In our Video Question Line, our instructors answer questions that we frequently receive from our students. The topic of the question line video below is regarding Derived Median Line (DML) Straightness and Rule #1. In this video, we answer the question, “Why doesn’t Rule #1 apply to the straightness of a Derived Median Line?”

Rule #1

To answer this question, let’s first look at GD&T Rule #1.

Rule #1 of GD&T, also known as the Envelope Principle, states that the form of a regular feature of size is controlled by its limits of size – its size tolerances. This means that the surface of the feature can’t extend past the envelope created by the Maximum Material Condition (MMC) of the feature. So, if a feature is at MMC – its maximum size, there is no room for form error. The form would have to be perfect for the feature to stay within the envelope.


Now, let’s take a look at Straightness. Straightness can be applied to either a surface or a feature of size.

If straightness is called out on a surface, it controls any linear element of that surface. This is illustrated in Figure 1, below.

Figure 1: Surface Straightness


However, if straightness is being called out on a feature of size, as indicated by the callout being attached to a size dimension, the straightness control is for the derived median line of the feature of size. The derived median line of the feature is the line made from the centerpoints of the cross-sections of that feature. If the feature has a bow in it, the derived median line will represent that. This is illustrated in Figure 2.

Figure 2: Derived Median Line Straightness


Derived Median Line Straightness overrides Rule #1

Figure 1 shows straightness being applied to the surface of the cylinder. According to Rule #1, the cylinder must fit completely within the .505 diameter envelope created by the MMC to meet specifications. Therefore, the larger the cylinder’s diameter, the less form error is allowed. If the cylinder’s diameter itself was .505, there would be no room for any form error.

Figure 2 shows straightness being applied to a feature of size, which means that it is being applied to the derived median line of the cylinder. This breaks the link between size and form, overriding Rule #1.  Regardless of the diameter of the cylinder, the derived median line would need to stay within a diametric tolerance zone of .003. The size dimension would still need to be held for the part to meet specification, but the size and form are held separately.

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