How to Draw a Spline in AutoCAD 2014

Most people use AutoCAD programs for precision drawing tasks: straight lines, carefully defined curves, and precisely specified points. You may believe that AutoCAD 2014 is not the appropriate program to free your inner artist — nonetheless, even meticulously created CAD drawings sometimes need free-form curves.

In fact, the advent of processes such as computer numerical control (CNC) machining and 3D printing mean that many consumer goods and even quite a few industrial products are taking on smoother free-form shapes. The AutoCAD spline object is just the tool for the job.

You can use AutoCAD splines in one of two ways:

  • Wing it. Eyeball the location and shape of the curve, and don’t worry about making it “just so.” That’s the free-form, sketchy, less-than-precise approach.

  • Be precise. Specify the control points and curvature characteristics precisely.

Beneath the easygoing, informal exterior of the AutoCAD spline lies the truly highly precise, mathematically defined entity known as the NonUniform Rational B-Spline curve — the NURBS curve.

Mathematicians and mechanical and industrial designers often care deeply about the precise characteristics of the curves they work with. For these people, the AutoCAD SPLine and SPLINEDIT commands include a number of advanced options. Look up spline curves in the AutoCAD online Help system if you need precision splines.

Drawing a spline is straightforward, if you ignore the advanced options. Follow these steps to draw a free-form curve by using the SPLine command:

  1. Start a new drawing.

  2. Click the Spline Fit button on the Draw panel slideout of the Home tab, or type SPL and press Enter.

    AutoCAD starts the SPLine command and prompts you to specify the start point of the spline. The command line shows

    Current settings: Method=Fit Knots=Chord
    Specify first point or [Method/Knots/Object]:
  3. Specify the start point by clicking a point or typing coordinates.

    AutoCAD prompts you to specify additional points:

    Enter next point or [start Tangency/toLerance]:
  4. Specify additional points by clicking or typing coordinates.

    After you pick the second point, press the down-arrow key to display additional options at the Dynamic Input tooltip. (You can turn on Dynamic Input from the status bar, if you need to.) The command line shows

    Enter next point or [end Tangency/toLerance/Undo/Close]:

    Because you’re drawing a free-form curve, you usually don’t need to use object snaps or other precision techniques to pick spline points.

  5. Press Enter after you choose the endpoint of the spline.

    AutoCAD draws the spline.

You can specify the start and end tangency of the spline to control the curvature of the start points and endpoints of the spline. If all you want is a swoopy, free-form curve, picking only points works well.

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After you draw a spline, you can grip-edit it to adjust its shape. If you need finer control over spline editing, look up the SPLINEDIT command in the AutoCAD online help system.

The Polyline Edit (PEdit) command can convert a straight-line-segmented polyline into a spline curve form.

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