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The geometric tolerance command has detailed rules for feature types with surface data, for features types without surface data, and for features types that you cannot use in a geometric tolerance command at all. Unfortunately, some feature types are only allowed to be used with a few tolerance types, and that information is not detailed here.Also, there are a few exceptions to the rules detailed here that were omitted for clarity but are rare enough that we did not document them.
There are several types of features that you can use with the geometric tolerance command:
Plane
Cross section of a plane (surface line)
Sample of a plane (surface point)
Cylinder
Cross section of a cylinder (surface circle)
Surfaceless axis
Width
Cone
Sphere
Surfaceless 3D point
Freeform
The documentation below talks about each PC-DMIS feature type and illustrates the kinds of PC-DMIS features that belong to the geometric tolerance command types.
With Surface Data
Plane feature types with surface data include:
Measured planes, auto planes (any measurement strategy), constructed BF planes, and constructed BFRE planes
No Surface Data
Plane feature types without surface data include:
Alignment planes, cast planes, mid-planes, perpendicular planes, parallel planes, primary datum planes, offset planes, translated planes, and generic planes
Only Allowed for Datums
Some plane types you can use only as datums, but not as considered features. These features include:
Alignment planes, cast planes, perpendicular planes, parallel planes, primary datum planes, offset planes, translated planes, and generic planes
The Geometric Tolerance dialog box has an interpretation message whenever you use a constructed line as a constructed feature or as a datum, letting you know whether the constructed line is being treated as a line on a surface (cross section of a planar surface) or as an axis of revolution (surfaceless axis).
With Surface Data
The geometric command always treats a line feature on a surface as a cross section of a planar surface. While you can measure a line on a non-planar surface in PC-DMIS, the geometric tolerance command always handles lines on surfaces as coming from a planar surface. These line types have surface data. They include these features:
Measured lines, auto lines (any measurement strategy), constructed 2D BF lines, constructed BFRE lines, and scan segment lines
Constructed 3D BFRE lines frequently have incorrect nominal values, even when you program your routine from a CAD model. Specifically, the nominal workplane of the constructed 3D BFRE line contains both the nominal surface normal and the nominal line vector. It is often nominally misaligned with the drawing. If you use this as a datum, it can frequently cause the following error message:
"Datum feature <feature name> is 2D. It needs a higher-precedence datum to constrain its workplane."
In most cases, we recommend using constructed 2D BFRE lines instead of constructed 3D BFRE lines so the nominal workplane of the line is correct.
No Surface Data
Some line features are cross sections of a plane, but they do not have surface data. These include:
Mid-lines, projected lines, and secondary datum lines
Some line features are surfaceless axes. These represent some axis of revolution and do not have a surface. These do not have surface data. These line features include:
Alignment lines, constructed 3D BF lines, cast lines, intersection lines, parallel lines, perpendicular lines, offset lines, and generic lines
Special Cases
Some line features deviate from the above interpretation in these special cases:
Constructed 3D BF lines and offset lines are cross sections of a plane and have surface data when they are constructed from points measured on a surface.
Parallel lines and perpendicular lines are cross sections of a plane, but do not have surface data when the resulting nominal surface normal matches the location reference (the second input to the construction).
With Surface Data
A point feature on a surface is always treated as a sample of a planar surface. While you can measure a point on a non-planar surface in PC-DMIS, the geometric tolerance command always handles points on surfaces as coming from a planar surface. These point types have surface data. These point features include:
Measured points, vector points (default strategy, not the self-centering one), surface points, edge points, high points, mid-points, and tertiary datum points
No Surface Data
Some point features do not have a surface and are constrained in 3 dimensions just like the center-point of a sphere. These are surfaceless 3D points. These point features include:
Projection points and generic points
A pierce point is constrained in two dimensions, so it is a surfaceless axis having a length of zero. For information on surfaceless axes, see the "Lines" above.
Not Allowed
Several point features are not allowed as inputs to the geometric tolerance command, neither as considered features nor datums. These include:
Corner points, auto angle points, intersection points, origin points, drop points, cast points, vector distance points, and offset points.
With Surface Data
Cylinder feature types with surface data include:
Measured cylinders, auto cylinders (any measurement strategy), constructed BF cylinders, and constructed BFRE cylinders
No Surface Data
Cylinder feature types without surface data include:
Cylinders and generic cylinders
Not Allowed
Projection cylinders are not allowed as inputs to the geometric tolerance command, neither as considered features nor datums.
With Surface Data
Circle features are treated as a cross section of a cylindrical surface. While you can measure a circle on a non-cylindrical surface in PC-DMIS, the geometric tolerance command always handles circles as coming from a cylindrical surface (with one exception detailed in "Circular Runout").
Circle feature types with surface data include:
Measured circles, auto circles (except the gage scan calibration measurement strategy), constructed BF circles, constructed BFRE circles, and scan segment circles
No Surface Data
Circle feature types that do not have surface data include:
Intersection circles, cast circles, tangent 2 lines circles, tangent 3 lines circles, tangent 3 circles, scan minimum circles, and generic circles
Not Allowed
Some circle feature types are not allowed as inputs to the geometric tolerance command, neither as considered features nor datums. These include:
Auto circles with the gage scan calibration measurement strategy, projection circles, cone circles, sphere circles, and cylinder circles
All width features have surface data.
With Surface Data
Slots and notches have surface data when you use them in a profile of a line tolerance.
No Surface Data
Slots and notches do not have surface data when you use them in a position tolerance or as a datum.
Be careful with slot and notch features.
You should only use them if you already know that the form of the features is very good. If you suspect that the manufactured form error might be significant, do not use a slot or notch command. Instead, measure a scan around the perimeter of the feature and then tolerance the form, orientation, and location of the feature with a profile of a line tolerance.
With Surface Data
Cone features with surface data include:
Measured cones, auto cones (any measurement strategy), constructed BF cones, and constructed BFRE cones
No Surface Data
Cast cones represent a conical surface but do not have surface data.
Generic cones don't have a diameter in PC-DMIS, and so they represent a surfaceless axis and do not have surface data.
Not Allowed
Projection cones are not allowed as inputs to the geometric tolerance command, neither as considered features nor as datums.
With Surface Data
Sphere features with surface data include:
Measured spheres, auto spheres, constructed BF spheres, and constructed BFRE spheres
No Surface Data
Sphere features without surface data include:
Cast spheres and generic spheres
Not Allowed
Projection spheres are not allowed as inputs to the geometric tolerance command, neither as considered features nor as datums.
The geometric tolerance command allows Freeform features as considered features for profile tolerances. These features have a number of measured points with matching nominal points and nominal vectors.
When there is no CAD model present, or when you don't use the iterate and re-pierce option, PC-DMIS calculates the deviations simply as the vector deviation of the measured point to the nominal planar surface defined by the nominal point and the nominal vector. This is the "piecewise planar" approximation.
When your measurement routine uses a CAD model, and you use iterate and re-pierce, PC-DMIS calculates the deviations to the CAD model (there is no piecewise planar approximation). For more information, see "Profile of a Line" and "Profile of a Surface".
PC-DMIS considers these feature types as considered freeform features:
Scan commands, constructed sets, constructed filtered sets, constructed adjust filter objects, auto Profile2D features (vision), and tori
Possible Surface Data
PC-DMIS considers some polygons as freeform features (with surface data) when you use them in a profile tolerance, but it considers them as circle features (lacking surface data) when you use them in a position tolerance. These feature types aren't really designed for position tolerances, but they are included as circles (lacking surface data) for migration purposes.
PC-DMIS considers slots and notches as freeform features (with surface data) when you use them in a profile tolerance, but it considers them as width features (lacking surface data) when you use them in a position tolerance.
Not Allowed
These features or types of features are not allowed as inputs to the geometric tolerance command, neither as considered features nor as datums:
Measured sets, area scans, projected ellipses, projected notches, projected slots, auto blobs, flush and gap features, load fixture features, face features, object features, constructed curve features, constructed surface features, and gage features
Several types of features are "reverse" features. These include:
Reverse planes, reverse cylinders, and so on
Possible Surface Data
Reverse features might have surface data. It depends on the input to the reverse feature. If the input has surface data, so does the reverse feature.
Possibly Allowed
You might be able to use reverse features as a considered feature or datum feature. It depends on the input to the reverse feature. The surface handling of the reverse feature is the same as the input feature. For example, a reverse line may be a surfaceless axis or a plane cross section. It depends on the input to the reverse feature.
Not Allowed
You cannot use reverse features that are constructed out of a different type of feature than the reverse feature as inputs to the geometric tolerance command, neither as considered features nor as datums. For example, you cannot use a reverse line constructed from a cylinder as an input to the geometric tolerance command.