CZ Common zone
Common zone · Modificadores de zona · ISO 1101
Example in plan
How to interpret it
On the drawing, “4× ⌖ ⌀ 0.1 CZ |A|B|C|” states a common zone requirement with a cylindrical tolerance zone of ⌀ 4 mm, measured from datums A, B, C in that order of precedence.
What controls
When the same feature control frame applies to several features, CZ indicates that they all share a single common zone: the individual zones are fixed relative to each other, at their theoretical location and orientation, and the part is only good if all the features fit within that arrangement at the same time. Without CZ, in ISO each feature is evaluated against its own zone separately and can shift independently of the others. On two coplanar faces with CZ flatness, for example, the requirement is no longer that each face is flat but that both fit between the same pair of planes, which also controls the step between them. What it does NOT do is enlarge the tolerance or replace a datum: if the frame cites no datums, the common zone still floats relative to the part, even though its parts are locked to each other.
How to verify
A single alignment for the whole group and a combined evaluation: in the software, the features must be declared as a single group with a common zone, not measured one at a time, reported, and started over with the next. If the software allows best fit, that fit must be done for the whole set, moving all the zones as a block, never each one on its own. In series production, a functional gauge with all the pins machined in one go checks the full requirement at once, because it physically embodies the fixed relationship between zones.
Common mistakes when interpreting it
- Measuring each feature on its own, realigning between one and the next: that defeats the requirement and passes bad parts.
- Assuming that without CZ the features in one frame are already evaluated together on an ISO drawing: by default each has its own independent zone.
- Thinking that CZ gives more margin: the width of each zone does not change; what goes away is each zone's freedom to move on its own.
How to get it in the workshop / if it comes out of tolerance
Hole pattern with CZ: the tolerance applies to the group, not to each hole individually. It affects how the gauge and the CMM evaluation are worked out. In manufacturing, the consequence is that what you need to look after is the relationship between features: machine the whole group in the same setup and with the same work offset, because every repositioning puts error exactly where it hurts most. ASME does not use this abbreviation: there, a single frame on several features with basic dimensions is already understood as a pattern, and the relationship is governed by the simultaneous and separate requirement rules.
If the features pass individually but the group fails, the cause lies in the relationship between them: number of setups, clamping repeatability, table rotation or a change of work offset between operations. Check the angular position of the indexer and the axis backlash before touching the program. If machining in several setups is unavoidable, leave a machined reference in the first one and use it to align the rest. And when the failure always concentrates on the same feature of the group, look at the tool and strategy for that particular feature, not the overall relationship.
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Other symbols in Zone modifiers
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