The datum features on a drawing are denoted with
a series of capital letters
. These letters are in boxes and tied to the datum feature with a black triangle. This letter will also show up in any feature control frame that uses this datum feature as a reference.
What is a datum plane?
A datum plane is
a plane on the target that is established as a datum
. It can be a target’s external plane (datum plane) or center plane (datum center plane). “Datum plane” is defined as follows in ISO and ASME standards: … A datum that is a theoretically exact plane.
What is a datum target symbol?
Datum target symbols
The symbol for a datum target is
a circle divided by a horizontal line
(see Fig. 22.36). The lower part identifies the datum target. The upper area may be used only for information relating to datum target.
How do you select the datum features?
Datums are important and
care must be taken
when selecting them. They must be easily identifiable on the part. When parts are symmetrical or have identical features that make identification of Datum Features difficult, the Datum Features should be physically identified.
What is a datum symbol?
Datums are indicated using the following symbols (datum symbols). Datum symbols use
either a white or black triangle
. The letters indicating the datums are always oriented to the direction the reader looks at the drawing. The position of the datum symbol on the drawing is also relevant to the target area.
What are the types of datum?
There are two main datums in the United States.
Horizontal datums measure positions
(latitude and longitude) on the surface of the Earth, while vertical datums are used to measure land elevations and water depths.
What are datum Features?
A datum feature is
a part feature (or FOS), that contacts a datum during measurement
. A datum is a theoretically exact plane (or axis or center-plane), from which dimensional measurement should be made. … Such gage surfaces are called datum simulator.
What is an example of a datum?
An example of a datum is
the assumed altitude of a specific piece of land used as the basis in calculating the altitude of the top of a mountain on the land
. … The singular form of data; for example, one datum. It is rarely used, and data, its plural form, is commonly used for both singular and plural.
What is a common datum?
A common datum means
a pair of single datums specified to function as one datum
. On the other hand, a datum system is a group of datums that is created by combining multiple datums.
What is the most commonly used datum plane?
The North American Datum of 1983 (NAD 83)
is the most current datum being used in North America. It provides latitude and longitude and some height information using the reference ellipsoid GRS80.
What is a movable datum target used for?
Movable Datum Target Symbols and Datum Target Spheres The movable datum target symbol can be used with a
spherical diameter specified for the datum target size
, and a circular phantom line is used to represent the spherical datum feature simulator as shown in Example 3-38.
What does Cylindricity mean?
Cylindricity is
a 3-Dimensional tolerance that controls the overall form of a cylindrical feature to
ensure that it is round enough and straight enough along its axis. … Cylindricity essentially forms a perfect cylindrical boundary around the object that the entire 3-Dimensional part must lie in.
What is the use of datum target in GD&T?
In GD&T, a datum target may be
used to specify a point, line, or area on a part to establish a datum
. This is sometimes necessary when a part feature is too large or naturally irregular to create a datum. Parts that are cast, forged, or molded often benefit from the use of datum targets.
What makes a good datum?
Datum features must be easily identifiable on the part. … Datum A makes a good primary datum for the
four holes because the primary datum controls orientation
, and it is desirable to have bolt holes perpendicular to mating surfaces. The hole locations are dimensioned from the bottom and left edges of the part.
How do you identify a datum?
The datum features on a drawing are denoted with
a series of capital letters
. These letters are in boxes and tied to the datum feature with a black triangle. This letter will also show up in any feature control frame that uses this datum feature as a reference.
How do I choose a datum point?
Creating a datum point by
selecting two end points
. Select two edges on the model; and enter the distance from the datum point to each edge, as shown in Figure 4. For detailed instructions, see Creating a datum point at a specified distance from two edges.
