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Datum Design Principles in 3DCS

by Benjamin Reese, on Jan 20, 2014 3:34:00 PM

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This article is from DCS's monthly article series the DE Focus included in each of DCS's monthly newsletter.

Datums are the origins for both Tolerances and Measurements. Figure 1 shows a datum example [ASME Y14.5-2009, 49]. Datums are established to ensure consistency between design intentions, manufacturing processes, and inspection plans.

Datum Design in 3DCS

Figure 1. Datums constrain Degrees of Freedom of a part


To make a datum, a feature or a group of features is designated for the datum. The selected feature is called a Datum Feature.


Datum Feature Selection

(S1) A good datum feature is a ‘functional’ feature that most influences the location or orientation of a component. [Paul Drake, ‘Dimensioning and Tolerancing Handbook, 1999, 5-61]. See Figure 2 for an example. A functional feature is often a mating feature used in assembling operations.

3dcs most influential feature

Figure 2. The Most Influential Feature

(S2) A good datum feature should be easily accessible for assembly and manufacturing. See Figure 3 for an example on an Engine Block.

engine block datum setup 3dcs

Figure 3. Engine Block with Good Datum Features

(S3) A good datum feature should be easily accessible for inspection and gauging. See Figure 4(a-b) for an example [Bruce Wilson, GD&T Application and Interpretation, 165].

GD&T Drawing With Good Datum Features

Figure 4(a). A GD&T Drawing with Good Datum Features


potential inspection setting 3dcs

Figure 4(b). Potential Inspection Setting




Datum Reference Frame


A Datum Reference Frame (DRF) is a set of three mutually perpendicular planes, established from datum features; see Figure 1 for a DRF example. A component, either a part or subassembly, is required to have at least one DRF to constrain the component. Figure 5(a-d) [ASME Y14.5-2009, 54] shows the steps in constructing a DRF.





three datum features 3dcs

Figure 5(a). Three Datum Features



theoretical inspection data 3dcs

Figure 5(b). Theoretical Inspection Gauge



datums derived datum features 3dcs

Figure 5(c). Datums derived from Datum Features



datum reference frame 3dcs

Figure 5(d). Datum Reference Frame




Note: A DRF with three datum planes is generally used to locate a component. It is also common that machined components use DRFs of one or two planes.




DRF Considerations


(C1) Identify the body of a part


The body of a part is the main portion of the part, excluding appendages and extensions. Datum Features should come from the body of a part.


(C2) Identify Functional Requirements


Find the gap and flushness requirements, as well as the mating features, flanges, holes, and slots.


(C3) Identify Process Requirements


Determine the process impacts, such as gravity, tool position, accessibility, checking repeatability, and historical part capabilities.


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