Optional Modifiers (ASME)
What do the available options mean?
1. Statistical — Adds the Statistical Tolerance symbol, , to indicate that the tolerance value has been assigned based on statistical methods. When you select this option, the following note will automatically be added to data set: FEATURES IDENTIFIED AS STATISTICALLY TOLERANCED SHALL BE PRODUCED WITH STATISTICAL PROCESS CONTROLS.
2. Projected Zone — Adds the Projected Tolerance Zone symbol, Ⓟ, to indicate that the tolerance zone is projected above/outside the part 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.
3. Tangent Plane — Adds the Tangent Plane modifier, Ⓣ, to indicate that the tolerance zone applies to the tangent plane of a feature surface (and not to the high and low points).
4. Free State — Adds the Free State modifier, Ⓕ, to indicate that the geometric 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.
5. Unequally Disposed — Adds the Unequally Disposed modifier, Ⓤ to indicate that a profile tolerance has an unequally disposed tolerance zone. When you select this option, the value that you specify represents the portion of the tolerance zone that is outward from the surface of the feature.
6. Separate Requirement — Adds the Separate Requirement modifier, SEP REQT, adjacent to the feature control frame. When you select this option, the feature (or pattern of features) will be a separate requirement from all other features in the model.
How do I decide which options to choose?
The symbol should be used when the dimension has been used in a statistical tolerance analysis (e.g. the square root of the sum of the squares of the individual tolerances). The symbol is not well-defined in the Y14.5 standard and is not commonly used.
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 within the location tolerance zone. See Projected Tolerance Zone.
The Ⓣ symbol is typically used when it is important to control a contacting plane of a surface and not the entire (high and low points) surface. When the tangent plane modifier is specified it may be necessary to add additional tolerances to constrain the form of the surface.
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 feature control frames where the intent is to have the tolerance apply without the restraint.
The Ⓤ symbol is typically used when you need to define a profile tolerance zone that is unequally disposed about 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.
The SEP REQT symbol is typically used when you want to remove a simultaneous requirement. Simultaneous requirements are generally more expensive than separate requirements. When considering whether a feature (or pattern of features) should be excluded from a simultaneous set, you should consider the following:
Does the position or profile tolerance that is applied to the feature reference a DRF with datum shift (i.e., it references one or more datum features of size with an MMC or LMC modifier)?
Is the feature intended to function separately from the other features that reference the same DRF?
If the feature meets both of these conditions, you should probably specify that the geometric tolerance is a separate requirement.