Ⓘ Independency
Independency · Modificadores de zona · ISO 1101
Example in plan
How to interpret it
On the drawing, “⌀ 20 h6 with Ⓘ and ○ 0.02” states a independency requirement.
What controls
Decouples size from form: with Ⓘ, the diameter or width dimension is checked as a local size (a two-point measurement at each cross-section) and form is controlled only by whatever geometric tolerances are applied, with no perfect-form envelope limiting both at once. It is the opposite of the envelope requirement Ⓔ, which requires the whole feature to fit inside a perfect cylinder at maximum material size. The difference between standards matters: in ASME Y14.5 the envelope is the default rule for rigid features of size, and Ⓘ exists precisely to switch it off on a specific feature; in ISO GPS the independency principle is the starting point and it is Ⓔ that has to be requested explicitly. What Ⓘ never does is enlarge the size tolerance: every local measurement still has to fall within its limits.
How to verify
With Ⓘ, size is measured point by point with a micrometer, bore gauge or probe at several sections and in several directions: what is judged is each local measurement. The envelope GO gauge (ring gauge for a shaft, plug gauge for a hole) is NOT the right instrument, because it checks the perfect envelope that Ⓘ has just switched off; using it rejects good parts. On a CMM you have to select the two-point (local) size mode in the software instead of the envelope or least-squares fit, and evaluate form as a separate characteristic. Without Ⓘ, on the other hand, the GO gauge is the correct check of the combined size and form.
Common mistakes when interpreting it
- Believing that Ⓘ gives more size tolerance: local measurements still have to be within the dimension; the only thing that goes away is the perfect envelope.
- Checking a dimension marked Ⓘ with an envelope GO/NO-GO gauge, which is exactly what the modifier cancels.
- Assuming the same default in both systems: in ASME the envelope is the rule unless Ⓘ is shown, and in ISO independency is the starting point unless Ⓔ is specified.
How to get it in the workshop / if it comes out of tolerance
Without Ⓘ, a shaft that comes out at its maximum diameter must also be perfectly straight and round, because the envelope is already used up; with Ⓘ, that same shaft may have bow or ovality as long as every local measurement is within size and the specified form tolerance is not exceeded. In practice, Ⓘ makes the part cheaper when the feature doesn't have to go into a hole (a shaft that just spins on separately mounted bearings, a pin that is welded afterwards). If the part does have to assemble with another one, switching off the envelope is risky: ask first.
If a part carrying Ⓘ is rejected, first check which criterion was used to measure size: a rejection from a GO gauge is not valid here. If it is really the form that fails, work on the process as you would for any straightness or circularity problem (overhang, steady rests, chuck clamping pressure, cutting edge condition). And if the feature has to mate with another one and still causes assembly problems even though it conforms, that is a sign the drawing should not carry Ⓘ: what is needed is the envelope requirement.
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