Part 3 The Coordinate System

Format for Entering Coordinates

Cartesian or Polar coordinates values are entered in two formats: Absolute and Relative.

ABSOLUTE format

Is a way of measuring distances from a fixed reference location (Origin Point), which is the 0,0 location of the coordinate system. This point should always be considered fixed at all times. In some CAD programs this point remains visible at the left bottom corner of the drawing area. In some others it is invisible. Absolute coordinates are primarily used to adjust the alignment of diagrams in a drawing, to align one drawing with another or to make plotting adjustments.

RELATIVE format

Is a way of measuring distances from the last point entered. All measurements are taken the same way as the absolute coordinates, the only difference being that the relative coordinates are measured from the last point entered instead of the origin point. When a point is entered, it becomes the reference for entering the next point and so on. This method of measurement is often used for drawing because it is always convenient to place the drawing objects relative to each other rather than a fixed reference point.

(Note: Each CAD program may have slightly different methods to enter absolute or relative coordinates).

Three-Dimensional Coordinate System

Cartesian Coordinates in 3D

In addition to the X and Y you also have the Z-coordinate. 3D coordinates are measured using all three axis: X, Y, and Z. The axes meet at a point in the shape of a tripod. This point is called the origin point, which is the 0,0,0 location of all coordinates. All distances can be measured using this point as a reference. 3D capabilities allow you to draw pictorial views such as isometrics, oblique views and perspectives. The views drawn with CAD are very accurate and provide a lot of flexibility in terms of editing and display. You can rotate a model on the screen just like an actual model, and display views from different angles.

Oblique Views

Oblique views are the simplest form of the pictorial views that can be drawn by using parallel projection lines from an elevation. There are standards established to draw oblique views at specific angles. A common standard used is to draw an oblique view by projecting lines at 45 degrees.

Isometric Views

Isometric views are more realistic than oblique views. The object appears to be tilted at a 30 degree angle on both sides.

An isometric is defined by three plans called isoplanes:

•Top isoplane,
•Right isoplane and
•Left isoplane.

The right isoplane is drawn with 30 and 90 degree angles, the left isoplane with 150 and 90 degree angles and the top plane with 30 and 150 degree angles. All distances are measured using 1:1 scale (actual size) to show depth, width and height. You can use simple 2D functions and draw lines at specific angles to complete an isometric.

3D Modeling

CAD’s 3D modeling capabilities allow you to create 3D images that are as realistic as the actual objects. These images are called 3D models because, just like a physical model, they can be rotated on the screen. You can display views from a 3D model, such as isometrics or perspectives, from any angle with a few simple steps.

Types of categories

Wire-Frame Models

When you draw a model with lines and arcs, they are called wire-frame models. These models appear to be made of wires and everything in the background is visible. This does not create a very realistic effect.

Surface Models

Surface models are more realistic than wire-frame models. They are created by joining 3D surfaces rather then bare lines and arcs. A 3D surface is like a piece of paper and can have any dimension and be placed at any angle to define a shape. Just like a paper model, you join surfaces to form a surface model. The views displayed from these models are quite realistic, because everything in the background can be hidden.

Solid Models