Essay: Typical Cad Command Structure

All systems have their own command structure and this is almost invariably of the form:
(VERB> NOUN> <MODIFIERS) Data Part>
The verb defines the action which is to be performed, such as ‘draw’, ‘erase’, ‘drag’. The noun describes the object upon which the action is to be performed, examples being ‘line’, ‘arc’, ‘circle’. Modifiers, which are not always needed, define how the action is to be performed, examples are ‘vertical’, ‘tangential to’, ‘parallel with’.
The Data Part of the command describes where on the drawing the action is to be performed —for instance, ‘X=2, Y=3’. Our three sample systems have this sort of command structure. In AutoCAD, the command DRAW CIRCLE CEN, DIA 2, 3.5 2 instructs the system to draw a circle of diameter 2 units at centre X=2, Y=3.5. In Personal Designer, the command INSERT CIRCLE D2:X 2 Y 3.5 does the same thing.
If typing such a command were the only way that a user could drive the drafting system then, because of the number of keystrokes required, drafting would be intolerably slow. Also because today’s systems are still of an unforgiving nature, the process would be error-prone.
It is fortunately possible to enter parts of commands in a variety of ways this has the adverse effect of obscuring the basic similarities between systems. Thus in Personal Designer, the user can pick, either from an on-screen menu or a tablet menu, the command sequence ‘INSERT CIRCLE’. In AutoCAD, commands can be selected in several ways. CADKEY permits the user to pick part commands by using the keyboard function keys -the sequence F1,F3,F2 would instruct the system to draw a circle of a given diameter and at a given center.It is largely in the convenience and power of command input that systems vary.
Menu organization differs from system to system. AutoCAD uses a normal menu which is displayed in a strip down the right hand side of the screen. A small root-menu is displayed first; this contains the main categories of commands, such as DRAW. If an item on the root menu is picked, then a further sub-menu is displayed and so on. A pull-down menu system is also available. Many users are now accustomed to using such systems where a list of items is shown, usually at the top of the screen, and if one of these is picked then a dependent sub-menu is displayed either in alphanumeric or icon form. These certainly help new users but tend to irritate the more skilled.
AutoCAD and Personal Designer also provide iconized on-screen menus. Examples of the layout of on-screen menus are shown in Figures.When menu cards are used, the card is divided into rectangular areas; these are also usually iconized. Often, only a selection of the commands is printed on the menu card and many systems permit users to define their own menu layouts which they may specialize to their own working preferences and fields of application. Although many of the large scale systems use menu cards, they seem to be giving way to on-screen menus for general use. Invariably, users can bypass the menu by typing commands and skilled users will develop a strategy for using a combination of methods of command input that suits their individual style.
AutoCAD default screen
Personal designer default screen
Cadkey default screen
1.5 Wire Frame Modeling
Wire frame model:
A wire frame model consist of points and curves only, and looks as if its made up with a bunch of wires. This is the simplest CAD model of an object. Advantages of this type of model include ease of creation and low level hardware and software requirements. Additionally, the data storage requirement is low.
The main disadvantage of a wire frame model is that it can be very confusing to visualize. For example, a blind hole in a box may look like a solid cylinder, as shown in the figure.
Wire frame model
In spite of its ambiguity, a wire frame model is still the most preferred form, because it can be created quickly and easily to verify a concept of an object. The wire frame model creation is somewhat similar to drawing a sketch by hand to communicate or conceptualize an object. As stated earlier, a wire frame model is created using points and curves only.
Advantages of Solid Modeling:
Unlike wire frames and surface representations which contain only geometrical data, the solid model uses topological information in addition to the geometrical information to represent the object unambiguously and completely. Solid model results in accurate design, helps to further the goal of CAD/ CAM like CIM, Flexible manufacturing leading to better automation of the manufacturing process.
Geometry: The graphical information of dimension, length, angle, area and transformations.
Topology: The invisible information about the connectivity, neighborhood, associatively etc.
Disadvantages of Wire frame Modeling:
• Ambiguity
• Subjective human interpretation
• Complex objects with many edges become confusing
• Lengthy and verbose to define
• Not possible to calculate Volume and Mass properties, NC tool path, cross sectioning etc.