How are datums used?
Symbol for a datum
A primary datum is the main locating surface used for alignment. Think of a box set on a table. In this case the bottom of the box is the primary datum. The table would be used to simulate the primary datum. In this example measurements could be taken from the table to other features of the box to verify they meet the required dimensions. The primary datum controls one axis of freedom. The box can move back and forth or spin in place on the table but it can not turn end over end. This is similar to the way a granite surface plate is used in machine shops all over the world.
What is a datum target?
A datum target is a specified location that is used to measure a part. It identifies the point(s) from which measurements will be taken. The use of datum targets or points provides a level of control over the datum surface. Some surfaces are incapable of being used like a normal datum. A very large datum surface or a surface that has a lot of variation in its form are two common reasons why datum targets are used. Datum targets are also used to mimic real world use such as contact points in an assembly.
Datum target symbol
The top half of the datum target symbol is often empty. The upper half lists the area of the datum target when specified. The lower half of the datum target symbol lists the datum target name.
Is a datum real or theoretical?
True position callout referencing datums A & B
Perpendicularity callout referencing datum A
Circular runout referencing datum A
Datum dimensioning vs chain dimensioning
Chain dimensioning is the process of dimensioning features of one another in a row. Datum dimensioning is used when features are referenced from a common point. Take a look at the differences in the examples below.
A disadvantage of chain dimensioning is that the tolerances stack up. This means that the location of the far right hole can actually vary by as much as +/- 0.040”. The example which uses datum dimensioning maintains the +/- 0.010” tolerance for each location. Chain dimensioning can be a perfectly acceptable way to tolerance your parts, just make sure you have taken into account the additional tolerance that can apply to the feature.