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After taking non-Tec-Ease, Inc. GD&T training, I find that most people remain confused regarding datums. This leads to incorrect drawings, improper set-ups and meaningless inspection reports. 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.
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. There were drawings of a milling machine and a lathe that contain the datum reference frame.
Think of the part as a blob of material. 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. This is done by selecting features on the part as datum features. A datum feature is something you can touch. These features are "named" using letters of the alphabet. Datum features are not datums. 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. When you refer to A, B, C, etc., you are referring to the features on the part. 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.
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. 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. If you are responsible for GD&T training, make certain the trainers and materials use the correct terms to avoid confusing your folks.