The Independency Symbol is used on drawings to declare that the requirement for perfect form at MMC or LMC is removed and the form tolerance may be larger than the size tolerance. The default rule in the ASME standards is to assume size will control form, this is known as Rule #1. However, when the Independency symbol is utilized next to a size dimension, the size and form of that feature are now controlled separately. Note this symbol only exists in the ASME Y14.5 Standards and not the ISO standards.
Applying the Independency Symbol:
In order to ensure that Size and Form are controlled independent of each other, the Independency Symbol is applied to the drawing using an encircled capital “I”. The symbol can be applied to an individual feature by locating it next to the tolerance dimension as seen below.
Note: Unless there is a supplementary method of control, like the flatness control above, the form of a feature is uncontrolled wherever the Independency Symbol is applied.
Voiding Simultaneous Control of Size and Form:
By negating the relationship between the control of size and form, the form is now able to deviate beyond the envelope of perfect form at Maximum Material Condition (MMC).
The thickness of the part above is controlled by the dimensional limits listed. Note the differences in the 2 examples shown. The left example shows the ASME default state, which is form being controlled by size via Rule #1 of Geometric Dimensioning and Tolerancing.
The example on the right utilizes the Independency symbol and applies a form tolerance to control flatness of the bottom surface. Note the flatness is only applied to the bottom surface since it is applied as a surface and not the thickness as a feature of size. The envelope no longer applies since the Independency Symbol decouples the form from the size dimension. However, flatness is used to apply tolerance for the form.
Note that the local part size must always measure within the stated size tolerances regardless of form and independency.
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