Optional Modifiers (ISO)
What do the available options mean?
1. Projected Zone — Adds the
Projected Tolerance Zone symbol, Ⓟ, to indicate that the tolerance zone is projected above/outside the
workpiece surface. When you select this option, you must also enter the minimum height of the tolerance zone. When a projected tolerance zone is used, it limits the orientation deviation of the toleranced
feature of size, usually a hole, without restricting the location deviation of the hole.
2. Free State — Adds the
Free State modifier, Ⓕ, to indicate that the
geometrical tolerance applies in the unrestrained condition. This modifier is only applicable for non-rigid parts for which there is a note that indicates that tolerances are specified for the restrained state.
3. Common Zone — Adds the
Common Zone modifier,
CZ, to the tolerance zone for geometrical tolerances applied to multiple surfaces to indicate that the geometrical tolerance establishes a common tolerance zone applied simultaneously for the indicated surfaces.
4. Not Convex — Appends the
Not Convex modifier,
NC, to the tolerance frame to indicate that the toleranced surface has only concave deviations from perfect flatness. (Available for ISO 1101:2012 only).
5. Specified Offset— Adds the Specified Offset modifier, UZ, to indicate that a profile tolerance has a specified offset. When you select this option, the value that you specify represents the offset of the theoretical profile, with the direction of the offset indicated by the sign. A "+" sign indicates "out of the material" and a "-" sign indicates “into the material”.
How do I decide which options to choose?
The Ⓟ symbol is typically used when the deviation in perpendicularity of threaded or press-fit holes could cause fasteners (i.e. screws, studs, etc.) to interfere with mating parts. In fixed fastener applications, the hole (threaded or press-fit) locates and orients the fastener. The interference can occur when the fastener is inclined with the location tolerance zone.
The Ⓕ symbol is typically used on features of
non-rigid parts. A non-rigid part is indicated by the inclusion of a
restraint note in the data set. When a restraint note is present, all dimensions are measured in the restrained state. The free state modifier is specified in
tolerance frames where the intent is to have the tolerance apply without the restraint.
The CZ modifier is typically used in situations in which there are multiple surfaces (e.g., coplanar planes) in the CAD model that are functionally a single surface. For example, for flatness applied to coplanar planar surfaces, the CZ modifier imposes a coplanar requirement in addition to the flatness requirement.
The NC modifier is used in applications where convex deviations cannot be permitted. The use of the NC modifier requires the toleranced surface to have only concave deviations from perfect flatness. The highest points of the toleranced surface must be around the perimeter of the real surface. Note that this modifier has been eliminated in ISO 1101:2017.
The UZ modifier is used when you need to define a profile tolerance zone that is offset from the true profile of a surface. Unless there is a functional reason to offset the tolerance zone from being centered about the true profile, this modifier should not be used.