MIL-D-28000
Class II - Engineering Drawings

3.2.2.1. Information requirements and data functionality. Currently, engineering drawings form the mainstay of product definition and form the accepted medium for viewing computer generated data. Geometry and text annotation form the majority of data items. Extensive use is made of dimensions, sectioned areas, text notes, and feature control symbols. Non-geometric information in the form of color, line weight, line font, and level is needed as well. Finally, relationships among the data help to structure the large quantities of data. Information requirements for this application subset include (but are not limited to):

1. Simple geometry in the form of points, lines, circular arcs, conic arcs, and spline curves.
2. Non-geometric attributes of color, line weight, and line font.
3. Data relationships, including the concept of subfigures.
4. Data organization methods such as level.
5. Part name, drawing number, formal identification, and drawing revision.
6. Annotation represented by the corresponding ASME Y14.26M entity type thus retaining its intended functionality.

3.2.2.2. ASME Y14.26M entity subset specification. Table II lists the entities of this subset. Only ASME Y14.26M entities which are enumerated in this table shall be used for representing engineering drawing product definition data. Other valid ASME Y14.26M entity types may be present in the file as described by 3.1.1. Additional requirements are placed on the global section of a file, and certain field value restrictions are placed on the range of parameter values in both the directory entry(DE) and the parameter data(PD) sections of a valid ASME Y14.26M file.

When the form column in Table II is blank for an entity which has multiple form numbers, all forms of that entity are included in the subset.

F000X.IGS - Null Entity

100 Series

F100X.IGS - Circular Arc

F102X.IGS - Composite Curve

F104X.IGS - Conic Arc

106 - Copious Data

Misc. 100 Series

124 - Transformation Matrix

126 - Rational B-spline curve

128 - Rational B-spline surface

Misc. 100 Series

200 Series

F202X.IGS - Angular Dimension

F206X.IGS -Diameter Dimension

F210X.IGS - General Label

F212X.IGS - General Note

F214X.IGS - Leader/Arrow

F216X01.IGS - Linear Dimension

F218X.IGS - Ordinate Dimension

220 - Point Dimension

F222X.IGS - Radius Dimension

228 - General Symbol

F230X.IGS - Sectioned Area

300 Series

400 Series

102 Composite Curve. Composite curves are intended for showing connectivity and continuity. They shall contain a minimum of two entities one of which shall be a curve the line entity included.

124 Transformation Matrix. Defining matrices are used to position an entity into model space from its definition space. When entities share the same plane of definition, they shall use the same transformation matrix to avoid multiple identical matrices being included in the file. If an entity contains translation information in its PD section, a transformation matrix shall not be used to translate the entity.

202 Angular Dimension. This entity shall have two leaders, and a vertex point. The Z displacement of the vertex point can be attained from any of the subordinates.

206 Diameter Dimension. This entity shall specify its arc center (may not be defaulted). The Z displacement of the arc center can be obtained from the subordinates. The arc center shall be valid. If multiple leaders occur, the first segment of each leader shall be collinear and opposite in direction.

212 General Note. General notes shall use a font code to minimize the number of text strings in the note. At least one string is required, but the number of different strings shall be minimized. Form numbers plus position information on each string shall be used. The Rotation Angle field shall contain the string angle. Transformation matrices shall not be used for string angles. Null strings are allowed and may be used to pattern a note into one of the standard forms.

216 Linear Dimension. When there are two witness lines, they shall be parallel to each other.

218 Ordinate Dimension. This entity shall not be used in place of the general label. The leader shall contain only one segment.

222 Radius Dimension. This entity shall specify its arc center (may not be defaulted). Z displacement of the arc center can be attained from any of its subordinates. Arc center shall be valid. If multiple leaders occur, the first segment of each shall be collinear. If two leaders are used, one and only one shall be of Form 4.

228 General Symbol. This entity shall be used for annotation. Flags in the subordinate entities (even geometry) shall be set to annotation. This entity shall not be used in place of a subfigure or group. All subordinate entities to the general symbol entity shall be coplanar.

230 Sectioned Area. This entity is used to provide for area fills. The normal distance between lines is constant and shall not be zero. Wider spaces between lines are to be considered blank lines with regard to normal distances. Boundary curve and islands are closed and not self- intersecting. Island interiors are mutually disjoint. Islands lie in the interior of the boundary curve. Boundary curve and islands can be logically dependent and do not need to be marked as annotation if they are not annotation.

404 Drawing. The drawing entity defines the basic engineering drawing format for each sheet. One drawing entity shall exist in the file for each sheet, or collection of sheets, of an engineering drawing. An example of a collection of sheets would be several B size sheets placed in an H size engineering drawing. All entities pointed to by the drawing entity shall be physically dependent and shall be flagged as annotation.

406 Form 5 Line Widening. The only allowed use of the line widening property is the case when physical model space width is important. It shall not be used to indicate plotter width or line weight.

406 Form 15 Name. This entity is used to convey the drawing name. A Drawing Name using Entity 406, Form 15 is required. If there is no name available in the architecture of the system, the preprocessor shall insert one, a reasonable default being global parameter 3.

406 Form 16 Drawing Size. A drawing size property shall be included for each drawing entity in the file. If drawing size is not in the architecture of the system, the preprocessor shall insert one and the drawing size for each sheet shall match the drawing size letter defined in the start section (see 3.2.2.4.1).

406 Form 17 Drawing Units. A drawing units property shall be included for each drawing entity in the file. If drawing units is not in the architecture of the system, the preprocessor shall use the defaults given in global parameters 14 and 15.

The simplest entity type shall be used to represent each piece of geometry, and zero size entities shall not be used. For instance, a B-Spline curve shall not be used to represent a circular arc. A zero length line entity or a zero diameter circle shall not occur in the file.

All 200 series entities shall be flagged as annotation and shall be parallel to the viewing plane, except for crosshatching. All dependent entities with parent 200 entities shall be flagged as annotation and shall be coplanar with their parent. The only exception is for geometry entities associated with the Sectioned Area (230 and 106 Forms 31-38). Any annotation entity which points to multiple leaders and multiple witness lines shall order those leaders and witness lines so that leader-1 corresponds logically to witness-1 and leader-2 to witness-2.