Useful for interfaces where form is not required.Īppears in the 2009 version of the standard, and refers to unequal profile distribution. Provides bonus tolerance only for a feature of size. Useful to maintain minimum wall thickness. is the default for ISO tolerancing.įor ANSI/ASME tolerancing this has to be specified. Figure 3-1 This section also introduces the feature control frame concept. Many additional symbols are listed and described in this section, including symbols for datums, modifiers, counterbores, countersinks, tapers, and others. This principle sets no limits to the number of errors of form possessed by individual features of a work piece. The table below shows symbols for the geometrical characteristics. Likewise a internal features minimum value defines a cylinder that has to fit inside the hole.Īpplies only when part is otherwise restrained. 'Datum Identification frame (Symbol) - The datum identification frame (symbol) contains the datum reference letter in a rectangular box, usually preceded and followed by a dash (any letter except I, O, or Q).' To my knowledge, ASME Y14.5M-1994 says that a datum feature symbol is A with a 60deg triangle for a 'arrowhead'. The upper value of a external feature defines a virtual cylinder that the entire feature has to fit inside. This symbol is used to identify a group of features when there is a requirement that they be teated geometrically as a single feature.įeatures identified as statistical toleranced should be produced with Statistical Process Control (SPC). We will discuss more involved applications in the coming posts.If no modifier follows a datum feature size, the datum feature applies regardless of material boundary. This blog post showed just a simple example of a datum reference frame and the related specification of tolerances for educative purposes. The Datum reference system itself is formed by the intersection of three datum planes A-B-C (in blue). The tolerance zones (profile and position – in red) are defined with respect to datum reference frame (DRF – at the bottom-left corner) through basic dimensions. The most important reference feature is usually identified with a capital A. True The datum feature symbol can be placed on the outline of a feature surface in the view where the surface appears as an edge. Symbol for a datum Datum names Datum names are identified with a capital letter. True Each datum feature on a part requiring identification must be assigned a different datum identification letter. The same figure as Figure 3, without the part itself. The datum feature symbol identifies a physical feature of an object. The distance between these two planes is 5 mm.įigure 3. Similarly, the upper surface of the part should be between the two planes defining the profile tolerance zone located by a basic dimension of 100 mm. This zone is defined by a cylinder of 5mm diameter. This means that once the part is immobilized through 3 datum planes, the center axes of the holes should be inside these tolerance zones located by 4 vertical and horizontal basic dimensions, 75-25 and 40-120 mm, respectively. Using this datum reference system as the common origin, the basic dimensions (the dimensions inside rectangular boxes) define the tolerance zones (in red) for position and profile tolerances. In Figure 2, the datum reference frame at the bottom-left corner is defined by the three orthogonal datum planes, A, B and C (in blue). The Datum reference system itself is formed by the intersection of three datum planes A-B-C (in blue) Section 6.4 DATUM FEATURE SYMBOLS The datum feature symbol has been changed in this latest revision of ANSI Y14.5 ( from the previous symbology in ANSI. The tolerance zones (profile and position – in red) are defined with respect to the datum reference frame (DRF – at the bottom-left corner) through basic dimensions. The part defines position and profile tolerances with respect to a datum reference frame through basic dimensions.įigure 2. Without this common coordinate system, product definition is unclear, rendering the inspection results unreliable.įigure 1 shows a visual representation of this concept. The table below shows symbols for the geometrical characteristics. It is the common coordinate system of all tolerance zones. survey station selection, field work procedure and conventional symbols. The main function of the datum reference frame is to specify a foundation for the inspection of the part. find the elevations of given points with respect to given or assumed datum. Is the condition of a surface, axis, or centerplane, which is at a specified angle from a datum plane or axis. The tolerance zones are defined with respect to the datum reference frame (DRF – at the bottom-left corner) through basic dimensions.Ī datum reference frame is a coordinate system against which the geometric dimensions and tolerances of a part are defined.
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