Linear Dimensions and How to Create Them

The simplest kind of Linear Dimension consists of a pair of points on a drawing, each of which relates to a point in the Design model. From each of these Dimension Points on the drawing, a projection line is drawn in a user-definable direction; between these parallel projection lines, dimension lines are drawn. Each dimension and projection line may have text associated with it. An illustration of a simple linear dimension between two Equipments is shown in Figure 12:2 below.

Figure 12:2. Single Value Linear Dimension

Such a dimension could be produced by entering the following commands, starting at Layer level:

The Dimension will appear on the display. If only two elements are hit a pair of Dimension Point elements will have been automatically created, with the last point becoming the current element.

In the above example the Dimension Points will be DPPT elements - the Dimension Point is defined by a p-point of the Design element, in this case the origin. Any p-point can be nominated as a dimension point by replacing the FROM ID @ syntax above by FROM IDP @/TO IDP @. The Dimension may also be defined explicitly:

The other two types of Dimension Point that may be owned by an LDIM element are:

• DPOI- allows you to dimension to/from any given 3D positions

• DPBA- allows you to dimension to/from ‘before’ or ‘after’ a Design element (in the Dimension direction).

DPOI elements would be created by a sequence such as:

NEW LDIM

FR POS @ TO POS @

DPBA elements would be created by a sequence such as:

NEW LDIM

FR BEFORE ID @ TO AFTER ID @

The effect of such a command will depend upon the dimension direction - see Principal Attributes of Angular Dimensions. shows an example of such a linear dimension, produced by the same command but with different dimension directions.

Constructed points (see Point and Line Construction) may also be used to create Dimension Points.

Note: In cases where the dimension value is less than 0.01 mm, the display of all dimension point graphics (see ) will be suppressed.

When dimensioning BEFORE or AFTER elements such as EQUI, STRU or SUBS, Draft will ignore those primitives with OBST (obstruction level) set to 0 or 1. When dimensioning BEFORE or AFTER a primitive, its OBST value will be ignored.

Figure 12:3. Single Value ‘Before/After’ Linear Dimensions

Figure 12:4 below shows examples of using ‘before/after’ linear Dimensions with a single element. (See Principal Attributes of Linear Dimensions for further information of DOFF and OSHT.)

Note: Under some circumstances the length of the projection line for a DPBA may be unsatisfactory. In this case changing the value of the NPPT attribute (which supplies a reference p-point) will correct the situation.

Figure 12:4. Before/After’ Linear Dimensions on a Single Element

Notes:Use of Radial Dimensions would provide a simpler method of drawing the two dimensions shown on the right-hand side of Figure 12:4 above- see Radial Dimensions.

Any combination of the above three Dimension-creating commands is permissible.

Each FROM command sets the Dimension Point’s DDNM (Design Data Name) attribute to the Name of the Design element. The DDNM attribute may be reset immediately to define a new Dimension Point. If the DDNM is set to refer to an element, which is not in the Id List referenced from the current VIEW, the Dimension will still be drawn.

The command:

CHECK REFS WARN

will cause a warning to be output in this situation, and the command

CHECK REFS FULL

will result in such Dimensions not being drawn.

The default reference-checking mode (that means, no checking) is given by

CHECK REFS OFF

Reference checking is also available for Labels - see Labelling.

It is possible to draw a Dimension that contains Dimension Points that are incompletely defined (for example, a DPPT with an unset/illegal DDNM). The Dimension is now treated as only containing the significant Dimension Points for both drawing the Dimension and for commands which rely on the drawn dimension (such as PLCL @ (see Figure 12:9 in Principal Attributes of Angular Dimensions) and DTOF @ (see Detail Attributes of Linear Dimensions).