Datums referenced on a drawing using Geometric Dimensioning and Tolerancing can be really confusing, even on simple parts! When you get right down to it Datums are not that difficult to understand when you consider what they are used for. I have received several questions about Datums, why and how they are used and why use more than one. I'm going to answer those questions using a simple part that has one hole and has three Datums referenced. But before I go into the explanation about multiple Datums I recommend that you read THIS post I wrote about a parts size and how the dimensions tolerances determine what a part can actually look like. For a quick review there are THESE posts that I wrote about Geometric Dimensioning and Tolerancing Basic concepts. This post is a quick overview of the basic concepts of using multiple Datums. There is a lot more to Datums than what I'm writing about here and I'll get into those more complicated concepts later!
Lets start with a simple drawing of a part that has one hole in it and the hole is positioned in relation to three Datums.
|Simple Part with Multiple Datums|
The drawing above has a hole in it that is True Position to three Datums in this order: A B C. The reference to Datum A is holding the location and orientation of the center axis of the hole to Datum A (bottom surface). Take a look at THIS picture from an old post if that doesn't make sense. Another way to look at this is Datum A is the surface that someone is going to measure the perpendicularity of the hole to. So Datum A in this case is a place to start taking measurements. We'll start taking measurements with Datum A because it's the first Datum and we will continue measuring the part with the remaining two Datums in order from left to right. The Datums don't have to go in alphabetical order, they are in used in the order that you reference them when you measure the part. The important thing to remember is Datums are used to measure a part.
So what are the other two Datums doing? I'll explain the below... ;-)