This ties into the second question as well.
#Primary datum how to
Your engineer’s understanding of how to measure parallelism is incorrect. In this example note the tangible Datum Feature is the surface of the bore itself while the Datum is the theoretically exact axis of that cylindrical surface. For example, the datum of the bore labeled as Datum Feature C above is going to be the axis of that bore, NOT the surface of the bore itself. These features will genera lly result in a datum that is an axis or plane ( midplane ). Placed as one half in an open-ended size dimension ( in line with the arrow).Ī few common Feature of Size Datum Features are bores, cylinders, slots, or tabs.Symbol placed on the arrows of a size dimension.Placed off a feature control frame that is applied to a feature of size.Symbol placed on a non-planar feature of size.Symbol placed on the leader arrow of a size dimension.Symbol is placed on a dimension line directly in line with the dimension arrows.ĭatum Feature Symbol on a Feature of Size When the symbol is indicated in the methods stated above t his means that the datum feature is the surface of the associated with that symbol. (Datum feature is geometrically controlled.) Placed off a feature control frame that is applied to a surface.Symbol placed on a size dimension but placed offset of the dimension arrows.Leader arrow with the datum feature symbol pointing to a planar surface.Symbol placed on an extension line off the datum feature surface.Datum feature symbol placed directly on a planar surface.A dashed leader line indicates the far side surface. Point with a solid or dashed leader line with the datum feature symbol placed on it.Datum features can be either a surface or a feature of size, depending on how the symbol is applied. It is important though that datum features are indicated correctly on the drawing to ensure that the right type of feature is being controlled. Īs stated before, datums can be points, axes, lines, and planes or a combination thereof. A feature control frame will reference as many datums features as necessary to lock in all the necessary degrees of freedom, th is creates what is known as a Datum Reference Frame. This letter will also show up in any feature control frame that uses this datum feature as a reference. These letters are in boxes and tied to the datum feature s with a black triangle.
#Primary datum series
The datum features on a drawing are denoted with a series of capital letters. How Datum Features are Shown on a Drawing They are usually important functional surfaces. ĭatums are theoretical and only simulated by Measurement Equipment (Gauge pins, Granite slabs, angle plates, computer-generated planes, etc)ĭatum Features are real, tangible features on a part where the measurement equipment would physically touch or measure. It is especially important to know this when studying the theories of Geometric Dimensioning and Tolerancing and while setting up a Datum Reference Frame. While they are related to one another they are indeed different entities. Its important to know the difference between Datum Features and Datums.
It is usually an important functional feature that needs to be controlled during measurement as well.Īll GD&T symbols except for the form tolerances ( straightness, flatness, circularity and cylindricity ) can use datums to help specify what geometrical control is needed on the part. They are the surface or feature where the other features are referenced from. You can think of them as anchors for the entire part. A datum feature is the tangible surface or feature of size ( comprised of multiple surfaces or revolved surfaces) that is indicated by the datum feature symbol. Datums are theoretically exact points, axes, lines, and planes or a combination thereof that are derived from datum features.
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