STANDARD. ISO. Second edition. Geometrical product specifications. (GPS) — Geometrical tolerancing —. Datums and datum systems. ISO is a geometrical product specification (GPS) standard and is to be regarded as a general GPS standard (see ISO/TR ). It influences the chain . ISO specifies terminology, rules and methodology for the indication and understanding of datums and datum systems in technical product.
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by ISO Geometrical Product Specifications (GPS). Yiqing Yan Terminologies and examples from ISO datums and datum systems. Abstract. In ISO the problem of datums and datum-systems established by cylindrical features is described graphically. The figures. tolerancing - Datums and datum systems (ISO ). Spécification géométrique systèmes de références spécifiées (ISO ). Geometrische .. full version online. I.S. EN ISO EN: COMBINED PDF.
The orientational deviation includes the form deviation Fig. If not otherwise specified orientational deviations are assessed over the entire feature. Orientational deviations are Properties of the surface 3 Fig. They are originated also by erroneous fixture of the workpiece after remounting on the machine tool.
Locational deviation is the deviation of a feature surface, line, point from its nominal location. The location is related to one or more other datum feature s. The locational deviation includes also the form deviation and the orientational deviation of the surface, axis, or median face Fig. If not otherwise specified locational deviations are assessed over the entire feature. Locational deviations are originated similarly as size, form and orientational deviations.
Waviness is mostly more or less periodic irregularities of a workpiece surface with spacings greater than the spacings of its roughness DIN The ratio between spacing and depth of the waviness is in general between and The waviness is assessed from one or more representative parts of the surface Fig. Waviness will not be dealt with in the following.
Roughness is periodic or non-periodic irregularities of a workpiece surface with small spacings inherent of the forming process. The roughness is assessed from one or more representative parts of the surface Fig. Due to the cutting process tear chip, shear chip the print will be modified. Other origins are deformations from blasting, gemmation with galvanizing, crystallization, and chemical effects mordants, corrosion.
Roughness ranges down to the crystal structure DIN Roughness will not be dealt with in the following. Surface discontinuity is an isolated imperfection of the surface like a crack, pore or lap.
In general it is not taken into account when assessing deviations of size, form, orientation, location, waviness and roughness. The definitions and sizes and permissibilities of surface discontinuities have to be dealt with separately.
At this time there are very few standards on this subject available, e. Surface discontinuities will not be dealt with in the following. Edge deviations are deviations of the workpiece edge zone from the geometrical ideal shape like burr or abraded edges instead of sharp edges. ISO 13 defines tolerances for edges and gives the drawing indications. They are dealt with in clause 22 of this book. The classification of surface irregularities as described above is useful for the following reasons: a The different kinds of surface irregularities have different origins in the manufacturing process.
In order to control the manufacturing process these kinds must be assessed separately. For example, on raceways of ball bearings, waviness has a great influence on lifetime and noise while roughness has little influence. In order to specify the permissible function-related deviations, the different kinds must be specified separately.
The ratio between spacing and depth of the irregularities also varies greatly. The smallest ratio between spacing and depth occurs with cracks and is in general smaller than Whereas the ratio between spacing and depth with form deviations is in general greater than Because of these wide ranges the requirements for measuring devices and for diagrams are quite different. For the assessment of different kinds of the irregularities deviations different kinds of measuring instruments with different magnifications and different profile diagrams with different ratios of horizontal to vertical magnifications are used.
The definitions of the different kinds of irregularities deviations are rather uncertain. There are no distinct borderlines. Therefore it was discussed in ISO to define borderlines in terms of defined spacing of irregularities or in terms of defined ratios between spacing and depths of irregularities or in terms of defined ratios between spacing of irregularities and feature lengths.
There is another distinction between micro and macro deviations.
Macro deviations are those, that can be assessed with usual measuring devices for the assessment of size, form, orientation and location e. Micro deviations are those that are assessed with roughness or waviness measuring instruments. Macro deviations are assessed over the entire feature length; micro deviations are assessed from a representative part of the surface.
Also, there is no distinct borderline because sometimes parts of the waviness will contribute to the result of the measured macro deviations and sometimes parts of the form deviations will contribute to the result of the measured micro deviations waviness. Figure 1. Workpieces always have deviations of size, form, orientation and location. When these deviations are too large the usability of the workpiece for its purpose will be impaired.
When during manufacturing attempts are made to keep these deviations as small as possible, in order to avoid the impairment of usability, in general the production is too expensive and the product is hard to sell. In general, competition forces the use of all possibilities for economic production, including possibilities, arising from current developments.
Therefore, it is necessary that the drawing tolerances define the workpiece completely, i. Only then is the manufacturer able to choose the most economic production method, e. Only completely toleranced drawings enable the production of workpieces to be as precise as necessary and as economic as possible.
This is necessary for competition. When all tolerances necessary to define the workpiece completely are indicated individually the drawing becomes overloaded with indications and is hard to read. Therefore general tolerances should be applied. General tolerances shall be equal to or larger than the customary workshop accuracy. The customary workshop accuracy is equal to those tolerances the workshop does not exceed with normal effort using normal workshop machinery.
Larger tolerances bring no gain in manufacturing economy. The normal workshop accuracy depends on the workshop machinery that produces the largest deviations disregarding exceptions which are to be dealt with in certain cases.
The customary workshop accuracy is in general the same within one field of industry.
For example, the customary workshop accuracy for material removal in the machine-building industry corresponds to the general tolerances ISO mH. The general tolerances shall be applied by an indication in or near the title box of the drawing. Tolerances that must be smaller have to be indicated individually, see Table 3. With the symbols of the left column of Table 3. The symbols of the right column cover special cases that occur very Table 3. Principles for geometrical tolerancing 9 Table 3.
These symbols are standardized in addition, and should be used in order to facilitate readability of drawings.
The proportions of the symbols for the drawing indications are standardized in ISO However, some CAD systems deviate from this standard. This may be tolerable as long as the drawing indications are unambiguous as demonstrated in this book. According to ISO , there are defined form tolerance zones within which all points of the feature must be contained.
Within this zone, the feature may have any form, if not otherwise specified. The tolerance value defines the width of this zone Fig. Form tolerances limit the deviations of a feature from its geometrical ideal line or surface form. Special cases of line forms with special symbols are straightness and roundness circularity Fig. Special cases of surface forms with special symbols are flatness planarity and cylindricity Fig. Orientation tolerance is the permitted maximum value of the orientation deviation see According to ISO , there are defined orientation tolerance zones within which all points of the feature must be contained.
A straight line and a plane 20 The common datum A-B is characterized by median straight line of the axes of two simultaneously associated Concepts and A B cylinders and of the plane containing the two axes of method these cylinders.
A plane The datum A is the median plane of the two plane tangents outside the material at a Concepts and distance of 10 which minimises the maximum method deviations. Flag for inappropriate content. Related titles. Jump to Page. Search inside document. Bikash Gaurav Bhuyan. Srinivasa Reddy Nallimilli. Lata Sharma.
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Siddharth Rajamohanan. Nicholas Teh. Faith Roundabout. Google Scholar. ASME Y ISO General terms and definitions Google Scholar. Indication in technical product documentation Google Scholar. Association methods for datums and datum systems for the assessment of geometrical tolerances Google Scholar. Metrological establishment of datums and datum systems for the assessment of geometrical tolerances Google Scholar.
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