Fastening & Attaching

Hello again!

So, on 2/10/15, we had a class where we learned about the various methods of fastening and attaching both rods and pieces of Delrin. These methods consisted of bushings to connect rods and pieces, and press-fitting, heat staking, and piano wire to connect multiple pieces.

Bushings:

Credit: Hannah Van der Eb

The first connection method that we learned about were bushings. Bushings are tiny, laser-cut circles of Delrin that can be slid onto rods. Bushings have a small drawback, however, in that they need to fit exactly in order to be properly used in their intended context. They can be tight-fitting or loose-fitting depending on their intended use. For example, a rod that needs to be held stiffly through a piece of plastic would need to have relatively tight bushings to hold it in place, while bushings used as buffers to keep multiple pieces on the same rod from touching each other could be looser. 

One problem when designing bushings is that what is designed in SolidWorks is not exactly what will be cut out on the laser cutter, so test plates of the bushings should be cut and tolerances of the bushings tested before the final design is replicated. The discrepancy between the planned design and the actual product is likely due to a number of factors, including the exact thickness of a Delrin sheet (which is why the same sheet should be used for the test plates and the final product), possible warping of the Delrin sheet, and the settings on the laser cutter as it is cutting, all of which could cause the plastic to melt a little bit around the edges of the cut. 

The average tolerances (from 6 measurements) that we measured using calipers for a 6.36 mm rod were as follows:

              Tight fit = 6.40 mm

              Medium fit = 6.52 mm

              Loose fit = 6.61 mm

Possible contexts:

• Tight: holding rods stiffly through pieces of material
• Loose: keeping apart multiple pieces on the same rod

Press-Fitting:

Credit: Hannah Van der Eb

Press-fitting is a method of connecting 2 pieces of Delrin by cutting notches in one piece and making the edge of the other piece into pegs that then can be fit into the notches. 

In order for press-fitting to work, however, the notches and pegs need to be exactly the right sizes, or they will either not fit together (too tight) or not connect at all (too loose). This method poses the same challenges as with bushings: what is designed in SolidWorks is not necessarily what is cut out on the laser cutter. Again, this means that we need to make test-plates of each notch and peg size and test their tolerances, so that the correct sizes can be determined for that specific Delrin sheet and those particular laser cutter settings. 

The average tolerances (from 3 measurements) of notches that we measured using calipers for a peg 3.15 mm x 12.63 mm were as follows:

            Tight fit = 3.16 mm x 12.65 mm

            Looser fit = 3.44 mm x 12.93 mm

This peg was then tested in various notches with the nominal (planned) widths etched into the plastic. (*Note: we had to convert from mm to inches here, as the nominal widths were recorded in inches.)  The average tolerances (from 3 measurements) of notches that we measured using calipers for a peg with a width of 0.124 inches were as follows:

            Tight fit = nominal of 0.115 in = 0.126 in, 0.117 in, 0.117 in

            Medium/loose fit = nominal of 0.125 in = 0.136 in, 0.135 in, 0.135 in

As our data shows, the nominal values are generally different than the actual values. Again, this could be due to the thickness of the Delrin sheet, warping of the Delrin sheet (causing the laser to be out of focus), or the settings on the laser cutter. Any of these problems could cause slight melting of the plastic around the cut, which would result in consequences in the fit of various pegs in various notches. 

Advantages: 

• Reversible, so mistakes can be fixed and designs can be altered as necessary
• Does not require any post-cut machining
• Possible to disassemble product

Disadvantages:

• Very difficult to get notches and pegs to have exactly the right fit to hold together, let alone be sturdy

Possible Contexts:

• When multiple iterations are expected
• When have enough time for precision cuts and testing
• When want product to be possible to disassemble

Heat Staking:

Credit: Hannah Van der Eb

Heat staking is a permanent method of connecting 2 pieces of Delrin by melting the end of a peg that is through a notch.

 Advantages:

• Strong
• Cannot be disassembled (safe, etc.)

Disadvantages:

• Not reversible -- mistakes cannot be fixed and designs cannot be altered
• Requires post-cut machining
• Cannot be disassembled (others can't study it, etc.)

Possible Contexts:

• When strength is preferred over reversibility
• When want product to be impossible to disassemble without damaging a part (e.g. to steal a part)

Piano Wire

Credit: Hannah Van der Eb

Piano wire is a method of joining pieces of Delrin by drilling a hole through overlapping sections (pegs) and then pressing piano wire through the hole. 

Advantages:

• Can be used to connect moving parts
• Relatively sturdy
• Reversible

Disadvantages:

• Requires post-cut machining
• Relatively difficult to make sure that there is a big enough gap between the pieces that the piano wire will fit, while also ensuring that the overlapping pegs are long enough to drill a solid hole through

Possible Contexts:

• When making a hinge or other moving parts
• When want a reversible connection that is sturdier than press-fitting 

Thanks for reading! See you next time, when we'll be applying some of these fastening and attaching techniques to our well windlass projects!