Computer Aided Draughting 411
dharm
d:\N-Design\Des21-1.pm5
Command: TEXT
Start point or Align/Centre/Fit/Middle/Right/Style:
By choosing:
Start point — Left justifies the text base line at the designated point.
A (Align) — prompts for two end points of the base line and adjusts overall character
size so that text just fits between these points.
C (Centre) — asks for a point and centers the text base line at that point.
F (Fit) — similar to ‘align’, but uses a specified fixed height.
M (Middle) — like ‘centre’, but centers the text both horizontally and vertically at the
designated middle point.
R (Right) — right justifies the text base line at that point.
S (Style) — asks for a new text style.
Null reply — places the new text directly below the highlighted text.
21.9 TYPES OF MODELLING
Geometric modelling provides a means of
representing part geometry in graphical
representation. This constitutes the most
important and complex part, in many
software packages. There are a variety of
modelling methods available in the industry
for the variety of functions. They are:
21.9.1 2D Wire Frame
This method consists of a range of 2D shapes
which can be used to develop basically the
outline of a part, which in most of the cases
is composed of lines and circles (Fig. 21.12), this is the easiest and most popular way to model
simple parts. They are easy to understand.
21.9.2 3D Wire Frame
This is similar to its 2D counter-part, except that it is drawn in 3 dimensions.
This is used in low cost designing systems. The complete object is represented
by a number of lines with their end point co-ordinates (x, y, z) and their
connectivity relationships. It is difficult to understand the outside of the solid,
represented by the wire frame model. Thus, the wire frame model is inadequate
for representing the more complex solids (Fig. 21.13).
21.9.3 Surface Modelling
It is constructed essentially from surfaces such as planes, rotated curved
surfaces and even very complex surfaces. These models are capable of
representing the solid, from the manufacturing point of view. No information
regarding the interior of the solid model could be available.
21.9.4 Solid Modelling
The best method for three dimensional solid construction is the solid modelling technique, often
called constructive solid geometry. In this, a number of 3 dimensional solids are provided as
primitives. From these solid primitives, the complex objects may be created by adding or subtracting
Fig. 21.12Fig. 21.13