The leg team actually finished all twelve legs at their next class session. We set up the tilting table on the drill press so that each student could step up, position their 2x2 leg, and make that very satisfying pull on the feed lever down to the depth stop. 30 seconds that make a big impression when you’ve never controlled a powerful motor before.

The second team was charged with making the L-girders. We needed six 8-foot L-girders made from the cheap 1x2 pine stock. The stock was slightly longer than 8 feet, which gave the team a bit of play when it came to getting the ends aligned.

But first we needed to have a quick introduction to structural engineering so they’d have a sense of the logic behind combining two pieces instead of using a single piece for the same function. We set up a 1x2 between two tables, laying on it’s wide side, and noted the substantial deflection at the center of the span. Then we flipped it to lay on it’s narrow side and saw that it barely deflected at all. An exaggerated view of the situation reveals why. As the beam starts to sag, the fibers at the top have to get shorter (compression) and the fibers at that bottom have to stretch (tension) because they are arcs with a common center but shorter and longer radii, respectively.

Since wood resists being squished or stretched, having more wood farther away from the neutral center line makes the beam stiffer. By adding another piece of wood to the depth of the already stiff orientation, we can make it even more resistant to bending.

Why lay the top piece flat? I made them a section sketch of the benchwork design so they could see how the L shape provided a convenient way to attach the cross members with screws up through the flange, in addition to the benefit of lateral stiffness. With our wildly irregular grade of wood, it also afforded an opportunity to use the curve of one piece to help straighten the curve of another.

Many classrooms today have a device called a document camera, which basically feeds a live video image of whatever you have on the table in front of you to an LCD projector. It’s a significant improvement over the older overhead projectors that use transparency film and dry erase markers because you don’t need to transfer content from books or other sources to film. Just shove it under the camera. Like the overhead projector, it retains the advantage over blackboards and whiteboards of letting you face the students as you write or draw. In five years of teaching I’ve only had a document camera for the last two and it has made a big difference in how I work. Lots of quick sketches and diagrams that respond to questions as they arise, and less reliance on rigidly planned handouts and worksheets.

These were the concepts that I planned to introduce, but once again I was impressed by how much more there was to learn in making these simple assemblies.

Here’s the quick sketch I made to show how the L-girders should go together (I described verbally that the mating surfaces would get a coat of glue before assembly):

When I thought of making L-girders using drywall screws, I imagined that each group of four or five students could knock one out in ten or twenty minutes, tops. I was incorrect.

Jeff