GDT Rule #1

#411 / Mechanical Engineering Easy

What is Rule #1 of Geometric Dimensioning and Tolerancing (GD&T)?

Expand Hint
Also known as the limits of size.
Hint 2
Also referred to as the Envelope Principle or Taylor Principle.
The limits of size (Taylor Principle, Envelope Principle, Rule #1) defines the form and size limits of individual, regular features. For most common situations, it requires perfect form at Max Material Condition (MMC) to ensure a component will fit inside an opening. Per ASME Y14.5:
“Where only a tolerance of size is specified, the limits of size of an individual feature prescribe the extent to which variations in its geometric form as well as size, are allowed.”
As a visual example, consider the following views on a drawing:
According to rule #1:
The feature’s form may vary within the size limits. At MMC, the form must be perfect. As the it drifts from MMC to LMC, the feature may be more bent, tapered, or out of round. This guarantees fitment at the next level assembly.
Per ASME Y14.5:
“Where only a tolerance of size is specified, the limits of size of an individual feature prescribe the extent to which variations in its geometric form as well as size, are allowed.”


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