Area Lamp - Part 3
Posted 12.26.2018 by Mario & Kitty Costello
Since we wanted the front and back to be a slot shape, the best approach would be to purchase a round tube and then cut it along its length, giving me the front and back half-circle sections. Those would then be buttressed by two flat plates to form the slot profile of the base. There were a few things to think about while material shopping and the specifics of the eventual base’s size and shape would be dictated to a degree by the following:
First, the finished part was going to be roughly 18” tall and roughly 8” wide by 12” to 18” long and were were going to be putting roughly 150 lbs. of concrete into it, so the material couldn’t be flimsy- it would have to relatively rigid and have some inherent strength to retain its shape and not bulge once we started to fill it. One of the ways to control this would be wall thickness- a thicker piece of material would be stiffer and we settled on 3/8” being a minimum.
Second, there is a concern about that cured concrete bonding with the mold material and making it difficult to remove once cured. You can apply a mold release, like a wax, that will prevent that bonding to any material, but it would be nice to find a material that had some non-stick properties on its own.
Third, the costs. Materials come in a wide range of shapes and sizes, but the prices vary widely and we didn’t want to spend hundreds of dollars just to build the mold. Tubing material in both metal and plastic jumps up dramatically in price one you pass 4” to 6” in diameter. Once we figured out what tube to work with, we would match the flat plate material so that the mold was homogenous.
It came down to some sort of plastic or aluminum with large diameter tubes in both being fairly expensive. Eventually we settled on a plastic called UHMW (ultra high molecular weight) that came in the shapes we wanted (flat plate and tube), was slippery, relatively easy to machine and affordable. If you have a white plastic cutting board at home it’s probably made from this material. The tube had an inner diameter of 7 1/2” and the wall thickness was 3/8”- pretty good.
One of the issues with materials in general is that they have varying degrees of geometric perfection. Flat plates aren’t always that flat and round tubes aren’t always that round. And once they are cut stresses imparted during the manufacturing process suddenly release and the material goes further out of whack. To get the tube into two halves, we would have to cut it length-wise on a mill with a slitting saw (which looks like a small circular saw blade) and we had worries about what this tube was going to look like once it was cut in half or if it possibly even that it would distort during the process of cutting and become unmanageable. We cut some plastic half-moon shapes to support and restrain the tube form and clamped it down to the mill table and commenced to cutting.
The tube definitely “sprang” quite a bit with both halves matching poorly once we tried to put the mold form together for an initial fit check. The lengthwise edges didn’t match up very well and there was no way it would work to try and clamp two plates to the edges and have it come out square. But we fixed the problem with some wood tools and some heat.
First, we established which of the circular edge faces were the most square by standing the two parts up on a table and putting a 24” square against the lengthwise edges. We marked these and made sure to always use the same edge face every time we checked them against each other. Then we used a miniature wood plane to selectively trim the face until we could get the long edges to be square when standing up on the table. This took quite a bit of time and we made quite a few chips, but the small plane, whose edge we had just sharpened, worked surprisingly well on the plastic.
To try to get the profile to be more circular, as opposed to the ellipse it had become, we made some aluminum hoops and put them inside the two parts and then clamped them together with hose clamps and put them in our oven at about 150-degrees F for about an hour. Then we would shut the oven off and let them sit in there and cool down slowly, the idea being that the material will become compliant during heating and then “relax” into the preferred shape during the cooling phase. We did this process a few times. Kitty was giving me the side eye during this whole thing- I’ve had issues with putting projects into the kitchen oven before and not monitoring the situation properly.
The tubes did end up conforming a bit after this process, but still had a slightly elliptical profile to them. But you can see how everything fits together much better than it did initially. After clamp the parts together, the parts were secured to the base with a healthy amount of hot glue.