Ø Understand the rules used in GD&T

Terms, Rules, and ConceptsFundamental dimensioning Rules• The fundamental dimensioning rules are a set of general

rules that apply to dimensioning and interpreting engineering drawings. These rules are automatically invoked where the ASME Y 14.5-2009 Standard applies to a drawing. The 16 fundamental dimensioning rules are paraphrased and illustrated in the charts on the following pages.

The engineering drawing in Figure 7-1 is used to illustrate how the rules apply. The letters on the drawing correspond to the letters used in the charts that relate the dimensioning rules to the parts of the drawing.

Rule #1

• Rule #1 is a dimensioning rule that ensures that individual regular features of size will assemble with on another.

• Where Rule #1 applies to a regular feature of size, it establishes an overall boundary (or envelope) for a regular feature of size that is equal to its MMC.

• Where Rule #1 applies to an external regular feature of size, the maximum boundary (or envelope) of the regular feature of size is equal to its largest size limit (MMC).

• Where Rule #1 applies to an internal regular feature of size, the minimum boundary (or envelope) of the regular feature of size is equal to its smallest size limit (MMC).

• To design two regular features of size that will assemble, the designer can use their MMC’s.

How Rule #1 affects the diameter of a pin. When the pin diameter is at MMC, the pin must have perfect form, which m e a n s p e r f e c t s t ra ightness, per fec t roundness(i.e., perfect cylindricity ). In theory, this would allow the pin to fit through a boundary equal to its MMC size.

Envelope Principle

W h e n t h e R u l e # 1 applies to a hole, the hole must have perfect form at MMC. For a hole, MMC is the smallest value, and p e r f e c t f o r m m e a n s perfect roundness and perfect straightness. In theory,, this would allow the hole to assemble with a pin equal to its MMC size.

On a planar regular feature of size perfect form refers to perfect flatness. In this figure, if the 10.2-10.8 dimension was produced at 10.7, then form variation equal to the amount of the departure from MMC (10.8-10.7=0.1) Is allowed. If this dimension was produced at LMC (10.2), form variation equal to the amount of departure from MMC (0.6) would be permitted.

Rule #1 applied to a Planar Regular Feature of Size :

Where Rule #1 Applies:

Where the ASME Y14.5 standard applies to a drawing, Rule #1 automatically applies to all individual regular features of size.

An irregular feature of size

(type A)

An angular dimension, not a

feature of size

Examples of where Rule #1 Applies

Rule #1 Boundary

Rule #1 and the interrelationship between features of size

In this picture, the dimensions shown are all regular features of size. The orientations between these features of size are labeled A,B, and C. Rule #1 does not affect the variation of these angles.

Exceptions to Rule #1

How to Override Rule #1

The Independency Concept

Summary

Key Points:

The end