Position

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Introduction

A position specification controls how much the considered feature or features can deviate from a specified position relative to zero or more datums.

For this geometric tolerance, these three aspects work together:

• Each considered feature and each resulting toleranced feature

• Each tolerance zone

• The datum features (if any are referenced)

To evaluate this tolerance, PC-DMIS converts each considered feature to a toleranced feature. You can find this described in "Deriving the Toleranced Feature”.

PC-DMIS then optimizes each toleranced feature into its respective tolerance zone. The optimization process respects whatever constraints each datum imposes. When there are multiple considered features, the optimization process simultaneously considers all features, so that all toleranced features are fitted into their tolerance zones at once. This process is similar to a physical gage, where all of the gage pins must fit into the workpiece's holes at the same time.

Allowed Feature Types

You can use these feature types:

spheres, 3D surfaceless points, cylinders, circles, cones, widths, slots, notches, constructed mid planes, constructed mid lines, and constructed mid points.

ASME position tolerances also allow 3D constructed BF lines. ISO position tolerances also allow planes, lines, and surface points.

3D constructed BF lines, spheres, cylinders, circles, cones, widths, slots, and notches have a toleranced feature that is different than the considered feature’s surface data. For information, see "Deriving the Toleranced Feature".

Tolerance Zone Shapes

Different tolerance zone shapes are allowed for different types of features. For information about the feature command types that refer to different feature types, see "Feature Types With and Without Surface Data".

Point-like considered features
When the considered feature is point-like, the tolerance zone shape may be planar, diametrical, or spherical. These are point-like considered features:

sphere or 3D surfaceless point

From left to right, the pictures below show FCFs with a planar, diametric, and spherical tolerance zone when the considered feature is a sphere:

Planar and diametric tolerance zones on point-like considered features need a specified tolerance zone orientation, because the feature does not have enough information to orient the zone correctly. In these cases, the Zone Orientation button in the Geometric Tolerance dialog box becomes visible. For information on how to use this button to change the zone orientation, see "Zone Orientation" in the "Feature Control Frame Tab" topic.

Axial Considered Features
When the considered feature is axial, the tolerance zone may be planar, diametrical, radial arc, or perpendicular to radial. These are axial considered features:

cylinder, circular cross section of a cylinder, cone, or surfaceless axis

A diametric tolerance zone is shown below, using the diametric zone symbol.

Planar zones, radial arc zones, and perpendicular to radial zones do not use any tolerance zone shape symbol and are shown below.

Planar tolerance zones on axial considered features need a specified tolerance zone orientation, because the feature does not have enough information to orient the zone correctly. In these cases, the Zone Orientation button in the Geometric Tolerance dialog box becomes visible. For information on how to use this button to change the zone orientation, see "Zone Orientation" in the "Feature Control Frame Tab" topic. This button also allows users to specify a radial arc or perpendicular to radial zone.

If a position tolerance controls the X component of the position, the tolerance zone surface normal vector should be X.

Plane-Like Considered Features
When the considered feature is plane-like, the tolerance zone is always planar and oriented parallel to the nominal surfaces. These are plane-like considered features:

plane, surface line, width, slot, notch, surface point, or mid-point

You can have more than one considered feature, but those features must all be the same type.

Be careful with slots and notches.

Actual Value and Measured Value

Actual Value:
Each considered feature has its own actual value. This is the size of the smallest tolerance zone that contains the actual toleranced feature. The zone is nominally oriented and located to each actual datum, with some exceptions detailed in "How PC-DMIS Solves Datums". When the position tolerance contains more than one considered feature, and the datum reference frame is not fully constrained, an optimization procedure must simultaneously fit all toleranced features in their respective tolerance zones if possible.

Measured Value:
Each considered feature has its own measured value. This is the size of the smallest tolerance zone that contains the measured toleranced feature. The zone is nominally oriented and located to each measured datum, with some exceptions detailed in "How PC-DMIS Solves Datums". When the position tolerance contains more than one considered feature, and the datum reference frame is not fully constrained, the PC-DMIS optimization procedure simultaneously fits all toleranced features into their respective tolerance zones in a proportional way, which guarantees all toleranced features will fit in their respective tolerance zones if at all possible.

Validity Rules

Considered features must have some nominal location and orientation to each datum feature.

For radial arc and perpendicular to radial tolerance zones on axial features, follow these requirements:

• The datum reference frame must establish a clear polar origin and polar axis.

• The axial features must be nominally parallel to the polar axis.

Allowed Modifiers

When the considered feature is a cylinder, sphere, or width, position tolerances allow a maximum material modifier to indicate the specification is at the maximum material condition (MMC). Alternatively, they allow a least material modifier to indicate the specification is at the least material condition (LMC). This means that as the unrelated mating envelope size (or unrelated minimum material envelope size for LMC) deviates from the MMC (or LMC), additional tolerance or "bonus" tolerance is added to the tolerance in the feature control frame, yielding a total tolerance. For more information on this bonus tolerance, see "Evaluating Size with the Geometric Tolerance Command".

When the considered feature is an Auto feature cylinder, you can use a projected-zone modifier . This projects (extrapolates) the measured feature axis as described in "Deriving the Toleranced Feature".

Exposed Options

When the considered feature has surface data and the toleranced feature differs from the considered feature's surface data (spheres, cones, cylinders, circles, widths), the feature math type controls how to compute the tolerance feature from the considered feature's surface data. For information, see "Deriving the Toleranced Feature”.

When at least one datum feature has surface data, the datum math type controls how to compute the measured datums from the datum features' surface data. For more information, see "How PC-DMIS Solves and Uses Datums".

Slots can be considered in either a lengthwise or a widthwise fashion, as described in "Lengthwise_versus_Widthwise_Slots".

Lower Segments of Composite Position

A position tolerance with multiple segments is called a “composite position”. Composite position tolerances are typically specified on a pattern of features. The first (or upper) segment of a composite position is the same as a single segment position as described in the preceding sections of this page. All lower segments of a composite position are subtly different. This is because the tolerance zones of the pattern have unlocked translation compared to the datum reference frame. However, the tolerance zones remain nominally located and oriented to each other.

The datum reference frames for the lower segments of a composite position follow these rules:

Report

Here is an example report for a position tolerance of two cylinders. The cylinders' size tolerance is in the upper label, and the diametric-zone position is in the lower label. The lower label includes information for the YZ positions of the cylinders, in the optimized frame (not in the current alignment).

In the lower label of the above illustration, the Y, Z, and TP rows for CYL1 and CYL2 summarize the position evaluation for each considered feature. The bottom row of each, labeled TP, behaves just like all of the reports for other geometric tolerances. For CYL2, the TP row includes the tolerance 0.200, the bonus 0.090, and the measured value 0.184.

In the lower label of the example report above, there are only three rows for each feature (Y, Z, and TP). For your reports, the axis column (AX) for your features might have some combination of several rows labeled X, Y, Z, PR, PA, and TP. When present, the X, Y, Z, PR, and PA rows give supplementary information as described below.

The Y and Z rows for CYL1 and CYL2 give supplementary information about the position evaluation. Those rows are intended to give a simplified representation of how the feature deviates from the ideal.

• The NOMINAL column of the supplementary information rows shows the nominal start point of the feature in question.

• The DEV column of those rows shows the worst deviation vector, among all of the toleranced feature's points.

• The MEAS column of those rows is the NOMINAL column plus the DEV column. In other words, it is the toleranced feature's worst point, after being projected to be as close as possible to the nominal start point, while preserving the worst deviation vector.

• Underneath the feature name, the report indicates which end or level of the cylinder was the worst. For example, it could say "(START PT)", "(END PT)", or "LEVEL#3".

The representation is simplified, because, as described in Deriving the Toleranced Feature, most feature types have more than one point in the toleranced feature. For example, cylinders typically have at least two points in the toleranced feature, the measured start point and the measured end point. By contrast, in some cases ISO cylinders have several points in the toleranced feature, one for each measured cross section.

If you turn on the textual analysis or graphical analysis, you can see all the points in the toleranced feature, together with their respective deviations. The supplementary information rows in the report has a MEAS column that does not necessarily appear in the textual analysis, because the supplementary information has been simplified to show deviation with respect to the nominal start point.