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Unit 6: Creating Basic Geometry


It has been estimated that 95 percent of all machined workpieces have holes in them. Visible holes are often represented by circles on an orthographic drawing. Along with circles, Lines and Arcs make up the majority of geometry in a typical mechanical drawing.


· Drawing circles with the CIRCLE command. · Drawing arcs with two common ARC commands. · Drawing a polygons with the POLYGON command.


In this unit you will apply some of the basic AutoCAD commands to create a Square Head Bolt and Nut shown in figure 6.1. To do this, you will apply the CIRCLE, ARC, and POLYGON commands.

Drawing Circles (CIRCLE)

As mentioned previously, as many of 95 percent of all machined workpieces have holes in them. The CIRCLE command is commonly used to represent visible drilled, reamed, and bored holes in a part. A circle may also be used to represent a countersink and counterbore.

Tutorial 6.1: Beginning the Square Head Bolt

In this tutorial you will set-up the drawing, and create the circle used to represent the Square Head Bolt. Begin by creating a new drawing. 1. Choose New from the standard toolbar. In the New Drawing dialog box, name the drawing BOLT and press Enter. 2. Select the Layer button from the Object Properties toolbar, create a new layer using the following parameters and set it as the current layer: Layer Name DR-BOLT_-UC Color Red Linetype Continuous 1

Figure 6.1 The CIRCLE, ARC and POLYGON commands are used to create the Square Head Bolt and Nut.

3. Choose Format/Units. Set the units to Architectural, 1/16" Precision. 4. Make sure the Draw floating tool bar appears on your screen. Select the Circle button. 5. Enter the absolute coordinates for the center point for the circle and the diameter. <Center point>: 3,4-1/4 <Diameter>: 3/4 6. Select the Line button from the Draw toolbar and enter the following points: First point: 1-15/16,3 next point: @13/16<270 next point: @2-1/8<0 next point @13/16<90 next point: 7. Press the Enter key at the Command: prompt to re-access the Line command and enter the following points: First point: 3,2-3/16 next point: @13/16<90 8. Press the Zoom All button on the Standard toobar. 9. Press the Save button to save your drawing. The next tutorial will continue from here. Your drawing should look like figure 6.2

Figure 6.2 Creating lines and a circle for the Square Head Nut and Bolt.

Drawing Arcs (ARC)

In addition to holes, many mechanical parts contain rounded corners. While AutoCAD contains 11 different methods for drawing an arc, the most common are the three point, startcenter-end, and start-end-radius. While the 11 different options can seem overwhelming, all of the methods are based on the starting point, starting direction, center point, included angle, endpoint, lengths of cord, and radius of an arc. Before deciding which arc command to use, evaluate the arc you need to draw. Determine what you know about the arc, and match up the known information to one of the different options. The different icons available for arc appear on the fly-out menu. The information needed to create the particular arc appears at the bottom of the AutoCAD display. For example, when you hold the pointing device over the Arc Start Center End button , it says "Creates an arc using the start point, center and end point" at the bottom of the AutoCAD display. 2

Tutorial 6.2: Creating Arcs with 3 Point and Start, End Radius

Using two common methods of drawing an arc, you will put the rounded portion on the nut. 1. Continue from the previous tutorial. 2. Select the 3 Points option from the Draw/Arc pull-down menu. <Start point>: 1-15/16,3 <Second point>: 2-1/2,3-3/16 End point: 3,3 3. Select the Start End Radius option from the Draw/Arc pull-down menu. 4. Establish the points of the arc. <Start point>: 4-1/16,3 <End point>: 3,3 <radius>: 7/8 6. Select Tools/Drawing Aids to access the Drawing Aids dialog box. Set your snap to 1/16" and grid to ¼". 7. Select the Line button from the Draw toolbar, and draw the line across the top of the bolt. You should be able to snap to the correct coordinates. Your drawing should now look similar to figure 6.3. Fig. 6.3 Drawing arcs with 3 Point and Start, End, Radius.

Section 3: Drawing Polygons (POLYGON)

Many mechanical parts consist of triangles, squares, and hexagons. With AutoCAD's POLYGON command, you can easily create closed figures bounded by three or more line segments. With POLYGON, you can create a figure having anywhere from 3 to 1,024 sides. When you create a polygon, it is drawn as a single closed polyline object. If you need to edit the polygon, you must first explode it. You can access the EXPLODE command by: · · Select the Explode button from the Modify toolbar. Enter explode at the Command: prompt.

After accessing explode, AutoCAD prompts Select objects:. Pick the polygon you want to explode and press Enter. The polygon now consists of individual segments. 3

Tutorial 6.3: Specifying a Polygon by Edge

In this tutorial, you will complete the Square Head Bolt by specifying a polygon by edge. 1. Continue from the previous tutorial. 2. Select the Polygon button from the Draw toolbar. 3. At the Number of sides: prompt, enter 4. 4. Next, you will specify the Edge option, and locate two corners of the polygon's edge. Specify center of polygon or [Edge]: edge First endpoint of edge: 3,3-3/16 Second endpoint of edge: 4-1/16,4-1/4 5. You have completed the Square Head Bolt and Nut, and your drawing should look similar to figure 6.1 6. Select the Save button to save your drawing.

Unit Review

The geometry of most mechanical parts can be made using the LINE, ARC, and CIRCLE commands. To speed up the drawing process, especially when you are required to create multi-sided polygons, use the POLYGON command. At times the multiple options available for these commands can seem overwhelming. Before constructing geometry, evaluate the object you need to create. Match up the information you know about the object with the commands available. Select the commands that will allow you to create the desired object as quickly and accurately as possible. Holding the pointing device over an icon will display the name of the icon next to it, as well as displaying additional information on the linetypes.



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