![]() Often, the first thing to do is determine which Datum Features will be used to locate and orient the part. When parts are symmetrical or have identical features that make identification of Datum Features difficult, the Datum Features should be physically identified. They must be easily identifiable on the part. Surfaces of sufficient sizes to allow repeatable measurementsĭatums are important and care must be taken when selecting them. When selecting Datums, designers should consider the following characteristics: You will usually wind up choosing those Datum featues that determine orientation, location, and functional relationships of part features and their mating parts. Datum Features (and their corresponding Datums) are selected on the basis of design requirements and functional relationships. It's easiest to think in terms of Datum Features on your parts when deciding which Datums to select. Machine Table, Surface Plate, Angle Plate, etc. Whatever setup you will use is going to include appropriate Datum Feature Simulators. Okay, one more way to think of a Datum Feature Simulator-think about how you would inspect a dimension or tolerance called out by GD&T. These physical devices can all have Datum Feature Simulators that are relevant to our discussions and knowledge of GD&T.Īnother way to think of a Datum Feature Simulator is that they are inverses of Datum Features. We might be talking about some metrology or measuring equipment-for example a granite surface plate or a Go-NoGo Gage. We might be talking about a feature on the CNC machine, such as the table on a VMC or spindle axis on a CNC Lathe. Let's make this easy-a Datum Feature Simulator is a real physical feature of the machinery that comes in contact with the part. When I first came across the term, my reaction was, "What an odd term-Datum Feature Simulators?" In fact, the term that was used before Datum Feature Simulator was added to the standard in 2009 was, "True Geometric Counterpart," which is only slightly less odd.Ī granite surface plate is a typical Datum Feature Simulator for planes. Now it's time to talk about Datum Feature Simulators. We learned that a Datum is an abstract geometric concept and that a Datum Feature is an actual feature on a part that corresponds. In the previous article, we looked at Datums-basic definitions, how they're shown on drawings, and so on. This tip was originally released in September 1998.CNCCookbook Beginner's Guide to GD&T: Datum Feature Simulators and ![]() This tip is in accordance with ASME Y14.5M-1994. Ask yourself if you need to know where this feature is relative to the primary and secondary datum features. Such cases are good to review with inspection prior to releasing the drawing.ĭepending on the shape of the part, the tertiary datum feature may require a location control (position or profile) relative to the primary and secondary datums, rather than perpendicularity. This ensures the part meets the functional requirements for fit. One solution is to add a note to the perpendicularity control that requires the inner boundary to be inspected instead of measuring the angle of the axis. The short length of the feature makes it difficult to reliably measure the perpendicularity of the axis. The same thing often occurs with thin parts, like sheet metal parts and circuit cards. One common case that presents a challenge for inspection is when the height of the secondary datum feature is very small, like the hole feature, datum B, in the following figure. For most parts the following flow chart should be helpful.Īlthough this flowchart won’t work all of the time, it does work on most parts. ![]() The 1994 standard, in section 4.3.3, states "Consideration shall be given to controlling the desired accuracy of the datum features by applying appropriate geometric tolerances." In spite of this, nearly all of the figures in the Standard omit these geometric tolerances. These datum features should be geometrically controlled. Many people seem to miss the point that there is a difference between a datum, that is theoretical, and a datum feature, that is an actual feature on a part. In addition to controlling the primary datum feature, secondary and tertiary datum features should be controlled with respect to the higher order datums. Tip #11 pointed out the importance of applying flatness to a primary datum feature that is a plane surface, if datum targets or a constraint note are not used.
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