Why is Geometric Dimensioning and Tolerancing (GD&T) important? (2/2)

Autodesk Support

Aug 9, 2019


Let’s have a look on another example. For describing a part according to its function, it is always important to see it in the context of the complete assembly (see Figure 9). In our case it is a clamp on a robot arm. We will define the adjustable clamp head in the center (see Figure 10).


Figure 9

 
Figure 10: adjustable clamp head

What is the function of that adjustable clamp head (Figure 10)? It is fixed at the mounting ring and brings the adjustable clamp finger at its correct position. Well, now that we've clarified the function, how do we start translating it to GD&T?

  1.  Where is the head mounted at the robot?
    It is the contact surface to the mounting ring. 
    So, let’s define this surface as primary datum A. To ensure that the part is in clean contact with its adjacent part, the surface must be of a certain quality. Here in our example a surface quality of Rz 16 (common value for contact surfaces without any further requirements) and a flatness of 0.1 mm. We choose 0.1 because it is the half of 0.2 (see below at secondary datum)
  2. How is the head mounted at the robot?
    It is fixed with six screws.
    Hole patterns should always be labelled with a position tolerance.
  3. How to make sure the finger is in the correct position and fits in the groove?
    The groove should be in the middle of our part. 
    1. What is the middle of our part?
      It is the center of the outer diameter 160. So, this is our secondary datum. To make sure that our secondary datum is not somewhere, we define it perpendicular to the primary datum A. The value 0.2 mm is a common value regarding to the general tolerances for metal-cutting production.
    2. How to make sure the finger slides in the groove with minimal clearance?
      If two components are to be joined exactly, a fit is required. Here in our case we define a horizontal as well as a vertical clearance fit based on the system hole basis fit H7/f8. To ensure that the shape and dimensional deviations are considered together, fits should be provided with the envelope principal.
      The remaining surfaces of the groove must not interfere with the fitting clearance and must be tolerated accordingly.
  4. How is the finger fixed in the groove?
    It is fixed with two screws each.
    Hole patterns should always be labeled with a position tolerance.
  5. What other contact surfaces does the head have with adjacent components?
    We continue with the same principal define our requirements for the component according to its function. 

You see, with answering a few simple questions you completely define a part with GD&T. And Inventor MBD supports you in defining your requirements according to current standards as you can see in Figure 11.

 
Figure 11

If you continue you can add all required product and manufacturing information in the model (Figure 12).


Figure 12

The Inventor integrated Tolerance Advisor gives you immediate feedback to your annotations. With the face status coloring you see what is fully, partly or not constrained. You can see the result in Figure 13.

 
Figure 13

And the decisions we made are not only for the head, they are for the mounting ring and finger as well (see Figure 14 and Figure 15).
 
Figure 14
 
Figure 15

Start now
With answering just a few questions about the function you can define a complete tolerancing concept of our assembly and bring the information in the 3D-model.
Now some of you might say, this is all terribly complicated! It is like learning a new language. As soon as you understand it, you have a wonderful toolbox to promote a uniform understanding and interpretation among all involved persons and departments. It might look complicated at first glance, but it's worth to learn it. So, what are you waiting for? Start now and describe your next part with GD&T.

 

back to Part 1

Learn more about Model Based Definition (MBD) in Inventor



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