# Inventor surfacing. Reading 2D Curvature Comb Graphs

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## Clean input sketch geometry is essential when creating smooth organic flowing shapes with Inventor. Surface curvature comb graphs are a valuable tool to use for evaluating the curvature of your 2D sketch geometry.

If you are not familiar with the term ‘Surface continuity ‘please start by reading my previous post ‘An introduction to Surface continuity’.

In this post we will learn how to create and evaluate 2D curvature combs. (A.K.A ‘Porcupine Spines’).

### Switching curvature comb graphs on

To switch on curvature of a given geometry, just right click and select ‘Display Curvature’ (You don’t have to be in sketch edit mode to do this).

**Note:** You need to pick a geometry with curvature (Arcs or splines) to display curvature. You won’t see the display curvature option when right clicking a straight line.

### Modifying your curvature combs

If the spines on your graph are too long/short or too close together/far apart, right click again and chose ‘Setup Curvature Display’.

This will bring up the Curvature Settings dialog, allowing you to tweak the Comb display to your heart’s content.

*If you are using Inventor 2014, and you don’t see this option – check out this fix on the Design & Motion Blog.*

### What does it all mean?

Curvature ‘spines’ are calculated as the Inverse of the radius of the curve at any given point. To put it simply, the curvier your curve is, the longer the spine will be.

The spines are at 90 degrees to the curve at each point (this is called the curvature Normal). The outline of the curvature graph is perpendicular to the spines. This is really helpful to know when coaxing Inventor to give you G3 curvature continuity!

### Understanding how to read a curvature comb graph

If the spines of your surface curvature comb graphs are all the same length, you can assume that the curvature of your geometry is constant.

A change in the length of your curvature combs tells you that the curvature is getting bigger or smaller. An increase in curvature is known as ‘Acceleration’.

If the smooth outer curve of your graph switches over to the other side of your geometry, you have an inflection.

If the outer curve of your graph is not smooth you have a discrepancy that may need to be fixed.

### Curvature between curves

Now you have some beautiful curves, you will want to create beautiful continuity between curves.

*The four continuity levels below are explained in more detail in this post. To learn how to create constraints between 2D sketch curves to maintain continuity, read this post.*

#### G0 = Continuity

Curvature spines are at an angle to each other

#### G1 = Continuity + Tangency

Curvature spines are aligned, but the amount of curvature varies

#### G2 = Continuity + Tangency + Curvature

Curvature spines are aligned and they are the same length at the point where the two curves join.

#### G3 Continuity + Tangency + Curvature + Acceleration

Curvature spines are aligned and the same length and the outer curves of the curvature graphs are tangent to each other.

### In conclusion

Applying and understanding curvature comb graphs are an essential skill when creating complex surface models with Autodesk Inventor. I hope that you can now read your porcupine spine graphs with confidence!