Design Curves for Irregular Shapes

Posted on 13 October 2017

One of the things that French curves excel at is creating smooth lines from a series of three points.

Last night I used one of our Design Curves to create an irregularly shaped seat for a dugout chair I’m constructing (full details on the seat here). While plotting the points that defined the seat’s shape was a bit of a trick with “ticking sticks,” the reasons the seat fit so well right off the saw was the Design Curve.

The reason these curves are useful is that their edge is a continuously changing radius. So when you have three points, you show the curve to the three points, then you rotate the curve to either increase or decrease the radius. When the edge of the curve touches all three points, you join them with a pencil line.

(Note: The proper way to get the smoothest line is to use the curve to line up three points and then connect only two of the points. Then move onto the next three points and repeat.)

You can work with two points or four (sometimes), but three points will always work.

After five minutes of work joining about 100 points I had a line that was much easier to follow than a freehand attempt to “connect the dots.”

— Christopher Schwarz

Note: The
Design Curve shown in the photos is one of our cosmetic rejects, which is why it looks so rough. I found it on the floor behind my workbench. The curves look much more like what is shown on the page in our store.

More Posts


  • Christopher Schwarz: October 13, 2017

    Yup! You are exactly right. I should have mentioned that is the method to make a more accurate curve.

  • David Evers: October 13, 2017

    I was always taught in drafting classes that you use a French curve by aligning the curve to three point, but only connect two; then you realign to the next three points and draw between two, …

Leave a comment

All blog comments are checked prior to publishing

Search our store