A Lesson in Fastening and Attaching

Heat Staking Machine

Tuesday, in class, we had what I would call our first formal lesson in engineering fundamentals. I learned a few ways of attaching and fastening materials, namely piano wiring, laser cutting and press fit, heat stakes and bushings. Interestingly enough, I've seen many of these methods and did not know what to call it.

Relative benefits and drawbacks of each:

• With piano wiring, you can either drill a fitted hole so that the pieces do not move or you can have a loose hole to make a hinge.
•  In both cases, if you do not measure the correct diameter (or length) of the wire, things can become messy. Additionally, although it is a strong attaching mechanism, if the holes in all the pieces are not aligned properly (or if the wire itself is not straight), the method will not work. Let's hope it does not become stuck in the holes.
• With laser cutting/press fit, you make openings to the exact dimensions of the piece you're inserting. Surprisingly, pressing or hammering the two into each other makes a very stable fastening. 
• However, this is not perfect: the laser cut could alter the actual dimensions of the piece (e.g. software issue, the laser's own width); with the right amount of force, the piece could become dislodged; and depending on your material's properties, the opening could expand or contract with environmental and temperature changes.
• Heat staking is fitting two piece together via a cut-out opening, melting down the plastic together and then cooling it quickly with air. This is quite effective and the shape of melted plastic bubble can be altered based on the press's shape.
• The downfalls? If you don't apply enough pressure, the pieces will be loose and/or might wiggle. Nonetheless, it's not as painfully precise as piano wiring and is a much more secure method than press-fit for permanently attaching pieces.

Calipers & Bushings (& Notches): Differences between a loose fit and a tight fit 

• Bushings are a unique but pretty common method. I'm quite sure I and others have done such a way when we were kids building forts or playing with Lego pieces. Essentially, bushings are a ring placed around a rod, which has been pressed between openings in two pieces, in order to prevent the rod from shifting horizontally. A loose fit will allow rotary motion while a tight fit will permanently fix the rod in place as a support.
• Callipers are very precise measuring tools that can evaluate depth, inner and outer width, as well as height. For the finest of wires, it is counterproductive to even try to use a ruler; a calliper can tell you measurements to the thousandths.
• In a fastening or attachment, a thousandth more or less than the dimension intended can cause a bushing not to fit on a rod tightly or loosely enough or even a notch to be too small/large for a press fitted piece. These tiny errors can make or break the success of a project.

SolidWorks and its problems

A simple exercise we did was to examine discrepancies between what we design/measure in a 3D software like SolidWorks and the actual laser-cut print. The results were almost devastating.

My partner and I experimented with notches and press fit. We designed two small pieces: one with a rectangular opening designed to fit the width/thickness and length of the other cut-out piece. When they printed out, the cut out piece was almost 1.5 times larger than the opening, and there was no way it would work. Revelations like that show how frustrating it can be to do such 3D drawing and laser cutting. 

• One reason we discussed about the difference could be the laser -- as small as it is -- has a width of its own. Though that might factor into very very tiny differences, it did not explain our "1.5x" issue.
• A second reason could be that the ratios and conversion to different units may have been muddled: a "lost in translation" issue. Such seemed more viable as when we pulled the file back up to laser print, we noticed how it rotated and scaled automatically -- and, additionally if the incorrect units are selected, it becomes even more misconstrued.  (Later on, in a project, we realized that the drawing mode was set to a ratio of 1:2, scaling the drawing being sent about half the original dimensions....) 

I'm sure professionals have to account for different types of materials and thicknesses, as different properties can affect how well and fast the laser cuts, but it seemed, for the material we're using, all materials can be just as errored as the others. Dimension shifts are definitely something that I need to keep in mind for future projects.