Imagine the designer defines the function of a component, the machinist knows what to pay attention to during production to make the part work and the quality engineer knows how to inspect the component. In this article you will learn why it is important that everyone has the same understanding of what finished parts should look like and how to use this method with MBD in Inventor. GD&T is an excellent tool for defining parts functionally.

Limits of dimensioning and tolerancing

The traditional way of dimensioning and tolerancing is limited if you want to ensure the part you design will fit in the assembly after manufacturing.
Let us have a look at the following example. Figure 1 shows a simple sheet metal part with the tolerance ±0,3 as it would like in the CAD-model.

Figure 1

Figure 2 shows the part as it could look like in reality after manufacturing.

Figure 2

Figure 3, Figure 4 and Figure 5 show how the component shown in Figure 2 can be measured. Surprisingly you have three different results with the same part. How can it be that all three figures are correct?

Figure 3

Figure 4

Figure 5

The dimension 20±0,3 must be fulfilled, but nowhere is it described how to measure this distance. In Figure 3 the part is aligned to the long side. In Figure 4 the part is aligned to the short side. In Figure 5 the part is aligned to best fit. Shape and position deviation are evaluated independently from each other.
This example might be a bit exaggerated, but it shows very clearly the challenge when exclusively using the traditional way of dimensioning and tolerancing. And remember, you still do not have any idea what the function of this part could be.
If you would use GD&T as shown in Figure 6 you would immediately know a lot more about this sheet metal part. It seems to be important that the two sides are nearly parallel, and it is probably fixed at the short side.

Figure 6

GD&T contains an exact instruction how to measure and eases work for quality control or inspection. First you must make sure your datum reference “A” fulfills the criteria of flatness (0.2 mm), then align the part on the short side and now you are ready to check if the long side is within the required tolerance zone (see blue zone in Figure 7).

Figure 7

If you describe your part with GD&T you would never get three so different measuring results as shown above with the traditional way of tolerancing.

How to use GD&T
GD&T is a symbolic language that explicitly describes nominal geometry and its allowable variation. This way of communication promotes a uniform understanding and interpretation among all involved persons, including a description how to measure the deviation.
To identify the deviation correctly it is important to describe how to position the part for measurement. This happens via datums. For making sure the positioning is repeatable the datums should fulfil a predefined quality.

MBD in Inventor supports you to define correct GD&T (see Figure 8). Let us assume you want to make sure that the long side is parallel to the short side within a defined tolerance. As first you define the short side as datum A. It is important to give every datum a profile tolerance, in our case a flatness of 0.2 mm. In the next step you give a positioning tolerance (0.6 mm to datum A) on the long side. To complete your annotation, you add the basic, nominal dimension 20 mm.
Figure 8

Learn how to define a complete tolerancing concept in your assembly in Part 2 of this article.

Learn more about Model Based Definition (MBD) in Inventor