Cad Programming

CAD programming

CAD systems are powerful software applications. They employ complex geometric modeling concepts, computer graphics algorithms, software techniques, design and analysis theories, manufacturing techniques, and database and management concepts. CAD software of any commercial system has been evolving over the years. The code itself is very long, complex, and difficult to follow.

Modifying and programming CAD software is a difficult, but not impossible, task. We can, and do, modify CAD software to meet our modeling needs. After all, it is engineers who work for CAD vendors and write the CAD software.

CAD software is written in a variety of languages. Legacy systems, such as Unigraphics and I-DEAS, started as Fortran code. Newer systems, such as Pro/E and SolidWorks, are written in C and C++. ACIS is written in C++ and also uses Scheme. Most of these systems use Java also in one form or another, especially for Web-based functionality.

How often do we need to program CAD systems? • In comparison to using the systems as they are, the need for programming is not great. This is not to say that customizing these systems is hardly needed. There are different levels of customization and programming, ranging from writing a simple macro to automate repetitive tasks to using the system API (application programming interface) functions to extend the system functionality.

Users who are interested in programming their CAD systems must be familiar with system functionality, structure, hierarchy, terminology, and philosophy. Such familiarity is a precursor to being able to use the programming tools and functions offered by a system. For example, those who want to modify a spline curve by programming it would have an easier time doing so if they have used the system spline command already.

What does it take to program a CAD system? • We need to know a programming language, geometry and graphics concepts, and database techniques. We may need more or less of each of these, depending on the programming or customization task at hand. For a programming language, C, C++, Visual basic or Java is good.

Visual Basic

C++

Relational and Object Databases • CAD systems utilize relational databases to store geometric, graphical, and other data that defines a CAD model. A model database is stored in the model or part file.

• A relational database is conceptually a set or collection of tables. A table is considered a relation. A database with four tables is considered a database with four relations. Each table stores data about a given entity. The data in a table is organized or stored as attributes in rows. Each row in a table is known as a record.

• For example, a database of a bank may have a table of customers (the entity). Each customer has a row (record) in the table. The record could hold the customer name and account number (the data attributes). Entities can be thought of as objects with attributes.

Relationships between tables can be of three types • one-to-one, one-to-many, and many-to-one. • In a one-to-one relationship, there is only one record in the second table for each record in the first table. In the one-to-many relationship, multiple records exist in the second table for each record in the first table. The one-to-many relationship is sometimes known as a parentchild relationship. The many-to-one relationship is opposite to the one-to-many relationship; for multiple records in the first table, only one record exists in the second table.

• SQL (Structured Query Language) is the language commonly used to query and update relational databases. • SQL can be used to retrieve, sort, and filter specific data of a database. SQL also allows users to define data in databases and later manipulate it. SQL was developed by IBM. • SQL uses set logic to query databases. It is an easy language to learn; it has an English-like syntax. • It requires few keywords and clauses to perform powerful data retrieval from the database. • Report formatting is also simple using SQL. Sample statements include CREATE DATABASE, CREATE TABLE, SELECT, ALTER, INSERT, DELETE, and UPDATE.

• The query SELECT FirstName, LastName FROM CustomerName; returns the two columns labeled FirstName and LastName in the table CustomerName as the query result. Note that the SELECT statement must end with a semicolon, as all SQL statements end with a semicolon. • We must create a database before we can use it. We need a database schema that defines the types of variables used in the database records in each table. We use this schema to create each table of the database. The following screenshot (of Microsoft Access) shows the definition of a schema for a bank database table.

CAD systems utilize various types of data structures and databases to store CAD menu and texts and their related data. Many systems use relational databases to store model data. They store the boundary representation of the model, that is, the faces, edges, and vertices of the model. In such as a database, we use three relational tables: Faces, Edges, and Vertices as shown in Figure 5.1. The Faces table links (points) to the Edges table, and the Edges table links (points) to the Vertices table. Each of the three tables has one record per entity (face, edge, or vertex). Each record has the geometric definition of each entity and other information, such as color and layer, that we do not show in the tables.

Object Definition • The basic unit of thinking, designing, building, and programming code is the object. Let us consider a define what an object is. If we think of what characterizes a car, we come up with make, model, year, color, drive, stop, maintain, and so forth. We can further divide these characteristics into two groups: attributes (properties) and behaviors. The attributes are make, model, year, and color. An attribute is just a value that we specify; it does not require any calculations — the year is 2005 and the color is red. Object attributes become its variables in programming.

The behaviors are drive, stop, and maintain. A behavior is doing something; it takes input and produces output. Driving a car requires a sequence of steps. Think of a behavior as an algorithm. Object behaviors become its methods in OOP. A method is a function associated with an object.

• An object is defined as an entity (construct) that has attributes and behaviors. The attributes are its variables, and the behaviors are its methods. • CAD programming utilizes OOP languages. We use these languages to create user-defined objects, or we use their built-in (predefined) objects provided by CAD systems. Understanding objects is a prerequisite to creating one's (custom) objects or using existing (predefined) ones.

Define objects. Write the definition of a spur gear Each of these models is an object that has attributes and behaviors. Table 5.1 shows some of them. It also shows their mapping to OOP variables and methods. • Spur gear object = {{root diameter, base diameter, addendum diameter, thickness, number of teeth}, {rotate, maintain}}

Figure 2.7

Parametric Design and Objects The philosophy of parametric modeling and design goes hand-in-hand with OOP. As shown in Figure 2.7, a parametric solid model is defined by its parameters P1 to P4, R, and an extrusion thickness T (not shown in the figure). The different values of these six parameters define a family of parts; each part in the family has specific values for the six parameters (variables). A CAD designer creates the solid sketch shown in Figure 2.7. When the designer sets the values of the parameters and regenerates the solid, a new part in the family is created. The six parameters define the attributes (variables) of the class. In this case, there are no methods for the class. Each part in the family is an instance of the class. Each part has specific values for the six parameters.

• Each time a designer assigns specific values to the six parameters of the Extrude class to create a part, the CAD system creates an instance — extrude1, extrude2, and so forth. When the designer deletes the part, the CAD system deletes the instance. However, the Extrude class is still there and can be instantiated to create a new part. • When the CAD designer clicks a parameter to assign it a new value (dimension), the designer is using the object concept of membership access. For example, selecting P1 of the instance extrude1 is equivalent to calling the extrude1. setPl { } method. Changing the current value of P1 to, for example, 5, and hitting Enter or OK is equivalent to calling extrude1.setPl(5).

Programming Levels The CAD programming can be divide into two groups. a. The first group is programming the threedimensional geometric and graphics concepts. b. The second group is programming the CAD systems themselves. Programming the first group is a prerequisite to programming the second group because we need to learn the programming concepts without having to deal with the high level of sophistication of the programming interfaces of CAD systems and the complex data structure and large size.

We can use one of two approaches to program the first group: programming languages or symbolic equation solvers. The approach of programming languages requires a programming language such as C, C++, or Java, and a rendering system to display the modeling and graphics results in 3D space. Two major rendering systems exist: OpenGL and Java 3D. The OpenGL-based system is used with C or C++. We write a C or C++ program to code the geometric and graphics concepts, and we call functions from the OpenGL library to render the results.

• The approach of symbolic equation solvers requires software packages that allow us to manipulate and solve the equations of geometric modeling and graphics concepts. These symbolic equation solvers include MATLAB, Maple, Mathcad, and Mathematica. The use of symbolic equation solvers is quite common in engineering practice. These solvers are used for teaching and research in various disciplines in academia and industry. • A symbolic equation solver solves differential and integral equations and performs linear algebra, including vector manipulations. It also solves matrix (system of) equations and performs matrix computations such as adding, multiplying, and inverting matrices.

• The second set of CAD programming tasks involves programming CAD systems themselves. This group builds on what we learned about programming the geometry and graphics.

Macros • A macro is a simple string of commands that records a sequence of actions that a designer performs from the time the designer turns on the macro command on a CAD system until it is turned off. The macro is saved in a file that can be played back to execute the sequence of commands. Macros are good to use to automate repetitive design tasks. Some macros can be edited to change parameters of a feature, thus enabling the designer to create a family of parts. • Different CAD systems use different languages to create macro files. For example, SolidWorks uses VB (Visual Basic) to record macros. We usually perform three activities: create, run, and edit. We first create a macro, and then run it. If necessary, we edit the macro file to make changes, save the file and run it again. Think of a macro as a simple computer program. Moreover, CAD systems allow their users to create a button to access the macro.

Example: Create, run, and edit macros. Show an example of creating, running, and editing a macro.

• Follow these steps: 1. Create macro. Turn on the macro command to begin recording. For example, click this sequence, or its equivalent: Tools (menu on menu bar) => Macro => Record. 2. Perform activities. Begin geometric modeling as if macro were not on. For example, select sketch planes, draw a sketch, and extrude it to create a feature.

Start and create macros

Begin geometric modeling

3. Stop macro and save it. Stop the macro recording by turning the macro off. Click this sequence, or its equivalent: Tools (menu on menu bar) => Macro => Stop. A window opens up asking for a macro Filename and a directory to save the macro in. 4. Exit the part without saving. This way we can run the macro to test it.

Macros and its program

5. Run the macro. Open a new part file. Then run the macro. The macro should execute its sequence of commands and create the extruded feature in the open part file. For example, click this sequence, or its equivalent: Tools (menu on menu bar) => Macro => Run. A dialog box opens to allow the selection of the macro, in case more than one exists.

Run macros

6. Assign a toolbar button to a macro. If needed, follow the system directions to add a macro button to the CAD system toolbar. For example, click this sequence, or its equivalent: Tools (menu on menu bar) => Customize => Macros (tab). Select the desired macro from the popup window shown in the screen-shot at right. After the procedure is complete, the macro is added to the toolbar.

7. Edit macro. If necessary, a macro can be edited if we know the language it is written in. We open its file, edit it, save it, and run it again. For example, click this sequence, or its equivalent: Tools (menu on menu bar) = > Macro => Edit. After you select the macro to edit, its file opens up in an editor for editing. For example, SolidWorks uses Microsoft Visual Basic editor. Recognize the VB statements and the SolidWorks API calls. Other systems use system editors. For example, Pro/E uses the vi editor on Unix systems. Users who know the APIs well can produce the macro files by writing the code manually. *application programming interface (API)

This example shows how to deal with macros in a system Independent fashion. As the example shows, dealing with a macro on a CAD system is an easy task to accomplish.

Example. Hands-on exercise: Apply the preceding seven macro steps to your CAD system to record the creation of an extruded feature using a rectangular cross section. Change the macro to accept input from the user for the rectangle's width and height, as well as the extrusion depth. For example, CATIA lets users use VB forms to receive input from them.

Thank you