It only applies when all three plaines are used. The 3-2-1 rule defines the minimum number of points of contact required for a part datum feature with its primary, secondary, and tertiary datum planes. In practice, we must eliminate 6 degrees of freedom before we can fully locate and orient a part within a Datum Reference Frame:Ĭontrolling 6 degrees of freedom means controlling 3 linear distances from Datum planes to establish an X, Y, and Z position and controlling 3 rotary positions to orient the part at that position. We refer to the translational degrees of freedom as X, Y, and Z and the rotational degrees as u, v, and w. In GD&T they are used to orient and locate tolerance zones.ĭatum Reference Frames and 6 Degrees of FreedomĮvery Datum exists within the context of some Datum Reference Frame. You may also come across the term Datum Target, which is defined as, “A specified point, line, or area on a part used to establish a datum.” Definition of a Datum Reference FrameĪ Datum Reference Frame is a coordinate system, and preferably it is a Cartesian coordinate system.Ĭoordinate systems are valuable because they’re used to locate objects. The standard itself defines a Datum Feature as, “An actual feature of a part that is used to establish a datum.” Edges on parts are wavy and are not perfect lines. Surfaces on parts are not perfectly flat planes. It is because real features are never perfect abstract geometrical concepts. The reason they’re not the same is perhaps small, but significant. When we refer to the tangible feature on the part that the Datum is associated with, we call it a Datum Feature. They correspond to points, lines, and planes. Datums are abstract geometrical concepts. Datums vs Datum Features and Datum TargetsĪt this point it is important to talk about the difference between a Datum and a Datum Feature–they are not the same though it is tempting to refer to them as such at first. In addition, the datum symbol could be shown on a side view by denoting the axis. – It can be placed on the Feature Control Frame for the hole.

– It can be placed on the leader pointing to the hole as it is in the middle case.

It is of course referring to the axis and not the surface of the hole. – It can be placed directly on the hole, as in the leftmost case. Referring to the example above, the “A” Datum can appear in these three different places on a drawing: The “A” Datum can appear in these three different places on a drawing… To establish a datum axis on a feature such as a hole, there are a number of ways to place the datum symbol: Placing the Datum on an axis is common with GD&T symbols that can have axis control like runout, perpendicularity, or concentricity. Remember: this places the Datum on the central axis through the feature, not the surface of the feature. Place the datum symbol on the dimension of a diametric tolerance for Axis control: However, for a round cylinder, the datum is the entire round surface. For any surface other than a round cylinder, the datum is strictly on the side where the symbol is shown. We can place the datum symbol either on the surface or on one extension line from the surface. Two examples of Datum Sybols on a Surface Let’s go through each one to see how it looks: Datum Symbol On DrawingsĪs we said in the definition, Datums are planes, axes, or point locations, and that’s exactly how they show up on a drawing. Nearly every GD&T symbol except for form tolerances ( straightness, flatness, circularity, and cylindricity) can use datums to help specify the geometric control that is needed on the part. Typically, multiple features will be referenced by each datum, so they’re a very important part of the whole thing. A Datum is a plane, axis, or point location that GD&T dimensional tolerances are referenced to.

Primary datum