In our on going series of covering the CMM basics we need to cover the all too familiar DFR, or the Datum Reference Frame. We see this DFR associated with a feature on the part but what does it mean? and how does it change my approach to programming this part?  We will attempt to answer these questions. This won’t be an attempt to address the overall make up of the DFR but we will look at the datum order and how that affects our programming

The Datum Order

In the above example, reading in the order that they are listed, we see that C is the primary datum. Datum A is secondary. The datum order plays a key role in revealing what the engineer is looking for. This shows the features of constraint and what priority each one has.  Here is the simple part we will be working with. We will concentrate on the 10mm diameter.

Again, this is a simple part but the principle applies to every feature that carries a DFR. Let’s look at Datum A it was established off Datums C, B, and D. The 10mm hole is called back to datums C and A. So why is C listed before A? And why does it matter? Datum A is a cylinder through the part and as a cylinder it has a 3d axis line. Meaning it can be used to create a new spatial orientation, or base plane. Your software may use a different name but the effect is the same. If you reoriented on Datum A if would take Datum A’s perpendicularity and form into the orientation and thereby effecting the 10mm diameter location. The engineer wants Datum C to be the primary datum. It might be a mounting surface or an assembly face, whatever the reason the engineer wants Datum C to be the surface the spatial orientation is on. Datum A is then used as the X, Y origin from which to locate the 10mm diameter.

With only two datums listed what about the axial alignment? As shown above Datum C is the spatial rotation, Datum A is the origin, the axial rotation is left to the rotation that set up Datum A since no realignment is called for.

  Datum A listed first

If Datum A was listed first then you would use Datum A to reorientate the part. Datum C would control the Z depth. In this case Datum A might have a fastener running through it and Datum C hangs out in space in the assembly and plays no crucial role other than to control depth.
 
Conclusion

The datum order is important to understand. It lists the order of datum constraints. You must first constrain the primary then the secondary datums, and tertiary if shown. Care needs to be given to ensure the part is inspected correctly and using the correct datum structure will guarantee you’ve written the program to meet the engineer’s needs.