So far, you’ve been working with polylines to generate spline curves. The advantage to using polylines for curves is that they can be enhanced in other ways. You can modify their width, for instance, or join several curves together. But at times you will need a more exact representation of a curve. The Splineobject, created with Draw> Spline, offers a more accurate model of spline curves, as well as more control over its shape.
Drawing a Spline
The next exercise demonstrates the creation of a spline curve.
1. Undo the changes made in the last two exercises,
2. Turn the Data layer on so you can view the data points. , .
3. Adjust your view so you can see all the data points with the elevation of 250.00.
4. Click the Spline tool on the Draw toolbar, or type Spl.J.
5 . At the Sped fy fi rst poi nt or [Obj ect] : prompt, use the Center Osnap to start the curve on the first data point in the lower-right comer . The prompt changes to Sped fy next poi nt or [Cl ose/Fi t
tolerance] <start tangent>:.
6. Continue to select the 250.00 data points until you reach the last one. Notice that as you pick points, a curve appears, and bends and flows as you move your cursor.
7. Once you’ve selected the last point, press .J. Notice that the prompt changes to Speci fy start tangent:. Also, a rubber-banding line appears from the first point of the curve to the cursor. As you move the cursor, the curve adjusts to the direction of the rubber-bandingline. Here, you can set the tangency of • the first point of the curve
8. Press .J. This causes AutoCAD to determine the first point’s tangency based on the current shape of the curve. A rubber-banding line appears from the last point of the curve. As with the_first point, you can indicate a tangent direction for the last point of the curve.
9. Press .J to exit the Spline command without changing the endpoint tangent direction.
You now have a smooth curve that passes through the points you selected. These points are called the control points. If you click the curve, you’ll see the grips appear pt the location of these control points, and you can adjust the curve simply, by clicking the grip points and moving them. (You may need to turn off the Data layer to see the grips clearly.)
You may have noticed. two other options-Fit Tolerance and Close-as you were selecting points for the spline in the last exercise. Here is a description of these options. Fit Tolerance Lets you change the curve so that it doesn’t actually pass through the points you pick. vVhenyou select this option, you get the prompt Enter Tolerance <0. 00 סס >:. Any value greater than 0 causes the curve to pass close to, but not through the points. A value of 0 causes the curve to pass through the points. (You’ll see how this works in a later exercise.) Close Lets you close the curve into a loop. If you choose this option, you are prompted. to indicate a tangent direction for the closing point.’
Fine-Tuning Spline Curves
Spline curves are different from other types of objects, and many of the standard editing commands won’t work on splines. AutoCAD offers the Modify :> Object :> Spline option (Splinedit command) for making changes to splines. The following exercise will give you some practice with this command. Youllstart by focusing on Splinedit’s Fit Data option, which lets you fine-tune the spline curve.
Controlling the Fit Data of a Spline
The following exercise will demonstrate how the Fit Data option lets you control some of the general characteristics of the curve.
1. Choose Modify :> Spline, or type Spe.J at the command prompt.
2. At the Se1ect Sp1i ne: prompt, select the spline you drew in the previous exercise.
3. At the prompt
Enter an option [Fit data/Close/Move vertex/Refine/~Everse/Undo]:
type F-I to select the Fit Data option.
Controlling Tangency at the Beginning Points and Endpoints
4. At the prompt
type T.J to select the Tangents option. Move the cursor, and notice that the curve changes tangency through the first point, just as it did when you first created the spline.
S. Press.J. You.can now edit the other endpoint tangency ..
6. Press .J again. You return to the prompt
7. At the prompt
type A.J to access the Add option.
8. At the Speci fy control poi nt <ex; t>: prompt, click the third grip point from the bottom end of the spline . A rubber-banding line appears from the point you selected. That point and the next point are highlighted. The two highlighted points tell you that the next point you select will fall between.these two points. You also see the Specify new point <exit>:prompl
9. Click a new point. The curve changes to include that point. In addition, the new point becomes the highlighted point, indicating that you can continue to add more points between it and the other highlight~ point ). .
10. Press .J. The Speci fy cont ro 1 point <exit>: prompt appears, allowingyou to select another point if you so desire.
11. . Press..↵ again to return to the prompt
1. At the prompt type L.J to select the Tolerance option. TIUs option sets the tolerance between the control point and the curve.
2. At the Enter fit tolerance <0.0000>: prompt,type30.J.Noticehowthe curve no longer passes through the control points, except for the beginning and endpoints (see Figure 13.22).The fit tolerance value you enter determines the maximum distance away from any control point the spline can be. 3. Type X.J to exit the Fit Data option.
You’ve seen how you can control many of the shape properties of a spline through the Fit Data option. Here are descriptions of the other Fit Data options you didn’t try in these exercises:
Delete Removes a control point in the spline.
Close Lets you close the spline into a loop.
Move Lets you move a control point.
Purge Deletes the fit data of the spline, thereby eliminating the Fit Data option for the purged spline. (See Wt:en Can’t You Use Fit Data? just below.)
When Can’t You Use Fit Data?
The Fit Data option of the Splinedit command offers many ways to edit a spline; however, this option is not available to all spline curves. When you invoke certain of the other Splinedit options, a spline curve will lose its fit data, thereby disabling the Fit Data option. These operations are as follows:
• Fitting spline to a tolerance (Spline/Fit Tolerance) and moving its control vertices.
• Fitting a spline to a tolerance (Spline/Fit Tolerance) and opening or dosing it.
• Refining the spline.
• Purging the spline of its fit data using the Purge option (Modify » Fit Data » Purge) or type Splinedit.J.
• Also, note that the Fit Data option is not available when you edit spline curves that have been created from polyline splines. See the Turning Objects into Polylines and Polylines into Splines sidebar earlier in this chapter.
Adjusting the Control Points with the Refine Option
While you are still in the Splinedit command, let’s look at another of its options, Refine, with which you can fine-tune the curve:
1. Type U.J.to undo the changes you made in the previous exercise.
2. At the prompt
[Fit data/Close/Move vertex/Refine/rEverse/Undo):
type R.J. The Refine option lets you control the “pull” exert i on a spline by an individual control point. This isn’t quite the same effect as the Fit Tolerance option you used in the previous exercise.
3. At the prompt
Enter a refine option [Add control point/Elevate order/ Weight/eXit) <eXit>: type W.J. The first control point is highlighted.
4. At the next prompt
Enter new weight (current – 1.000) or [Next/Previous/Select Point/
eXit] <N>: press .J three times to ~ove the highlight to the fourth control point.
5. Type 25.J. The curve not only moves closer to the control point, it also bends around the control point in a tighter.
You can use the Weight value of Splinedit’s Refine option to pull the spine in ‘ tighter. Think of it as a way to increase the IIgravity” of the control point, causing the curve to be pulled closer and tighter to the control pointContinue yOur look at the Splinedit command by adding more ‘control pointswithout actually changing the shape of the curve. You do this using Refine’s Add Control Point and Elevate Order options.
1. 1Ype 1..1 to return the spline to its former shape.
2. lYpe X.J to exit the Weight option; then type A.J to select the Add Control Point option.
3. At the Sped fy a po; nt on the sp 1;ne <ex; t>: prompt, clickthe secondton last control point toward the top end of the spline . The point you select disappears and is replaced by two control points roughly equidistant from the One you selected . The curve remains unchanged. Two new control points now replace the one control point you selected.
4. Press .J to exit the Add Control Point option.
5. Now type E.J to select the Elevate Order option.
6. At the Enter new order <4>: prompt, type 6.J. TIle number of control points increases, leaving the curve itself untouched.
7. Type X.J twice to exit the Refine option and then the Splinedit command.
You would probably never edit contour lines of a topographical map in quite the way these exercises have shown. But by following this tutorial, you have explored all the potential of AutoCAD’s spline object. Aside from its usefulness for drawing contours, it can be a great tool for drawing free..form illustrations. I is also an excellent tool for mechanical applications, where precise, non-uniform curves are required, such as drawings of cams or sheet metal work.