Datum reference frame is a point as the result of the intersection of all the datum planes/features represented by datum simulators (see figure 7 right). In figure 7 right, a datum reference frame is shown. The reason datum A should have the tightest tolerance and manufacturing accuracy is because in any manufacturing processes, such as machining and inspection processes, such as by using CMM, the surface of datum A on the part should touch the main table of the machine tool or the CMM. In addition, the tolerance of datum A should be the tightest among the other datums (in this case datum B and datum C) since datum A is the reference and each small deviation on the manufacturing of datum A will propagate and amplified to the deviation to other features. For datum A, since it is the main and first datum, only un-related GD&T tolerances applied as datum A, such as flatness, roundness and cylindricity. In figure 7 left above, datum A is applied to the back surface of the part because it has the largest surface area on the part. Figure 1: (Left) A datum system and (Right) a datum reference (datum reference frame).
In general, datum A commonly applied to the most stable feature on a part, such as the surface with the largest surface area or as the base of the part. A datum can be, for example, a surface or an axis from a symmetrical feature or other geometrical features.ĭatum A is the main datum on GD&T tolerances on a part and also the main reference for the other datums (in this case datum B and datum C) on the part. In figure 1 left below, a datum system containing three datums: datum A, datum B and datum C is shown. This datum reference is the main reference for all GD&T tolerance on a part or component.įigure 1 shows a datum system and datum reference/datum reference frame. These datums will intersect each other and the point of all the intersections is called datum reference (datum reference frame). This datum system is commonly called datum A, B and C (sometimes until D or E depending on tolerance situations and requirements).
DATUM IN DRAWING VERIFICATION
Datum and datum referenceĮspecially for GD&T tolerance with relations (for example orientation, location and run-out) that require datum, a datum system is necessary to be the reference point for all GD&T tolerance verification and comparison. This material condition concept can be a huge saving in the quality inspection process of parts.įinally, real examples showing the benefits of using MMC and LMC symbols in feature of size are presented at the end of this post. Material condition is a condition to explain how a larger or smaller tolerance values can be obtained when a feature of interest is made smaller or bigger that its nominal size. All feature positions will be based on this datum reference. Because, all GD&T conformance and non-conformance decisions are based on verifying features with respect to one or more tolerance zones.ĭatum reference provides the origin of coordinates of all GD&T tolerances on a part. Tolerance zone maybe the most fundamental building block of GD&T. The understanding of these three elements is essential to thoroughly understand GD&T. The elements are datum reference (datum reference frame), tolerance zone and material condition.Īll GD&T tolerances are built on top of all these three elements. This post explains some fundamental elements of GD&T.
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