11/27/2022 0 Comments Datum in drawingIt appears that in your case the designer has used both approaches which I would personally just put down to bad practice. However, it then goes on to say that the drawing can be simplified by only referencing a single letter or by referencing the entire group as B-B. The standard says that for common datums there should be as many datum references, separated by hyphens, as there are features, so in this case there should be B-B-B-B-B. DATUM IN DRAWING ISOAccording to ISO 5459-2011, your datum B callout is defining the pattern of 20xØ13.5 holes as datum B. You need to get clarification from whoever issued the drawing but I believe in this case B and B-B mean the same thing. You could have a cylinder and a plane or, as in this case, a pattern of cylinders and there are many other examples also. Although two concentric features combining to form an axis of revolution would be one allowable case, it is not the only valid application. A common datum is a collection of features that, when combined form any of the allowable invariant classes. If you are responsible for GD&T training, make certain the trainers and materials use the correct terms to avoid confusing your folks.B-B means that it is a common datum. They form the important functional feature in a drawing having said that not all form of tolerance depend on it, namely circularity, flatness, straightness and. It acts as a reference on that drawing object so that relative dimension tolerances included in Feature Control Frames can refer to this and other placed datum. When you refer to the datum planes in the datum reference frame, it is better to refer to them as the 1st, 2nd and 3rd planes of the datum reference frame. So, whenever you refer to the datum feature labels such as A, B or C, keep in mind that you are referring to the datum features on the part that are not perfect features. There have been countless disagreements and even lawsuits where the datum feature labels like A, B and C have been used on the drawing to label centerlines and theoretical planes. When you refer to A, B, C, etc., you are referring to the features on the part. Processing equipment such as gage pins, angle plates, surface plates or calculated planes and axes (in the case of automated measuring machines) use the datum features on the part to line the part up with the datum planes that exist in the processing equipment. These features are "named" using letters of the alphabet. A datum feature is something you can touch. Datum features are real, tangible features on a part and are usually important functional surfaces. This reference system is called a Datum Reference Frame (DRF). This is done by selecting features on the part as datum features. Datums are used in GD&T drawings to create a reference system for inspecting a manufactured part. In order to inspect this blob, it must be possible to repeatedly and reproducibly align the part to the origins of measurement in the processing equipment. There were drawings of a milling machine and a lathe that contain the datum reference frame. Way back in the 1966 Y14.5 standard it was made clear that datums are theoretical and are assumed to exist in the processing equipment. How many folks will point to a feature on a part and refer to it as a datum? It isn’t! This Tip is intended to clear the air on datums. This leads to incorrect drawings, improper set-ups and meaningless inspection reports. GD&T training, I find that most people remain confused regarding datums.
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