Lesson42 – Editing Existing SPLINEs

You can select a SPLINE when no command is running to highlight the SPLINE & display the SPLINE grips.

  • if you selected the topographical SPLINE from the previous exercise the grips would look like the illustration below.
  • a grip would appear at each SPLINE point that you supplied when you used the Spline command.

You can Stretch these SPLINE grips using grip editing techniques to change the shape of the selected SPLINE object.

  • click on a grip to make it hot then click at another point to Stretch the SPLINE grip to a new location.
  • press <Esc> twice to clear the grips.
  • this grip editing technique is the same as for editing other AutoCAD objects like LINEs & ARCs.

Control frames are simple shapes with straight edges and control frame vertices have an influence on the SPLINE shape.

  • vertices in control frames are like magnets that attract the actual SPLINE towards each vertex.

You can turn the SPLFRAME system variable to 1 (On) to make the special SPLINE control frame visible (default is 0 or Off).

  • it is good CAD practice to turn SPLFRAME Off as soon as you are finished working with these control frames.
  • you must Regen the drawing to see a change in SPLFRAME.

If you select a SPLINE when control frames are visible you may see grips on the SPLINE and control frame.

  • you can Stretch the control frame grips using grip editing techniques to change the shape of SPLINEs.

Grips in the SPLINE object will disappear after you stretch any control frame grip for a SPLINE because the fit data is purged.

  • when you purge fit data the Fit data option is no longer available in the Splinedit command which limits future editing options.

Splinedit lets you edit SPLINEs with more options compared to editing SPLINEs with grip editing techniques.

When you invoke the Splinedit command you are prompted to select a SPLINE object and the desired edit option.

  • you can type keywords for these edit options or right-click in the drawing area to select an option from a shortcut menu.
  • grips for the control frame are displayed at the first prompt even if the control frame is not visible (SPLFRAME = 0).
  • you do NOT have to turn SPLFRAME On but it helps to at least imagine these grip points being connected by the control frame.

Command: SPLINEDIT
Select spline: (select a SPLINE)
Enter an option [Fit data/Open/Move vertex/Refine/rEverse/Undo]: (right-click for shortcut)

The Fit data sub-options are practical for many common tasks while the Refine sub-options are more advanced editing tools.

  • the following pages summarize the Splinedit options.

Use the Fit data option to edit the data that was used to generate the current shape of the SPLINE.

  • this option is not available if you have purged fit data from the SPLINE or have used grip editing to stretch the control frame.

Command: SPLINEDIT
Select spline: (select a SPLINE)
Enter an option [Fit data/Open/Move vertex/Refine/rEverse/Undo]: F
Enter a fit data option
[Add/Open/Delete/Move/Purge/Tangents/toLerance/eXit] <eXit>: (select another option)

  • use Add to insert new SPLINE points.
  • use Delete to remove existing SPLINE points.
  • use Move to change locations of existing SPLINE points.
  • use Purge to remove the fit data from the SPLINE.
  • use Tangents to change tangent directions for the SPLINE. use toLerance to change the tolerance value for the SPLINE.

When you edit an open SPLINE there will be an option to Close it and when you edit a closed SPLINE there will be an Open option.

  • these options are available in the main Splinedit prompt and with the Fit data sub options.

  • The Move vertex option lets you move control frame vertices
  • this is like using grip editing on control frame vertices and you willpurge the fit data when you use this option.
  • use the Next, Previous or Select point options to select the desired vertex grip (make it hot so you can Stretch it).

Command: SPLINEDIT
Select spline: (select a SPLINE)
Enter an option [Fit data/Open/Move vertex/Refine/rEverse/Undo]: M
Specify new location or [Next/Previous/Select point/eXit] <N>:

Refine is an advanced option for refining SPLINE shapes.

  • Add control points lets you add new control frame vertices.
  • Elevate order lets you change the mathematical order for the SPLINE (e.g. 4 is the default order for quadratic SPLINEs).
  • Weight lets you change the influence that each individual control point has on the SPLINE.

Command: SPLINEDIT
Select spline: (select a SPLINE)
Enter an option [Fit data/Open/Move vertex/Refine/rEverse/Undo]: R
Enter a refine option [Add control point/Elevate order/Weight/eXit] <eXit>:

Use the rEverse option to change the sequence of points

  • the start point becomes the end point and the end point becomes the start point.
  • this change may be required if the sequence is important when you use the SPLINE object in another application.

Command: SPLINEDIT
Select spline: (select a SPLINE)
Enter an option [Fit data/Open/Move vertex/Refine/rEverse/Undo]: E
PRACTICE EDITING SPLINES

» 1) Close drawings from the previous exercise if they are open.

» 2) Open the T208_3.dwg drawing in your personal folder.

3) Pick Modify + Spline and select the SPLINE object. Right- click in the drawing area to invoke a shortcut and select the Fit data option. Right-click again and select toLerance. Enter 50 as the new Fit tolerance. Press <enter> to eXit the Fit data mode. Press <enter> again to complete the Splinedit command. The equivalent command line history is also shown below.

Command: SPLINEDIT
Select spline: (pick the SPLINE)
Enter an option [Fit data/Open/Move vertex/Refine/rEverse/Undo]: F ↵Enter a fit data option [Add/Open/Delete/Move/Purge/Tangents/toLerance/eXit] <eXit>: L ↵Enter fit tolerance <1.0000E-10>: 50
Enter a fit data option [Add/Open/Delete/Move/Purge/Tangents/toLerance/eXit] <eXit>: ↵
Enter an option [Fit data/Open/Move vertex/Refine/rEverse/Undo]: ↵
Command:

4) Type SPLFRAME at the command line and enter 1 to make the SPLINE control frame visible. Then pick View + Regen to force a regeneration.

Command: SPLFRAME
Enter new value for SPLFRAME <0>: 1
Command: REGEN
Regenerating model. Command

5) Type SPLFRAME at the command line and enter 0 to make the SPLINE control frame invisible again. Then pick View + Regen to force another regeneration.

Command: SPLFRAME
Enter new value for SPLFRAME <1>: 0
Command: REGEN
Regenerating model.
Command

6) Pick Modify + Spline and select the SPLINE again.

7) Right-click to invoke a shortcut and select Move Vertex. Press <enter> a few times to repeatedly use the <Next> option until the grip near P1 is selected. Then move your crosshairs near P2 and left-click to move this control frame vertex to the new location. Right-click to invoke a shortcut and select the eXit option. Press <enter> to complete the Splinedit command.

8) Pick Modify + Spline and select the SPLINE again. Right- click in the drawing to invoke a shortcut. Then press <Esc> twice to cancel the command.

9) Close this drawing without saving your changes.

10) Open the T208_4.dwg drawing in your personal folder.

11) Pick Modify + Move and select the POINT object near P1 that has been placed at the mid span and at the same elevation as the ends of the cable. Press <enter> to continue and enter 0, – 69.5145 as the displacement and press <enter> for the second point of displacement to move this POINT to the new calculated sag at mid span.

The location for this POINT has already been calculated using the equation on the following page.

  • now you can edit the SPLINE using Splinedit & change the location of the SPLINE data point to match this POINT object.
  • this POINT was created as a construction object because you can snap to the POINT using a Node osnap.

12) Pick Modify + Spline and select the cable SPLINE. Right- click in the drawing area to invoke a shortcut and select Fit data. Right-click again and select Move. Press <enter> to use the <Next> option so the middle point is selected. Move your crosshairs to the POINT object & left-click to use the Node osnap for the new location. Right-click to invoke a shortcut and select eXit to exit the Move mode. Do not exit the Fit data mode yet.

13) Right-click in the drawing to invoke a shortcut and pick Tangents. Enter @ –3965,525 as the starting tangent then enter @3965,525 as the ending tangent and press <enter> to eXit Fit data and <enter> to terminate the command.

The new horizontal component (H) for tangent directions (at the cable ends) was found using simple trigonometry. (40002=V2 + H2).

  • the vertical force (V=525) is due to the weight of the cable (it did not change) so H=3965 is the new horizontal component.

the new mid-span deflection was calculated using an equation from a design handbook (15860 = 4H).

Y = X (2100 – X) + 15860 Inches
Y = 69.5145 inches at mid span

More practice?

14) Use Dist & appropriate osnaps to find the cable sag at the vertical LINE near P1 and the corresponding horizontal distance between the LINE and the cable start point. The dimensions in the illustration below shows these distances to be Y = 37.2035 and X = 334.3772.

15) Compute the theoretical value for Y at X = 334.3772 using the equation on the previous page.

Y = X (2100 – X) + 15860 Inches
Y = 334.3772 (2100 – 334.3772) + 15860 = 37.2247
This is a within 0.0212 inches of the theoretical value.

18) Compute the theoretical value for Y at X = 525 (1/4 span).
Y = X (2100 – X) + 15860 Inches
Y = 525 (2100 – 525) + 15860 = 52.1359

17) Pick Modify + Copy and select the POINT object near P2 at the start of the cable. Press <enter> to continue and enter a displacement of 525,–52.1359 then press <enter> at the prompt for the second point of the displacement.

The cable is symmetrical about the mid span point so you could create the corresponding sag point at the 3/4 span point with Mirror.

  • in the next step you will Mirror the Last POINT about a vertical LINE at the mid span POINT.
  • then you will edit the SPLINE again to include these two new theoretical POINT locations as two new SPLINE data points.

18) Pick Modify + Mirror and enter L to select the Last POINT then press <enter> to continue. Left-click using the Node osnap near P1 as the start of the mirror line. Turn POLAR On in the status bar (if it is not already On) then invoke a Polar tooltip angle of 90 degrees and left click to use that point as the other end of the mirror line. Press <enter> to keep the original POINT object.

19 Pick Modify + Spline and select the cable SPLINE again. Right-click in the drawing area to invoke a shortcut and select Fit data. Right-click again and select Add.

20) When you are prompted to select a control point left-click on the grip point at the start of the cable near P2 and then right-click & select After. Then move your crosshairs to the POINT object near P3 to invoke a Node osnap and left-click to add this location as a SPLINE data point. Press <enter> to return to the main Add mode.

21) Pick the mid span grip point near P4 as the next control point then invoke a Node osnap at the POINT object near P5 and left-click to use this as another new SPLINE data point. Press <enter> four times to eXit Splinedit.

22) Use Dist with Intersection osnaps to find the sag for the cable at the vertical LINE near P1 again. The dimension illustrated below shows this distance to be 37.2231.

You already computed the theoretical sag at this point as 37.2247 (see step 15) and the previous error at this point was 0.0212 inches.

  • by adding only two new SPLINE data points you reduced the error for the SPLINE at this point to only 0.0016 inches.
  • this new error is less than 10% of the previous error (the new SPLINE is at least 10x more accurate at this point).

» 23) Save the changes to your drawing and Close the file.

Leave a Reply

Your email address will not be published. Required fields are marked *

Show Buttons
Hide Buttons
SUBSCRIBE TO OUR NEWSLETTER

It's FREE!

Sign up and receive an email every time a new tutorial is published