Fastenings and Attachments

Joining methods: I will discuss the different methods I have used to join pieces of Delrin and their respective benefits and drawbacks.

• Heat Staking: This method is probably the most effective just in terms of joining two pieces together. The pieces are fused together and are inseparable afterwards. However, this could be problematic if you need to later take it apart or unseal the pieces for any reason. Also, it can be difficult to heat stake oddly shaped or very large pieces.
• Hinge: Using a drill press to make a hinge is also an effective way to join pieces if you want the pieces to rotate about an axis. However, this method requires you to be extremely precise in order to ensure you make the hole big enough, make the piano wire long enough, get the correct alignment, etc.
• Press fit: This method is the trickiest to achieve perfectly, because it will probably take a few attempts to get the correct fit. However it may also be the most sensible option for attachment because it can be easily taken out for disassembly.

Bushings: There can be loose or tight fitting bushings, which can both be useful for different purposes. For example, a loose fitting bushing may be useful for a pulley, where the center must spin around an axis. On the other hand, if you want to keep a certain piece locked onto a rod, you could press a tight fitting bushing onto the rod and against the piece you want.

Tolerances: Below are the tolerances my partner and I measured for different bushings and press fit holes.

• Bushings: For a rod that is 6.35 mm in diameter, we found a good tight fit bushing to have an inner diameter of 6.38 mm and a good loose fit bushing to have an inner diameter of 6.55 +/- 0.05 mm. 
•  Press fit holes: For a tab measuring .1221 inches in width, we found a good tight fit hole to be .1225 in and a loose fit to be .1317 in.

We also noticed a significant discrepancy between SolidWorks measurement of hole width and reality. For a sheet of thickness 3.19 mm SolidWorks proposed three different hole widths: .135 in, .125 in, and .115in. In reality, we measured the widths to be .1425 in, .1350 in, and .1260 in respectively. This tells us that SolidWorks measurements are not 100% accurate. Moreover, the discrepancy will differ depending on the thickness and type of material due to the level of accuracy of the laser cutter. It is important to note that you must make test models of press fit holes to get the right fit before cutting out your entire piece so as to not waste material and time.