![]() In the actual production, the supplier inspector didn’t have definitive instructions on how to hold the part. A centerline is theoretical and intangible. The real problem with this part is the datum label attached to the centerline on the customer drawing, because it didn’t specify which tangible feature would serve as the datum feature to inspect the tight tolerance. Figure 1 is a simplified illustration of the part.Ī tight tolerance is fine as long as the product function justifies it. The original part by a customer was a lens barrel in a space telescope on which the opening at the left interfaced with a lens, which is why the total runout tolerance was controlled tightly at 0.0006 inch. This actually turned out to be a million-dollar problem. (Image courtesy of a Tec-Ease GD&T tip video.) We can begin by asking the question: Do you see any problem with the GD&T definition in Figure 1? In the figure, datum symbol A is attached to a centerline and then is referenced in a total runout tolerance.įigure 1. So let’s look into this important topic in this article. ![]() Datum features provide similar references in a feature control frame. When we locate a physical address on a map, we reference the country, state, city and street of the address. Datum features are the foundation of composing and interpreting geometric dimensioning and tolerancing(GD&T) definitions. This SOLIDWORKS MBD forum post provides a recent example. If you are responsible for GD&T training, make certain the trainers and materials use the correct terms to avoid confusing your folks.A frequent question about 3D annotations is how a datum symbol can be attached to a centerline, an axis or a middle plane of solid bodies. 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. This is done by selecting features on the part as datum features. 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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