Asklemmy

When I was in college I was taking trigonometry and structural engineering. In structures we were studying trusses. In trig we were solving for angles and distances using systems of equations.

At the same time I was living in a little studio apartment with old fixtures, a creaky hardwood floor, a view over the street below. It had those multi-pane windows with white paint just slathered onto the muntins, and streaking the edges of the panes.

The kitchen lacked counter space, but also lacked space overall. So I built myself a folding countertop.

It’s a piece of plywood about two feet by three feet. Grain showing, stained with polyurethane. When it’s unfolded and ready to be used, there’s a straight wood bar with a hinged connection near the end of the plywood piece, attached underneath, and the other end is a hinged connection at the wall.

That would be a static truss if the counter were always extended open. Those hinges would never rotate because it’s a triangle.

The hinges made it a truss by offering no resistance to turning. The fact that it was a truss allowed me to calculate how much load it could hold. At the end of the countertop, I should have been able to apply 700 lbs of force straight down before it broke. (But the actual number was probably less; that’s what my gut says. I sat on it but I never wanted to bounce on it)

The trick was getting it to fold. See, to make it fold I had to put a hinge in the middle of that diagonal member (this structure was double, one on each side of the plywood, but I’ll just describe one).

I tried to set it up with angle hinges, like you’d see on a door. Two flat pieces connected by a line about which they rotate. But that didn’t work. I can’t recall why.

Instead I had to use an axle based hinge. The member coming from the plywood down was made of three pieces glued together, with the middle piece shorter to present a forked pair of layers. Those were circular at the end. In the center of that circle was a hold passing horizontally through the room. I built the hinge out of various washers and a big bolt. I had to keep the connection from being tight, like a bolted connection normally would be, because I didn’t want the upper wood squeezing the lower wood.

I can’t remember the exact sequence of the metal parts of that hinge, but it allowed the whole thing to loosely rotate without any part of it being in danger of eventually coming lose and unscrewing. The bolt extended into space after leaving the joint toward the inside, and that’s where the stabilizing bar comes down to hold that joint in place.

The stabilizing bar is quite thin. It doesn’t need to carry much load at all; it just keeps the long diagonal truss member from starting to buckle at the hinge I just described.

The exact position of that hinge along the main member was precisely constrained by this:

When extended, the sum of the two sub-members had to be the distance from the bottom of the wall attachment point, to the point where it met the plywood.

When folded, the difference between the two member lengths had to equal the distance between the lower wall attachment point and the connection with of the now-hanging plywood.

It was fun to make, and even if it was tiny I got to solve a system of equations to figure out the dimensions, and it expanded my ability to cook in that little kitchen, making my life better.

I baked cookies, hosted dinners, perfectly inoculated like 100 shroom cakes with one infection.

I think they made me remove it when I moved out. Went against code somehow.