This week in How to Make (almost) anything, the class explored press fitting, a method of fastening that uses material geometries to connect components. In lecture, a variety of strategies for accomplishing this task were demonstrated, with advantages and disadvantages displayed. Needless to say, there would need to be some experimenting with tollerances and methods before a final design could be selected.
Using Solidworks, I generated a hexagon (shown below) to be used as a test bed for a variety of pegs and notches in the hopes of developing "the perfect fit". Dimensions for the cardboard were taken by measuring a random sample with calipers and averaging the results (~0.150"), and subsequent widths were decided based off of an estimate for cardboard's compliance.
As can be seen in the above image, some of the prongs employed a hook method, of varying agression, some of the hooks exteded below the surface of the cardbord to create an interference fit, and one of the pegs features a curved wall, which was intended to ensure a solid press fit. Holes were also tested at the center of the "female" test piece to determine what geometry would work best if closed on all sides.
Though a variety of options showed merit, the curved walls was by far the most promising, and tollerent of variatons of cut quality and cardboard. This was incorperated into my final design.
Excitedly searching the internet for inspirations for a model off of which to base my experiments on "peg in hole" construction, I stumbled across this glorious image of an old steam locamotive.
Ambitious I thought, but I can do it justice. Taking my newfound knowlege from the test pegs above, I began work in SolidWorks using the "Sheet Metal" feature. This magnificent program allows one to loft complex constructions into curved shape, and the system thickens the shell into a sheet metal while providing bend lines to approximate the shape. Consequently, I was able to build the train largely as I would any other CAD model, and the flatten at the end to get the laser cut parts. An example of this flattening is shown in the image below.
After several hours of making up arbitrary dimensions based proportionatley off of the steam tank, I had the following CAD model, and shortly thereafter, A piece of cut cardboard, shown below.
Remarkably, and a testament to both the accuracy of the laser cutter and the SolidWorks "Sheet Metal" feature, assembly was simplly a manner of folding and pressing, and soon I held in my hands a surprisingly sturdy model train. It wasn't a particularly good rendition of the above locomotive, but it would certainly delight my young cousin for the 3-4 seconds it would last before being smashed. Rupert would enjoy it after that.
Tune in next week, when I might develope some better web design skills!