![]() See the image below to see a reference frame with a rectangular cube arranged in it. First, we take the brick and set it in our datum reference frame and position it so it conforms to the engineer’s needs. That means we have to apply the 3, 2, 1 rule to it. Now if we want to determine if the real world brick is true to what the engineer wants we have to measure the real world item in a perfect world frame of reference. The second image is the real world version of the brick, one that’s made from clay in a crude mold and fired in an oven to make it hard. This first image shows a drawing of the brick and because it’s presently an idea we can call it perfect. To get an idea of how this is accomplished we need to have a part that gives us a taste of what a real-world part looks like when it’s adapted to a perfect world datum reference frame. That rule assumes that any real-world part will first need to be landed or fixed in a datum reference frame arrangement in order to measure how that part’s features conform to their requirements. To prove this point our Y14.5 standard describes the 3, 2, 1 rule. For this reason when a datum is defined by the design or drawing it is one thing and when it becomes a feature on a real-world part it is something else. ![]() This statement can be argued with because practicality sometimes rules but the truth is that the closer we look, the closer we inspect a real-world feature, the more differences we’ll find. I’ll borrow a comparison to say actual part features are like snowflakes, no two are alike. That uniqueness makes each part, as the term implies, different. Like some other things we’ve looked at an actual part feature has uniqueness. In the case where a feature is used as a datum, it is called a datum feature. All feature definitions must have applied tolerances and those tolerances always manifest as imperfect features. However when that frame is adapted to a real-world part the story changes. A datum reference frame is composed of three mutually perpendicular planes, both perfectly perpendicular with each other and each perfectly planar. Datums in the perfect world are defined as perfect things. Datums, as we’ve seen with some of our other terms, have different definitions in the perfect world and real world. Datums provide engineers and countless manufacturing technicians a starting point or reference, fixed on a feature or features of a part, that is used to create and orient the 3-dimensional space in which that part exists. Accomplishing the definitions of both the geometry and tolerancing requires a fairly complex set of rules and procedures which include the very fundamentally indispensable use of datums.
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