Open C GD&T

Tolerance Features

A Tolerance Feature is the basic building block of the Geometric Dimensioning Module. A Tolerance Feature contains information about the size tolerance and geometric form of a feature, as well as, what geometric tolerances, datums and datum targets are applied to the feature. The new tolerance feature type is recognized as a unique object by the part database and is not confused with "old" GD&T notes. Tolerance features are always associated to actual model geometry.

Tolerance Features are made up of a number of attributes. Some of these attributes are mandatory to the creation of a Tolerance Feature, others are optional. The following is a list of the valid attributes of a Tolerance Feature.

Face Lists - In the Geometric Tolerancing Module all tolerance features are related to actual model faces. In many cases, a single tolerance feature may reference multiple model faces. The Face List of a tolerance feature, is the set of all model faces which are constrained by the tolerance feature. Figure 1-1 illustrates different Face Lists.

Figure Face Lists

All tolerance features, except the multiple datum type, must include a Face List. Only one unique Face List may be associated with a tolerance feature.

Features - The Feature of a tolerance feature allows the user to apply a more definitive shape to the tolerance. Different Feature types include planes, axes, axis of patterns, tabs, slots, holes, pins, etc. The valid Features used by the geometric tolerancing module are defined in the enumerated variable type UF_GDT_feature_type_t. A tolerance feature must have a feature defined. Furthermore, each geometric tolerance applied to a tolerance feature may also have a feature defined.

A tolerance feature may not be created without at least one of the following:

Size Tolerances - A dimension where the total amount by which the dimension is permitted to vary is noted. A tolerance feature may or may not include a size tolerance. Tolerance features with a size tolerance are considered Features of Size. Only one unique Size Tolerance may be related to a tolerance feature, though the Size Tolerance may include multiple dimensions (i.e., patterns.)

Datum Identifiers - A theoretically exact point, axis, or, plane derived from the true geometric counterpart of a specified datum feature. A Datum Identifier is the origin from which the location or geometric characteristics of features of a part are established. If datum targets are to be used with the Datum Identifier, the identifier must also be defined as primary, secondary, or tertiary. A tolerance feature may or may not include a Datum Identifier. Only one unique Datum Identifier may be associated with a tolerance feature.

Datum Targets - A specified point, line, or area on a part used to establish a datum. A tolerance feature may or may not include Datum Targets. Datum Targets cannot be included in a tolerance feature without an associated datum. The minimum number of Datum Targets is defined by the type (primary, secondary, or tertiary) of the parent datum. Otherwise any number of Datum Targets may be specified for a tolerance feature.

Geometric Tolerances - The general term applied to a tolerance that controls the form, profile, orientation, location or runout of a feature. In the Geometric Tolerancing Module Geometric Tolerances are manipulated through their related feature control frame. A tolerance feature may or may not include a Geometric Tolerance. Any number of Geometric Tolerances may be applied to a tolerance feature.

Multiple Datums - A Multiple datum is a special composite tolerance feature. Multiple datums are the composite two or more datum identifiers. They do not have a defined face list, but have a unique feature type. If a tolerance feature is a Multiple Datum it may not be of any other type.

Pattern Types - A tolerance feature may exist as part of a pattern. Tolerance features that are part of patterns should be specified when they are initially created.

Datum and Datum Target Symbols

Datum and Datum Target Symbols are respectively the graphical representation of Datums and Datum Targets on a drawing or view. The symbology of Datums and Datum Target Symbols is illustrated in the following figure.

Figure Datum and Datum Target Symbols

Leaders attaching Datum and Datum Target Symbols and their actual format may vary according to different international standards.

Feature Control Frames

A Feature Control Frame (FCF) is the graphical representation of a Geometric Tolerance on a drawing or view. Geometric Tolerances are always instanced as a Feature Control Frame. The basic parts of a Feature Control Frame are:

Characteristic. The type of feature control frame. A characteristic is an absolutely mandatory portion of the frame.

Tolerance Zone. The tolerance zone of an FCF is made of a number of different parts. The first part is a zone shape symbol. this symbol calls out the shape of the tolerance zone. The second is the actual value of the tolerance zone or the tolerance value. The final part of the frame contains the modifiers. Modifiers include Material Condition, Tangential, Projected and Statistical symbols.

Datum References. Datum References call out the datums which constrain the tolerance. They may also have a material condition modifier. Some Geometric Tolerance types may not require a datum reference.

There are three basic types of feature control frame.

Single Frame - Single Frames are the most basic type of FCF. They are made up of a single characteristic and tolerance zone callout with up to three datum references.

Composite Frame - Composite Frames have a single characteristic, but multiple lines of tolerance zone and datum reference callouts inside of multiple frames. Composite frames can be further subdivided into Composite Tolerance Frames and Non-composite Tolerance Frames. Composite Tolerance Frames have callouts that contain actual tolerance data (position callouts for patterns and profile callouts that control position). Non-composite Tolerance Frames have callouts that contain zone refinement information (projected tolerance zones and unit length bases callouts). Though graphically containing multiple frames, Composite Frames have only one tag per callout.

Multiple Frame - Multiple Frames have multiple characteristic, tolerance zone, and datum reference callouts. Each unique characteristic in a multiple frame denotes a unique FCF.

Figure Typical Feature Control Frames.

Leaders attaching Feature Control Frames vary according to the standard.

Miscellaneous

Tolerancing Standards - The various governing bodies for standards in dimensioning and tolerancing have published guides to help in standardizing the usage of GD&T symbols. The NX Geometric Dimensioning Module supports the following standards:

ANSI Y14.5M - 1982
ISO 1101 : 1983
ASME Y14.5M - 1994

Tolerance Module Routine Naming Convention - All routines in the Open C API for the Geometric Tolerancing Module begin with the UF_GDT prefix. This prefix is followed by a verb descriptor. These verb descriptors have the following meanings.

add: Add routines add a new attribute to an existing tolerance feature.

ask: Ask routines query the Geometric Tolerancing Module for an existing value.

check: Check routines make calls to the VHOST Checker to test the validate of a callout.

create: Create routines create a new tolerance feature with the specified attributes.

is: Is routines return a logical value for a given query.

set: Set routines edit the existing values inside the Geometric Tolerancing Module.

uf_gdt.h

You must include the header file uf_gdt.h in your Open C API program. This header file includes the type defined enumerated variables and structures used by the various UF_GDT routines. In addition, this header file defines error codes.