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2024 | OriginalPaper | Chapter

6. Datums and Datum Systems

Author : Stefano Tornincasa

Published in: Technical Drawing for Product Design

Publisher: Springer International Publishing

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Abstract

A datum system has the purpose of defining a set of two or more ideal features established in a specific order (for example, a system made up of a triad of mutually orthogonal planes) that allows not only the tolerance zones to be orientated and located, but also their origin to be defined for the measurement, and the workpiece to be blocked during the control. When it is not desirable to use a complete integral feature to establish a datum feature, it is possible to indicate portions of the single feature (areas, lines or points) and their dimensions and locations using datum targets. This chapter illustrates the main differences between the ISO 5459 and ASME standards for the specification of datums and highlight some theoretical and mathematical concepts. This section also provides simple rules to follow whenever choosing functional datums to ensure the part will function as intended, with the least possible amount of variation.

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Footnotes
1
Constrained least squares simulator L2 (See Sect. 6.3).
 
2
Datums and datum systems are situation features, not coordinate systems. Coordinate systems can be built on datums.
 
3
The term “theoretical datum feature simulator” was replaced in the ASME Y14.5:2018 standard with the words “true geometric counterpart”.
 
4
A similar perfect feature counterpart of the smallest size that can be contracted about an external feature or of the largest size that can be expanded within an internal feature so that it coincides with the surface at the highest points.
 
5
ASME Maximum Material Boundary, MMB.
 
6
ASME Least Material Boundary, LMB.
 
7
ASME Regardless of the Material Boundary, RMB.
 
8
Crosses are not used in this case in the ASME standard.
 
9
In ISO 5459, the direction is only given by the moveable modifier, and not by the leader line.
 
Literature
1.
go back to reference Krulikowski A (1999) Advanced concepts of GD&T, textbook based on ASME Y14.5M—1994. Effective Training Inc. Krulikowski A (1999) Advanced concepts of GD&T, textbook based on ASME Y14.5M—1994. Effective Training Inc.
2.
go back to reference ISO 5459:2011 Geometrical product specifications (GPS)—geometrical tolerancing—Datums and datum systems ISO 5459:2011 Geometrical product specifications (GPS)—geometrical tolerancing—Datums and datum systems
3.
go back to reference ASME Y14.5-2018 Dimensioning and tolerancing. Engineering product definition and related documentation practices. American Society of Mechanical Engineers. New York, USA ASME Y14.5-2018 Dimensioning and tolerancing. Engineering product definition and related documentation practices. American Society of Mechanical Engineers. New York, USA
4.
go back to reference Krulikowski A (2010) Alex Krulikowski’s ISO geometrical tolerancing guide. Effective Training Inc. Krulikowski A (2010) Alex Krulikowski’s ISO geometrical tolerancing guide. Effective Training Inc.
5.
go back to reference Fischer BR (2009) The journeyman’s guide to geometric dimensioning and tolerancing: GD&T for the New Millennium. Advanced Dimensional Management Press Fischer BR (2009) The journeyman’s guide to geometric dimensioning and tolerancing: GD&T for the New Millennium. Advanced Dimensional Management Press
Metadata
Title
Datums and Datum Systems
Author
Stefano Tornincasa
Copyright Year
2024
DOI
https://doi.org/10.1007/978-3-031-51187-5_6

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