Technology & Engineering
Datums
Datums are reference points or surfaces used in engineering and technology to establish a common point of origin for measurements and alignments. They provide a consistent framework for designing and manufacturing components, ensuring accuracy and compatibility in assembly and production processes. Datums are essential for maintaining precision and standardization in various industries, including manufacturing, automotive, and aerospace.
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5 Key excerpts on "Datums"
- James D. Meadows(Author)
- 1998(Publication Date)
- CRC Press(Publisher)
Da-tums are origins of measurement. These imaginary Datums do not exist in the real world, so we use manufacturing and inspection equipment to simulate their exis-tence. We recognize that this equipment is not perfect, as are the imaginary da-tums, but is the best we can do to recreate the perfect geometric counterpart of the datum surface(s). This simulation equipment creates a reasonable facsimile of a point, axis, or plane from contact with the datum feature surface. In making this contact, the part is stabilized. We then orient or locate other feature surfaces, axes, centerplanes, or tangent planes from these simulated datum points, axes, and planes. Datum features are either permanent on the part or temporary (to be re-moved or altered during the manufacturing process). Temporary Datums are of-ten used in-process from which to measure other features. These other features are sometimes used as datum features and establish the permanent Datums to be used in the final inspection process. Sometimes, because of rough surfaces—such as forgings or castings— where machining is out of the question because of cost factors, datum target points, lines, or areas are used instead of entire surfaces from which to establish Datums. This is also sometimes done with weldments and similar parts prone to warpage or other distortions. Datum targets are even occasionally more func-tional than would be the use of entire surfaces for the establishment of Datums for orientation or location. These portions of surfaces will often act to create a datum reference frame to eliminate the six degrees of freedom (three rotational Datums 179 and three linear) a part experiences in free space. A drawing should be a physical description—a factual statement—of how a part works, and the datum selection is key to making it such. Concessions can and should be made to manufacturing for ease and consistency of set-up when these concessions do not impair the functionality.
eBook - PDF- John Bossler, John D. Bossler, James B. Campbell, Robert B. McMaster, Chris Rizos, John D. Bossler, James B. Campbell, Robert B. McMaster, Chris Rizos(Authors)
- 2010(Publication Date)
- CRC Press(Publisher)
17 3 Datums and Geospatial Reference Systems John D. Bossler and Richard Snay This chapter introduces and defines the concepts of Datums and geospatial refer-ence systems . It also discusses various applications involving Datums and geospatial reference systems and, in tabular form, provides a list of the major Datums used in the world as of this writing. A datum, in general, is any quantity or set of quantities that may serve as a refer-ent or basis for the calculation of other quantities. 3.1 GEODETIC Datums According to the Geodetic Glossary (1986), a geodetic datum is “a set of constants specifying the coordinate system used for geodetic control, i.e. for calculating coor-dinates of points on the earth.” Geodetic Datums have also been referred to as geo-spatial reference systems. Datums may be classified as geometric, geopotential or some combination thereof. A purely geometric datum is concerned only with those spatial relationships between terrestrial points that are independent of the Earth’s gravity field, for example, (a) the straight-line distance between two points, (b) the length of the geodesic connecting two points on an ellipsoidal surface, and (c) the CONTENTS 3.1 Geodetic Datums ............................................................................................ 17 3.1.1 Horizontal Datums ............................................................................. 18 3.1.1.1 Horizontal Datums with Multiple Ellipsoids ....................... 19 3.1.2 Vertical Datums .................................................................................. 19 3.2 Geospatial Reference Systems ........................................................................ 20 3.2.1 Modern Geometric Reference Systems .............................................. 20 3.2.1.1 Reference System Definition ................................................
- Alex Krulikowski(Author)
- 2012(Publication Date)
- Cengage Learning EMEA(Publisher)
The datum system is comprised of two major parts. The first component is identifying the features or features of size that are to be used to contact the measurement equip-ment. The second component is communicating the order in which the part surfaces contact the Datums or measure-ment equipment. The datum system constrains a part to restrict or remove some or all of the available degrees of freedom relative to a datum reference frame. This defines the relationship between the part, the geometric tolerance zones, and the datum reference frame. Figure 13-2 shows an example of the datum system. FIGURE 13-2 Datum System Example The datum system provides three important benefits to in-dustry. Each of these benefits helps to improve communica-tions and reduce costs. The datum system benefits include: • Precise communication of geometric relationships for proper fit and function • Repeatable dimensional measurements • Dimensional measurements that correlate to the fit and function requirements Benefits of the Datum System Design Tip Whenever dimensioning the loca-tion of a feature of size, do not use implied Datums. Datum Terminology A datum feature is a feature or feature of size that is identi-fied with either a datum feature symbol or a datum target symbol. A datum is a theoretically exact point, axis, line, plane, or combination thereof, derived from the theoretical datum feature simulator. It is the origin for dimensional measure-ments. There are two types of datum feature simulators: theoretical and physical. A datum feature simulator (theoretical) is the theoretically perfect boundary used to establish a datum from a specified datum feature. A datum feature simula-tor (physical) is the physical boundary used to establish a simulated datum from a specified datum feature. A simu-lated datum is a datum established from a physical datum feature simulator. The terms above describe both theoretical elements and physical entities used in the datum system.
eBook - ePubThe 3-D Global Spatial Data Model
Principles and Applications, Second Edition
- Earl F. Burkholder(Author)
- 2017(Publication Date)
- CRC Press(Publisher)
8 Geodetic Datums Introduction As stated in the Preface, the reader is encouraged to consult the NGS web site (https://www.ngs.noaa.gov/ (accessed May 4, 2017)) to obtain information about current implementation of Datums as related to spatial data applications and to plans for replacing both the NAD 83 and the NAVD 88 Datums. The revisions to material on geodetic Datums in this second edition are intended to reflect the impact of current practice on applications to the GSDM. However, due to the fluid nature of evolving practice and policy, it is impossible to make an unequivocal reading of the “crystal ball.” Nonetheless, the anticipation is that the GSDM will be compatible with implementation of the 2022 geodetic Datums. A datum is a reference to which other values are related. In surveying, a vertical datum could be as simple as an arbitrary bench mark assigned a height of 100.000 meters (or feet, etc.). A horizontal datum could be defined by a stake pounded in the ground for a Point of Beginning (P.O.B.) and assigned arbitrary coordinates such as east = 5,000.000 meters, north = 10,000.000 meters. If both horizontal and vertical values are assigned to the same point, the result could be called a 3-D datum. The definition of units of measurement, orientation of azimuth, and coordinate system are all implicit in such a definition. Another important implicit assumption is that horizontal is perpendicular to the local plumb line or, as is the case with the GSDM, horizontal * is taken to be perpendicular to the ellipsoid normal through the standpoint or through the P.O.B. as defined in Chapter 1. Presumably, rectangular Cartesian coordinates, either 2-D or 3-D, are used to describe the location of all points with respect to the datum origin and with respect to each other
eBook - ePubGeometric Dimensioning and Tolerancing
Applications and Techniques for Use in Design: Manufacturing, and Inspection
- James D. Meadows(Author)
- 2017(Publication Date)
- Routledge(Publisher)
Many skills that are essential to communicating between job categories are not taught in colleges and universities across the nation. Worse, they are often not taught--and are even discouraged--on the job. It seems hard to believe, in this day, that the concept of teamwork is often paid only lip service. But, still, it is sadly true. For this reason, people recall with great fondness the days when white collar and engineering types were required to spend time on the shop floor. Sometimes training courses can help to bridge that gap. Training that brings people of different technical disciplines together to learn a technical skill can have a powerful side effect. Parts become more functional and producible. Design drawings don’t ask for the impossible. Datum features are accessible and repeatable. Costs of part production and inspection are reduced.One must remember that Datums are imaginary perfect points, axes or planes established from actual part features. These perfect geometric references act as origins of measurement. The datum features are simulated with manufacturing and inspection equipment such as machine tables, chucks, surface plates, angle plates and vee-blocks. These datum feature simulators are used because they are as close to perfect as has been made or has been deemed cost effective.Datum features are chosen based on part functionality and their interrelationships with mating features. They are used to tie a part down. A free-floating part has 6 degrees of freedom: 3 are rotational degrees of freedom and 3 are linear. A datum reference frame consists of three mutually perpendicular planes. Once a part is locked in a simulated datum reference frame, all 6 degrees of freedom are eliminated. For planar surfaces:- The primary datum plane
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