Rule #1 in GD&T
Mufaddal Rasheed 
What is Rule # 1 and why is it important in GD&T ?
What is Rule #1
When a size dimension and tolerance is applied to a feature of size. The Feature of size should have perfect form at Maximum material condition.
Also called as Taylors principle.
In the above image the pin when manufactured at size of 11 mm (MMC) it should have perfect form. It can have deviation in form as shown by the left image (bowed pin) if the size is lesser than 11 mm .
- Where only a tolerance of size is specified, the limits of size of an individual feature prescribe the extent to which variations in its form – as well as in its size- are allowed
- No element of a feature shall extend beyond the MMC boundary of perfect form.
- The form tolerance increases as the actual size of the feature departs from MMC toward LMC.
There is no perfect form boundary requirement at LEAST MATERIAL CONDITION
This is an idealistic rule, as in reality perfect form cannot be achieved.
What can override Rule #1?
- If a Geometrical (form, orientation, location ) control is provided explicitly i.e. flatness, circularity, cylindricity , profile
- By mentioning a note which says explicitly that Perfect form at MMC is not required
Learn more about how to Interpret GD&T callouts from this Free e-book on GD&T interpretations
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Categories: : GD & Tolerancing
