Today I was doin’ some cipherin’ as I looked through an old textbook and found myself using a tool that I’ve carried from office to office for over 30 years. In all those years, I’ve almost never used it for its intended purpose. In fact, I’m pretty sure it broke shortly after I bought it. And yet it’s never let me down in the alternate job I’ve assigned it.

I was reading a paper on the design of grinding disks (you only wish you had my job) and came across a formula for stress in a rotating disk that didn’t seem right. I grabbed out one of my favorite texts, Cook and Young’s Advanced Mechanics of Materials, flipped to the page with the right formula, and started noodling with the algebra to see why the paper’s formula was different.1 Because the book wouldn’t lie flat, I pulled my trusty flexible French curve out of the top drawer of my desk and laid it across the pages. As it has for three decades, the French curve did perfectly a job for which it wasn’t designed.

Flexible French curve in action

A regular French curve is a flat plastic drafting tool with several edges of varying curvature. By combining different edges, a smooth curve of any shape can be drawn. It was the pre-computer Bezier curve.

Regular French curve set

Image taken from Genesis Art Supplies

A flexible French curve is a single tool meant to take the place of several regular French curves. The idea is to have a device that can be bent into any shape and traced.

In my experience, flexible French curves are better in theory than in practice. Mine is made of several thin mild steel strips set parallel to one another and enclosed in soft black plastic. When it was new, I could bend it into almost any shape (although it was never good at tight curves), but after a few bends back and forth, the strips inside the plastic broke in fatigue. This left kinks and bumps in the curve, rendering it useless for drafting.

Flexible French curve detail

But it could still be bent into a gentle, if bumpy, arc, and because its innards were steel, it was pretty hefty. Those two properties combined to make it perfect for holding books open—an essential device for graduate school. People who’ve seen it in action have often thought that was what it was made for.

I could buy a bookweight that would do the same thing and would certainly look classier. But a bookweight would cover up too much of the page and, more important, wouldn’t have the history of my old flexible French curve.

  1. Turned out the paper’s author used a plane strain formulation instead of plane stress. For a thin disk! You’re stunned, I know. 

5 Responses to “Repurposed”

  1. Dave Rogers says:

    For some reason Professor Higgins, Eliza Doolittle, and “The strain in Spain stays mainly in the plane.” comes to mind.

    Now I’ll have that in my head all morning!

    I haven’t done any engineering in, well, since college. But I still recall some of the rules. Flow never crosses a streamline. The velocity of the fluid at the boundary is equal to the velocity of the boundary. (The “no-slip boundary condition.”) Wave power varies as the cube of the height. And the perennial favorite, “F=ma and you can’t push a rope. All else is derived.”

    Oh yeah, and the angle of repose of sand is 45 degrees.

    I think.

    The good old days, when engineers were people who worked with atoms, not with bits, and architects designed buildings and not databases.

    Hope your iMac arrives soon.

  2. Terrence Dorsey says:

    Nice idea.

    I use a book-width sheet of 1/4-inch plexiglass scrap. Heavy enough to hold down most books and transparent.

  3. Carl says:

    I keep my books open with a “Page Boy® Folding Book Holder.” See for a description, but I can’t seem to find any place online that sells them now. Maybe it was discontinued?

  4. Bruno says:

    What is the brand/model name of the blue mechanical pencil on the picture?

  5. Dr. Drang says:

    Bruno, it’s a Staedtler Mars 780 lead holder. When my eyesight was better, I used 2H lead, but now I use HB to get a darker line. I’ve resigned myself to sharpening more often.