feature axis is controlled by the perpendicularity tolerance.
The two planar surfaces that are datum feature C are then controlled using a profile of a surface tolerance relative to the datum reference frame A,B.
The two opposed parallel planar surfaces are not controlled as a feature of size. The planar surface on the right is instead controlled with a profile of a surface tolerance. The profile tolerance references datum feature A. The form, orientation, and location of the planar surface on the right is controlled relative to datum A, which is a plane that is constrained by the high points on datum feature A. This approach leaves the form of datum feature A uncontrolled, so the flatness tolerance must be added to datum feature A. The tolerances applied to datum features B and C in the second drawing are the same as specified in the first drawing.
In the first two examples our tertiary datum feature is controlled relative to a datum reference frame established by both the primary and secondary datum features. Our third example shows a tertiary datum feature that is controlled with reference to only one other datum feature.
In this drawing the primary datum feature is a cylindrical hole. This feature of size needs only a size tolerance applied to control both size and form per Rule #1.
Datum feature B is the 60 mm width of the upper portion of the part. This is also a feature of size, with the size tolerance controlling size and form, and a perpendicularity tolerance controlling the orientation of the feature center plane.
As we can see by examining the configuration of the datum features, datum feature C needs a profile of a surface tolerance, since it can be located relative to datum axis A, there is no need to reference datum feature B in this tolerance. Datum center plane B would provide no constraint of this profile tolerance zone,
So, to properly apply tolerances to a set of datum features, we must examine the configuration of the datum features to determine which tolerances to apply to each, and also which higher precedence datum features we must reference for each orientation or location tolerance.
This tip is in accordance with ASME Y14.5M-1994, Y14.5-2009, & Y14.5-2018
This tip was originally released in August 2003