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# Geometry for Modeling and Design

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## 4.21 User Coordinate Systems

Most CAD systems allow you to create your own coordinate systems to aid in creating drawing geometry. These are often termed user coordinate systems (in AutoCAD, for example) or local coordinate systems, in contrast with the default coordinate system (sometimes called the world coordinate system or absolute coordinate system) that is used to store the model in the drawing database. To use many CAD commands effectively, you must know how to orient a user coordinate system.

Most CAD systems create primitive shapes the same way each time with respect to the current X-, Y-, and Z-directions. For example the circular shape of the cylinder is always in the current X-Y plane, as shown in Figure 4.75.

To create a cylinder oriented differently, create a user coordinate system in the desired orientation (Figure 4.76).

To create the hole perpendicular to the oblique surface shown in Figure 4.77, create a new local coordinate system aligned with the inclined surface. After you have specified the location of the hole using the more convenient local coordinate system, the CAD software translates the location of the hole to the world (default) coordinate system.

Many CAD systems have a command to define the plane for a user coordinate system by specifying three points. This is often an easy way to orient a new coordinate system—especially when it needs to align with an oblique or inclined surface. Other solid modeling systems allow the user to select an existing part surface on which to draw the new shape. This is analogous to setting the X-Y plane of the user coordinate system to coincide with the selected surface. With constraint-based modelers a “sketch plane” often is selected on which a basic shape is drawn that will be used to form a part feature. This defines a coordinate system for the sketch plane.

A user or local coordinate system is useful for creating geometry in a model. Changing the local coordinate system does not change the default coordinate system where the model data are stored.