8-1 Unit 8 Drawing Accurately OVERVIEW When you attempt to pick points on the screen, you may have difficulty locating an exact position without some type of help. Typing the point coordinates is one method. AutoCAD has several other methods to help control the movement of the cross-hair. Another method is the snap function introduced in Unit 2, Creating Your First Drawing. This unit introduces another method for accurately locating precise points: the object snap, or osnap, function. OBJECTIVES Understand the relationship between object points and object snap Draw with the temporary object snap modes Draw with the running object snap modes Control the aperture size for object snap INTRODUCTION A competent draftsperson can locate a point on a drawing by using a straight edge and rule with a fair amount of accuracy. Using the point as a reference, the draftsperson can draw more objects. One of the first steps in creating accurate drawings is to locate precise points. One method of locating precise points in AutoCAD is to type the coordinates, as you did in Unit 4, Basic CAD Drawing Techniques In most cases, however, you do not have such complete information that you can type all the points. Regardless of the information available, typing points is tedious work and prone to typing errors. The object snap, or osnap function is another method of accurately locating precise points. Object snap is a very useful tool. The term object snap refers to a function in which the crosshairs are forced to snap exactly to a specific point or location of an existing object. One of the primary advantages of using object snap is that you don t have to pick an exact point. Suppose you want to draw a line beginning at the exact intersection of two previously created lines. If the intersection point of the lines is not on the snap increment, you may try to guess and pick the intersection point with the crosshairs. Unfortunately, you ll probably miss. Using object snap, you just pick somewhere near the intersection of the lines. The crosshairs automatically snap to the exact intersection. AutoCAD offers several object snap modes to help locate specific points on existing geometry accurately.
WORKING WITH ENTITY POINTS AND OBJECT SNAP 8-2 When you create geometry in AutoCAD, vectors are used to display the objects drawn on the graphics screen. A vector may be defined as a quantity completely specified by a magnitude and a direction. Using vectors allows AutoCAD to store a great deal of information about each object. A line, for example, has a starting point, ending point, and specific length; and the line points in a certain direction. Different types of objects have different types of information associated with them. A circle, for example, has a center point and radius. The vector information stored with each object enables AutoCAD to perform calculations on the object. By accessing the information stored with each object, you can locate endpoints, midpoints, intersections, center points, and other geometric information. The object snap function allows you to access this information and select these exact points when creating geometry. It is very important that the new user of AutoCAD understand it is never accurate to just guess or eyeball locations and points. What may appear accurate on the screen will probably be inaccurate when plotted, and will definitely be inaccurate in the drawing database. Generally, the plotting device has a higher level of precision than the computer display and therefore can display mistakes that otherwise seemed fine on the computer display. FOR THE PROFESSIONAL It is very important to maintain an accurate computer database for all of your drawings. When you begin creating a drawing on the computer, it is very important that information be entered accurately. Lines that don t meet and circles that are not tangent can have grave consequences. If a CAD drawing was to be used later to create a toolpath for a Numerical Control machine, or used to calculate a bill of materials, integrity of the drawing database is essential. Just because a drawing looks good on the screen and plots correctly is no guarantee it is accurate. The professional CAD draftsperson will use the SNAP function, coordinate entry, and object snaps to create accurate drawings. Before discussing the object snap modes, you should learn about the objects on which you ll use the object snap modes. Figure 8.1 shows the most common graphic objects and their potential snap points. Understanding Object Points and Object Snap Before discussing the Object Snap modes you should learn about the objects on which you ll use the Object Snap modes. Figure 8.1 shows the most common graphic objects and their potential snap points. Fig. 8.1 Object snap points on eight common graphic objects.
8-3 SKILL BUILDER Object snaps are not commands but modes used in connection with AutoCAD drawing and editing functions. Entering int or cen at the Command: prompt displays an error message. FOR THE PROFESSIONAL Object snaps can also be picked from the cursor menu, by holding down the SHIFT key while using the right button on the pointing device. The cursor menu appears at the location of the crosshairs. AutoCAD has 13 different object snap modes. The next sections explain each object snap and provides examples. Each object snap can be abbreviated to its first three letters. Notice that the first three letters are capitalized in the following sections. These abbreviations are the only letters you need to enter to access the corresponding object snap mode. The following section explains the service performed by each object snap mode. An alternative to typing the first three letters is to select the corresponding object snap button. You can find the object snap buttons by right-clicking on the Object Snap button located at the bottom of the drawing window. When you right-click on the Object Snap button, a fly-out menu appears, displaying all of the object snap modes. The fly-out menu is shown in figure 8.2. Fig. 8.2 Right-clicking on the Object Snap button on the status bar shows all the object snap modes in a fly-out menu.
8-4 ENDPoint A common task when you are creating geometry is to connect a line, arc, or center point of a circle to the endpoint of an existing line or arc. To select the endpoint, move the aperture box past the midpoint of the line or arc toward the end you want to pick and select. Figure 8.3 snaps a new line to the endpoint of an existing arc. Fig. 8.3 Object snap a new line to the endpoint of an arc. MIDpoint To find the midpoint of a line or arc, use the MIDpoint object snap mode. For example, to connect a line from any point to the midpoint of an arc, place the aperture anywhere on the arc and pick. The line automatically snaps to the midpoint of the arc (see fig. 8.4). You can also snap to the midpoint of a line (see fig. 8.4). Notice that you don t need to pick anywhere near the midpoint of the arc or line to snap to the midpoint. Fig. 8.4 Object snap to the midpoint of a line or arc. Fig. 8.5 Object snap to the center of a circle or arc. CENter The CENter option enables you to snap to the center point of an arc, circle, or donut. Figure 8.5 shows a line snapped between the center of circle A and the center of arc B. To select the center point, you must select the arc or circle itself, not the center.
Fig. 8.6 Object snap to a point object, referred to as a node. 8-5 NODe In addition to drawing geometry such as lines, circles, and arcs, you can define point objects in AutoCAD. A point object is referred to as a node. You can use point objects singularly or with other commands such as DIVIDE and MEASURE. Unit 12, Advanced Drawing Techniques, covers point creation and the use of points with the DIVIDE and MEASURE commands. Using the NODe object snap finds a point object (see fig. 8.6). Fig. 8.7 Using object snap QUAdrant to locate one of the quadrants on a circle. QUAdrant AutoCAD defines a quadrant as a quarter section of a circle, donut, or arc. Using the QUAdrant object snap mode, you can find the 0, 90, 180, and 270 degree positions on a circle, donut, or arc (see fig. 8.7). When snapping to a quadrant, locate the aperture on the circle, donut, or arc closest to the proposed quadrant. Figure 8.7 shows the endpoint of a line located at one of the quadrants in circle A. Fig. 8.8 Using the INTersection object snap mode. INTersection The INTersection object snap mode snaps to the point where objects cross each other. The INTersection mode also allows you to snap to the imaginary intersection of two objects that do not actually intersect but would if one or both were extended. The INTersection object snap mode requires that there be a point (real or imaginary) in two or three-dimensional space where an intersection occurs. To select an intersection point with one pick, the intersection of the two objects must be inside the aperture box Figure 8.8 shows snapping line C to the intersection point of circle A and arc B. You can also select the intersection of objects one at a time. This method works well when geometry is crowded together and snapping to an intersection point is difficult. When you use INTersection and select a single object, AutoCAD prompts you to select the second object. The crosshairs will snap to the intersection of the two objects selected or to the point where they would intersect if one or both were extended.
Fig. 8.9 Using the INSertion object snap mode to snap to the insertion point of text. 8-6 INSertion When you create text in AutoCAD, you must first specify its location. The location point where you initially create the text is the insertion point. Other AutoCAD objects that have insertion points are shapes, blocks, and attributes. Figure 8.9 shows a snap to the insertion point of text. PERpendicular A typical geometric construction is to snap a point on a line, arc, or circle that forms a perpendicular from the current point to the selected object. You can perform this step easily, using the PERpendicular object snap. Figure 8.10 shows how the PERpendicular object snap mode is used to create a line perpendicular to line B. Fig. 8.10 Using the PERpendicular object snap option. Fig. 8.11 Using the TANgent object snap to draw a line tangent to a circle and arc. TANgent Locating tangent points on arcs and circles can be difficult if you use manual drafting procedures. AutoCAD greatly simplifies this process by providing the TANgent object snap mode. TANgent enables you to snap to a point on an object that forms a tangent between the object and another point. Figure 8.11 shows a new line tangent to arc A and circle B. SKILL BUILDER AutoCAD sometimes has problems finding a tangent point unless you are zoomed in close to the selected object.
Fig. 8.12 Using the NEArest object snap mode to draw a line to an arc. 8-7 NEArest The NEArest object snap mode snaps to a point on a line, circle, arc, or other object that is nearest to the center of the target box, where the crosshairs intersect. Figure 8.12 shows how NEArest is used to connect a line from a point to a location on an arc. APParent intersection. This Object Snap snaps to the APParent intersection of two objects which mayor may not intersect in 3-D space. For APParent intersection to work in 3-D space, the objects must appear to intersect from the current viewpoint. PARallel Snap to PARallel remembers the angle of a selected line and lets you track along that angle. The PARallel snap places a small yellow cross (by default) somewhere on the object (but not at the end of the object). Use the following procedure to use the PARallel object snap: 1. Start the line command. Make sure there is at least one other linear object to snap to. 2. At the Specify first point: prompt, select the beginning of the line as shown in Figure 8.14. 3. Pick the PARallel object snap. 4. Move the cursor over the line you want your new line to be parallel to. A marker in the shape of a small cross will appear on the linear object. 5. As you move your cursor to a point that would be near parallel, a tracking line will appear along with the PARallel object snap icon. 6. Enter a length for the new line segment, or pick a second point. Fig. 8.13 PARallel snap remembers the angle of a selected line and lets you track along that angle.
DRAWING WITH OBJECT SNAP MODES 8-8 Object snaps are essential part of creating accurate drawings. Object Snaps allow you to specify precise locations on existing objects. You can specify an object snap whenever you have a command activated and are prompted for a point. By default, a marker and a tooltip are displayed when you move the cursor over an object snap location on an object. This feature, called AutoSnap, provides visual confirmation that indicates which object snap is in effect. The marker for an endpoint object snap is a square as shown in Figure 8.14. There are two main types of Snap modes: Temporary and Running. Fig. 8.14 The Drafting Settings dialog box can be used to set different running object snaps. A Temporary Object Snap mode is used for one selection only. A Running Object Snap mode has selected Snap mode(s) activated every time you make a selection. Setting Running Snap Modes If you need to use one or more object snaps on a regular basis you can turn on running object snaps. If you repeatedly need to locate the endpoint of an existing line, for example, you could set an endpoint object snap. You can select one or more running object snaps on the Object Snaps tab in the Drafting Settings dialog box shown in Figure 8.15. Once you have selected the Object Snap modes click the object snap button on the status bar or press F3 to turn running object snaps on and off. If several running object snaps are activated more than one object snap may be eligible at a given location. You can press Tab to cycle through the possibilities before you specify the point. Running snap modes are set thorugh the Object Snap tab on the Drafting Settings dialog box. You can access the Object Snap tab in the Drafting Settings dialog box by: Right-click on the Object Snap button located in the status bar and selecting Settings in the pop-up menu. Type osnap at the Command line. Select the Tools/Drafting Settings pull-down menu. In the Drafting Setting dialog box select the Object Snap tab. Fig. 8.15 The Drafting Settings dialog box can be used to set different running object snaps.
8-9 Setting Temporary (Interrupt) Snap Modes Temporary, or Interrupt snap modes are available for one selection only. When AutoCAD prompts you to indicate a point, you can use a temporary Object Snap mode to help you locate that point precisely on existing geometry. Selecting a temporary object snap mode will override any running object snap modes. When you use one of these methods, the Object Snap mode is activated for one selection only. Enter the first three letters at a prompt line during a command sequence Hold the SHIFT key down and right-click to access the Object Snap pop-up menu. Right-clicking on the Object Snap button on the status bar to access the Object Snap pop-up menu. Tutorial 8.1: Using Temporary Object Snap Modes to Create a Circle Top Window. In this tutorial, you will use temporary Object Snaps to create the circle-top window shown in Figure 8.16. Because this symbol is for an architectural drawing, you need to change the units. After you have completed the drawing, save the file as CTWINDOW. 1. Click the Application menu, then select New/Drawing. In the Create New Drawing dialog box select the Start from Scratch button, Imperial (feet and inches). 2. From the Format pull-down menu, select Units. The Units Control dialog box appears. Change the units to architectural with a precision of 0-0. 3. Draw the first horizontal and vertical line. Select the Line button from the Draw/Home ribbon. Specify first point: 2,3 Specify next point or [undo]: 5 <TAB>,0 Specify next point or [undo]: 5 <TAB>,90 Specify next point or [undo]: press Enter 4. Use the zoom command to adjust your display to see the lines. Fig 8.16 Drawing a circle-top window using temporary Object Snap modes. 5. Draw the second horizontal line. Select Line from the Draw/Home ribbon. Specify first point: 7,3 Specify next point or [undo]: 5 <TAB>,0 Specify next point or [undo]: press Enter Next you will draw the inner arc. 1. Select the Start Center End option from the Arc pull-down menu on the Draw/Home ribbon. <Start point>: mid AutoCAD is now waiting for the midpoint of an object. 2. Move the aperture box to the middle of line C,D and click the Pick button (see Figure 8.17). <Center>: int 3. Now select point C (refer to Figure 8.17). <End point>: mid 4. Move the aperture box to pick the middle of line A,C (refer to Figure 8.17). Your drawing should now look like that in Figure 8.18.
8-10 Fig 8.17 Use Object Snap to create the inner arc of the circle-top window. You will now draw the right half of the outer arc. 1. Select the Start Center End option from the Arc pull-down menu on the Draw/Home ribbon. <Start point>: end 2. AutoCAD is now waiting for an end of object selection. Pick point D, as shown in Figure 8.17. <Center>: int 3. Pick point C (refer to Figure 8.17). <End point>: end 4. Move the target box to point B and pick that point (refer to Figure 8.17). Fig 8.18 The circle-top window with the inner arc complete. Continue with the second (left) half of the outer arc. 1. Select the Start Center End option from the Arc pull-down menu on the Draw/Home ribbon. <Start point>: end 2. Move the target box to point B and pick that point, as shown in Figure 8.17, and click the Pick button. <Center>: int 3. Pick point C (refer to Figure 8.17). <End point>: end 4. Move the aperture box to point A and pick that point (refer to Figure 8.17). 5. The drawing should now look like that in Figure 8.19. Fig 8.19 The circle-top window with the outer arc complete.
To complete the tutorial you will draw the two diagonal lines. 1. Select Line from the Draw/Home ribbon. Specify first point: int 2. Move the aperture box to point C, as shown in Figure 8.20, and pick it. Specify next point or [undo]: mid 3. Select the middle of the outer arc at point E (see Figure 8.20). 4. At the next Specify next point or [undo]: prompt, press Enter to end the Line command. 5. Issue the line command again. Specify first point: cen 6. Move the aperture box to point F, and pick that point (see Figure 8.20). This selects the center of the arc. Specify next point or [undo]: mid 7. Pick point F (refer to Figure 8.22). 8-11 8. At the next Specify next point or [undo]: prompt, press Enter to end the command. 9. Select the Save button and name the drawing CTWINDOW. You have now completed the circle-top window. Figure 8.20 Drawing diagonal lines for the circle-top window. Setting the Object Snap Variables The Temporary and Running object snap modes have several variables you can set, available in the Drafting tab of the Options dialog box (see Figure 8.21). You can access the Drafting tab of the Options dialog box by: Select the Options button from the Object Snap tab on the Drafting Settings dialog box. Select Options from the Tools pull-down menu. This will access the Options dialog box where you can select the Drafting tab. Fig 8.21 The Drafting tab in the Options dialog box displays various settings that can be use to fine tune Object Snaps.
The variables in the Drafting tab of the Options dialog box that affect object snaps are described as follows: 8-12 Marker. When checked, a small geometric symbol is displayed on the object at the selected snap location when the cursor moves over the snap points of the object. The geometric symbol displayed on the object reflects the type of Object Snap. Figure 8.14 shows the different geometric figures for each of the object snaps. For example the geometric figure for Endpoint Object Snap is a square, Midpoint is a triangle, and so on. Magnet This causes an automatic movement of the cursor that locks the cursor onto the selected snap point. Display AutoSnap tooltip When the tooltip is activated, a small flag appears that describes the part of the object you are trying to snap to. In Figure 8.14, the Endpoint flag is displayed near the crosshair. Display AutoSnap Aperture Box Off by default, this option causes a box to appear at the center of the cursor after activating one or more object snaps. AutoSnap Marker size Moving the Marker s1ider increases or decreases the size of the geometric symbol, or marker, that is displayed on the object at the selected snap location. AutoSnap Marker Color This option accesses a drop-down menu that allows you to change the color of the marker. Controlling Aperture Box Size The aperture box appears on-screen when you use any object snap mode. You can enlarge or reduce the size of the aperture box. Do not confuse the aperture box with the pickbox. The aperture box appears on-screen only when object snap modes are used. The pickbox appears on-screen when you issue any command that activates the Select objects: prompt. The pickbox is displayed in place of the crosshairs. Enlarging the aperture box makes it cover a wider area, enabling you to place and pick objects more easily. Enlarging the aperture box makes it cover more potential pick points. However, these enlargements can force AutoCAD to process the information longer before determining the best point. When working on a large drawing, using a large aperture box can slow down the computer significantly. Reducing the size of the aperture box makes it cover a narrower area. This lessens the potential pick process, speeding up AutoCAD s selection of the best pick point. The disadvantage to a small aperture box is that you must place it more precisely because it covers less area. Try experimenting on your own to see how changing the target box size affects point selection. You can change the aperture box size with one of two methods: At the Command: prompt, enter aperture, or at any prompt enter aperture. Enter the aperture box size in pixels. You are limited to a number between 1 and 50 pixels. In the Drafting tab of the Options dialog box, move the scroll bar in Aperture Size between Min and Max, watching the sample target box change
8-13 Tutorial 8.2: Using Running Object Snap Modes to Create a Divided Octagon In this tutorial, you will use running Object Snaps to create the divided octagon shown in Figure 8.22. To create the octagon, follow these steps: 1. Click the Application menu, then select Open/Drawing. Open drawing 08dwg01. You should now have the drawing shown in Figure 8.23 in your drawing area. 2. Type osnap at the Command line to access the Drafting Settings dialog box. The Object Snap tab should be displayed. 3. In the Object Snap tab, choose Endpoint. This sets the running Object Snap to the END of the Object Snap setting. Make sure Object Snap On is checked. Your dialog box should look like Figure 8.24. 4. Choose the Line button from the Draw/Home ribbon. Fig 8.22 Draw a divided octagon using running Object Snap modes. 5. At Specify first point: prompt, move the aperture box and pick point A (see Figure 8.25). 6. At Specify next point or [undo]: prompt, move the aperture box and pick point B. 7. Continue responding to the Specify next point or [undo] : prompts by moving the aperture box and picking each point (refer to Figure 8.25). 8. After you finish going all the way around and return to point A, respond to the next Specify next point or [undo]: prompt by pressing Enter. You now have the divided octagon shown previously in Figure 8.22. Fig 8.23 Open the 08DWGO1 drawing in preparation for creating a divided octagon. Fig 8.24 Select Endpoint object snap in the Drafting Settings dialog box. Fig 8.25 Select the endpoints to create the divided polygon.
8-14 Using Multiple Running Object Snap Modes In addition to using single Object Snap modes, you can direct AutoCAD to use multiple Object Snap modes. When you use multiple Object Snap modes, AutoCAD chooses the point closest to the center of the pickbox that meets the criteria of one of the set Object Snap modes. You can activate multiple Object Snap by accessing the Osnap tab in the Drafting Settings dialog box (refer to Figure 8.24). When you have the dialog box displayed, select the Object Snap modes you want. FOR THE PROFESSIONAL Initially it seems like a good idea to simply check all of the Object Snap modes and have all of them available when creating geometry. When you select geometry in AutoCAD by using object snap modes, AutoCAD searches for the best solution to your request. In a simple drawing, this process happens very quickly. As drawings become more complex, however, finding the object snap may take some time, especially if there is a lot of geometry close together. Activate only the running object snaps that will be used the most in a given situation. You can always use quick object snaps in special situations. Tutorial 8.3: Using Multiple Running Object Snap Modes In this tutorial you will work with multiple running object snap modes. Follow these steps: 1. Click the Application menu, then select New/Drawing. In the Create New Drawing dialog box select the Start from Scratch button, Imperial (feet and inches). 2. At the Specify first point: prompt, move the crosshair to the lower-left portion of the screen and randomly pick a point near the location of A (see Figure 8.26). 3. At the Specify next point or [undo]: prompt, move the crosshair to the upper-right portion of the drawing area and randomly pick point B. 4. At the Specify next point or [undo]: prompt, press Enter to exit the command. You should now have a line similar to that in Figure 8.26. 5. Right-click on the OSNAP button located at the bottom of the drawing window. In the pop-up menu select Settings. 6. Make sure you have the Object Snap tab selected. Select the Endpoint and Midpoint options, make sure Object Snap On is selected and click OK (see Figure 8.27). 7. Choose the Line button from the Draw toolbar. 8. At the Specify first point: prompt, move the crosshair close to point B (see Figure 8.26). When the marker appears, pick that point. 9. Now move your crosshair away from the line. Note that the line begins at the exact endpoint of the previous line. 10. At the Specify next point or [undo]: prompt, enter u to undo your last selection. 11. The From point: prompt appears again. This time, instead of selecting the endpoint, move your crosshair toward the center of the line. Note that a triangular marker automatically appears. Pick the point. Now move your crosshair away from the line. This time, note that the line begins at the middle of the previous line. 12. At the Specify next point or [undo]: prompt, press Enter to exit the line command. This completes the Multiple Running Object Snap Modes tutorial.
8-15 Fig 8.26 Creating line A,B. Fig 8.27 Select Endpoint and Midpoint Object Snap modes in the Drafting Settings dialog box. SUMMARY Proper use of Object Snaps increases the accuracy of your drawings as well as simplifies the creation of geometry. Object Snaps ensure that no points are guesses and all points are precise. When used with coordinates, Object Snaps will help you create drawings that are accurate and useful. Running Object Snaps are active whenever you are prompted for a point. By default, a marker and a tooltip are displayed when you move the cursor over an object snap location on an object. Running Object Snaps are set in the Object Snap tab of the Drafting Settings dialog box. Running Object Snaps can be turned off by selecting the OSNAP button at the bottom of the drawing window, or pressing the F3 key. Temporary, or Interrupt Object Snaps are active for one operation only. An interrupt object snap overrides any active running object snaps. Interrupt Object Snaps can be activated for one operation by typing the first three letters of the Object Snap, holding the SHIFT key down and pressing the right mouse button, or right-clicking on the OSNAP button. Keep in mind Object Snaps work off of existing geometry. Object Snaps are only as accurate as the integrity of the geometry they are snapping to.
EXERCISES Fig 8.28 Loading the Osnap drawing. 8-16 Load drawing file ex8-1.dwg shown in Figure 8.28. Use the appropriate Object Snap modes to complete the drawing as shown in Figure 8.29. Fig 8.29 Completed Osnap drawing.