This is the forth in a series of Tips and Tricks that will show you how to create and use 2D and 3D Splines. If you haven’t read the Drawing 2D Splines, Constraining Splines, and The Process of Drawing Splines Tips and Tricks, you might want to read them before proceeding.

In this short exercise you will learn how to use Spline Commands to control the shape of splines.

Create a Fully Constrained Spline Loop

If you’re not sure how to create a fully constrained spline loop, read The Process of Drawing Splines Tips and Tricks.

Fit Methods
Inventor uses three different Fit Methods to fit the shape of splines through spline points. By default the Smooth method is used to create a spline with smooth continuity between spline points. You can also use the Sweet Method and AutoCAD Method.

To access these methods, right click on the spline and select a Fit Method. The Sweet Method uses the Smooth Method, but also provides better curvature distribution. This method also takes longer to calculate, and increases the size of the file. The AutoCAD Method uses the same spline fitting method AutoCAD does.

Bowties
Bowties are used to define the shape of the spline near spline points, and they can take three forms: a Handle, a Handle and Curvature, and a Handle and Flat.

First we’ll add a Handle and dimension it. Right click on the top spline point and select Bowtie – Handle.

Once you select the command a handlebar is attached to the spline point.

The handlebar can be used to change and constrain the shape of the curve near the spline point. You can change the angle of the handlebar by dragging one of the grips, and if you do, be sure to add an angular dimension to control the angle. To keep the spline symmetrical, add a Horizontal constraint to it.

The length of the handlebar defines the relative length of the curve that is tangent to the handlebar. For this example, when you add a dimension to the length of the handlebar, the distance is .476. This is a unitless value that describes how closely the spline hugs the handlebar.

Increasing this value increase the length of the tangent portion of the spline making the spline appear to hug the handlebar closer.

You can also define the radius of the spline near the spline point by changing the bowtie to a handlebar and curve. To do this, right click on the handlebar and select Bowtie – Curvature.

With this option you can control the radius of the tangent portion of the spline. The radial dimension in the following example is .467 inches. So this dimension uses the standard units for the document.

The last option consists of a handlebar and flat curve. Before you evoke this command, delete the radial dimension. Once you’ve done that, right click on the spline point and select Bowtie – Flat.

If you try to add a dimension to the flat bowtie you’ll find that you can only add a radial dimension.

That’s because the flat option essentially sets the radius of the curve approximately to infinity. In this case, it has the same effect as setting the radius of the curvature option to 393.701. You can change this value back to .467 to have the same results as setting the bowtie to handle and curvature.

Why is this important?
While splines are more difficult to define the exact size and shape than lines and arcs, knowing these commands give you more control over them. This will ultimately help you take more control over your solid models.