Part 4: Meet the Natives - Simple 2D Objects

The CAD Drawing Components

You create a CAD drawing by drawing individual components often referred to as objects or entities. They make up such lines, arcs, dimensions and symbols. In drawing any component of the drawing, you need to access a particular drawing commands within the software program you are using.

The figure below illustrates the basic drawing components of CAD. There are separate commands to draw text, dimensions, patterns, symbols, and other objects in the drawing. The command(s) can be entered by typing it in the command line, selecting it from the menu, or by clicking on a specific tool button.

Circles

The next geometric entity is the circle. Most often indicating opening, holes, shafts, etc. It is defined by X,Y coordinate for center and a radius R. You snap to existing geometry for the center point with recognition of: intersections, midpoints, etc. and also snap to a point on the edge of the circle. You can create fillet geometry be selecting 2 existing lines (not necessarily perpendicular to each other), entering a radius, and then selecting from many choices within CAD program. You can also select 3 existing lines that have tangency with the desired circle.

There are many different ways to construct circles using a CAD system. See figure below.

•Selecting the center of the circle and then specifying the radius.
•Selecting the center and one point on the edge of the circle.
•Selecting 3 points on the edge of the circle.
•Selecting 2 points on the edge and then a radius.
• other ways . . .

Lines

Lines are probably the most used and important geometric entity in the CAD system. They are used for the object within the drawing. These object or “geometry” lines will be for straight edges of the part and for locations where a curved surface meets a flat surface. Generally a line is straight. In 2D CAD, it can be assumed that these lines are going to just lie in a X,Y plane.

The line geometry are defined by two X,Y coordinates (See figure below). You snap to existing geometry for the points with recognition of: intersections, midpoints, centerpoints, etc. You can then snap to enter X and Y for the start point, then enter an angle or slope value or direction for the line parallel or perpendicular to the existing geometry.

Summary:

CAD allows the user to create simple 2-D objects. These basic objects may consists of lines, rectangles, regular polygons, circles, ellipses, arcs, dimensions, and text.

Arcs

CAD provides many ways to draw arcs. Arcs are drawn so accurately that a number of engineering problems can be solved graphically rather than mathematically. Suppose you need to measure the circumference of an arc, just pick it and the exact value is displayed.

NOTE: The arcs are drawn in a counter-clockwise direction from starting point to end point.

Arcs are created by specifying various combinations of endpoint, center, start point, radius, angle, chord length and direction values. See the following figures below.

Flexible Curves aka “Splines”

CAD allows you to create flexible curves (often called splines) that can be used to draw almost any shape. A spline is a smooth curve that passes through or near a set of points that influence the shape of the curve. See figure below.

Ellipses

A common geometric entity is the ellipse. The ellipse is basically oval-shaped. It is usually described by a major axis and a minor axis. The major axis is the longer or stretched distance, the minor axis is the shorter or compressed distance. The geometry is defined by X,Y coordinates for center, direction and length of the X-axis, and length of the Y-axis direction of ellipse. You then enter the X- and Y- values directly for the center and axis lengths. Or you can snap to existing geometry for the center point with recognition of: intersections, midpoints, etc. and select a length for the major and minor axis. See figure below.

Check out the following examples of 2D object drawings

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Part 4: Meet the Natives - Simple 2D Objects