《ISO 5459 - 3》.pdfVIP

German Comment on ISO CD 5459-3 ISO/TC 213 N 443 ISO/TC 213/WG 2 N 158 The aim of datum targets is to define a set of small supports of theretically exact form, orientation and location relative to each other on which the workpiece can rest unique in orientation and location without any further constraint. Orientation and location of tolerance zones are related to the set of datum targets. In order to keep the workpiece in a unique orientation and location the number and the location of the datum targets must be chosen according to the degrees of freedom of the workpiece. A realisation of the datum targets may be small supports in a manufacturing or inspection device. Clause 5 of the CD does not meet this philosophy. 5.1 requires filtration, 5.3 requires that the associated ideal feature is of the same invariance class as the corresponding non-ideal feature. For the use of datum targets, there shall be no filtration. The selection of datum targets is a kind of filtration in itself. The associated feature, e.g. 2 points in a plane associated to a cylindrical feature as secondary datum, see figures 3 and 4, and the datum feature may be of different invariance classes (plane and cylinder in figure 3 and 4). For clause 5 of ISO 5459-3 we propose the following content: ISO 5459-3 Proposal 5 Datum targets 5.1 General A datum target is a point or straight line or limited small area of a plane on which the workpiece feature can rest. Datum targets can be used to establish a single datum or a common datum or a datum system. Datum target systems define a set of datum targets (small supports) of theretically exact orientation and location relative to each other on which the workpiece can rest unique in orientation and location without any further constraint. Orientation and location of tolerance zones are related to the set of datum targets. The relationship between the datum targets is given by theoretically exact dimensions. The r