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Datums
Published in James D. Meadows, Geometric Dimensioning and Tolerancing, 2017
The establishment of datum reference frames has come under close scrutiny recently by members of the ANSI and measurement communities. The Dimensioning and Tolerancing Standard (ANSI Y14.5) makes statements to the effect that: When magnified, flat surfaces of manufactured parts are seen to have irregularities; contact is made with a datum plane at a number of surface extremities or high points.Therefore, it is necessary to establish a method for simulating the theoretical reference frame from the actual features of the part.Planes are simulated in a mutually perpendicular relationship to provide direction as well as the origin for related dimensions and measurements.A datum feature is selected on the basis of its geometric relationship to the toleranced feature and the requirements of the design. To ensure proper part interface and assembly, corresponding features of mating parts are also selected as datum features whenever possible.If not sufficiently accurate, datum features from which datum planes and datum axes are formed may need to be controlled by specifying appropriate geometric tolerances. Where control of the entire feature becomes impractical, use of datum targets may be considered.But, if datum targets are not used, datum planes and axes are established by datum feature surface extremities or high points. For example: a planar primary datum feature relates the part to the datum reference frame by bringing a minimum of 3 high points on the surface into contact with the first datum plane. The part is further related to the frame by bringing at least 2 high points of the secondary planar datum feature into contact with the second datum plane. The relationship is completed by bringing at least l high point of the tertiary planar datum feature into contact with the third datum plane.
Interoperability of CAD geometry and product manufacturing information for computer integrated manufacturing
Published in International Journal of Computer Integrated Manufacturing, 2020
Satchit Ramnath, Payam Haghighi, Adarsh Venkiteswaran, Jami J. Shah
The GD&T global model, Constraint Tolerance Feature Graph (Wu, Shah, and Davidson 2003) was originally designed to include information about nominal geometry (features), constraints (including dimensions, mating conditions, assembly constraints), tolerances (including datum reference frames), and degrees of freedom (DoFs) in textual format. This information forms the first four sections of the CTF file, following the ‘File name’, as shown in Figure 2. The first five section (A to E) of a CTF graph are summarized as follows:
STEP AP 242 file-based automatic tolerance analysis of mechanical assembly using unified Jacobian Torsor Model and direct linearization method
Published in International Journal of Computer Integrated Manufacturing, 2023
Bala Murugan S, R. Manu, Deepak Lawrence K
The entity of geometric tolerance is found using ‘name of geometric characteristic‘+_TOLERANCE (i.e. PARALLELISM_TOLERANCE). If the geometric tolerance has datum reference, then it has 3 sub-entities. If the geometric tolerance is without datum reference, then it has 2 sub-entities. The tolerance value is stored under the entity LENGTH_MEASURE_WITH_UNIT, and the datum is stored under the entity DATUM. Let us consider the below line taken from the STEP file #1615=PARALLELISM_TOLERANCE(“Position1@Plane2”’,’,#1614,#1605,(#1613));