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!

Bottle Opener

Hello again friends!

Our first assignment in ENGR 160 was to make a bottle opener that could open a non-twist-off bottle cap. The bottle opener had to be cut on the laser cutter from a single sheet of Delrin (the brand name for Polyoxymethylene - a type of plastic) no greater than 6"x6". I worked with Angel Kuo, a classmate. 

Process

We started out by brainstorming approximately 20 ideas in about 10 minutes. As some of these ideas were completely ridiculous, or did not follow the directions, or wouldn't work in any way, shape, or form, we ended up narrowing our list down to 10 different designs:

Original Brainstorm

We further narrowed down this list based on various physical aspects to maximize the likelihood that the bottle opener would easily open the bottle and not break or degrade while doing so. The first 4 designs (left to right) were discarded because our Delrin sheets were not strong enough to withstand the necessary force at the fulcrum of the lever (where the edge of the handle presses onto the edge of the bottle cap), while still remaining thin enough to fit under the edge of the bottle cap.

Engineering Analysis

We analyzed the potential deflections of the handles using a cantilever beam deflection equation:

deflection = (FL^3)/(3EI)

F=force, L=length, E=Young's modulus, I=area moment of inertia

In this equation, the Young's modulus (E) is set because it is dependent on the material stiffness of the Delrin, and the force (F) is uncontrollable because it varies depending on the user. The only variables that we could actually control when designing our bottle opener were the length (L) of the handle and the area moment of inertia (I), which is dependent on the stiffness of the cross-sectional area. To get as small of a deflection as possible, the handle length for these designs needed to be as short as possible, as the area moment of inertia (I) would be rather high in a design where the pressing surface of the bottle opener was across the thickness of the Delrin sheet. 

We knew that the Young's modulus (stress/strain)(E) for Delrin was 410,000 psi and that the proposed length (L) for each handle was around 2" and the width was around 1". The area moment of inertia (I) for a 2" by 1" handle would be ((LW)/12) = (1in*2in^3)/12 = 2/3 in^4. The total force (F) applied to the bottle opener would depend on the user and was thus uncontrollable. Thus,

deflection = (F lbs*(8 in)^3)/(1,230,000(lbs/in^2)*(2/3 in^4) 
= (F*512 in^3)/(820000 in^2)
= 0.000624F in

Original Brainstorm

The next 3 designs and the design in the middle of the bottom row were discarded because they were all intended to hook under the edge of the bottle cap and roll down the outside of the bottle, pulling off the cap. However, we realized that this would only pull a small section of the bottle cap edge out and away from the bottle, rather than pulling the bottle cap all the way off. 

Both of the remaining designs (bottom left and bottom right) were very similar in function: the hook went underneath the edge of the bottle cap and pulled up while the top section of the bottle opener pressed on the center of the bottle cap, creating a fulcrum around which the cap bent. We ended up choosing the design on the bottom right because it was both more aesthetically pleasing and less likely to chip/break or to pierce the bottle cap. We knew that if we chose either of these designs, we would use the thickest possible Delrin sheet (1/4"), so that the bottle opener would be as strong as possible. Because the pressing surface of these bottle openers is the thickness of the Delrin sheet, the pressing surface could be as large as possible and it still wouldn't affect the ability of the bottle opener to hook under the edge of the bottle cap.

Engineering Analysis

We again calculated the possible deflection of the length of the bottle opener, which was analogous to the beam of a cantilever:

deflection = (FL^3)/(3EI)

F=force, L=length, E=Young's modulus, I=area moment of inertia

Again, we could only control the length (L) of the handle and the area moment of inertia (I). To get as small of a deflection as possible, the handle length for these designs needed to be relatively short, but could be somewhat longer than in our other designs, as the area moment of inertia (I) was lower because pressing surface of the bottle opener was parallel to the thickness of the Delrin sheet. 

We knew that the Young's modulus (stress/strain)(E) for Delrin was 410,000 psi and that the proposed length (L) for the handle was around 2" and the width of the cross section was 1/4". The area moment of inertia (I) for a 2" by 1/4" handle would be ((LW)/12) = (1/4in*2in^3)/12 = 2 in^4. The total force (F) needed to pry off the bottle cap would remain constant between each design. Thus,

deflection = (F lbs*(8 in)^3)/(1,230,000(lbs/in^2)*(2 in^4) 
= (F*512 in^3)/(2460000 in^2)
= 0.000208F in

This is less than the handle deflection of our other designs, so we picked this design to move forward with. 

After choosing our preliminary design, we started refining and went through a couple of iterations:

Bottle Opener Design Iterations

1. This was our original design, copied from our brainstorm.
2. We lengthened the whole design in order to create more leverage when opening the bottle.
3. We thickened the hook in order to reduce the likelihood of it breaking, as the hook is where a large part of the force is concentrated.
4. We decreased the diameter of the circle at the top of the opener, so that it stayed on the bottle cap as it was rolled over.
5. We decreased the distance between the hook section and the circle section, so the circle hit the center of the bottle cap while the hook was under the edge of the cap. We also thickened the whole bottle opener to increase its strength and rounded out the whole design to make it both more aesthetically pleasing and more comfortable to hold. 

Our next step was to create a prototype out of foam core:

Foam Core Prototype 1

We held this prototype up to a sample bottle and realized that the circle still did not hit the middle of the bottle cap, but was instead closer to the edge, which would make it more difficult to bend the cap and pop it off of the bottle. So we made another iteration of our foam core prototype:

Foam Core Prototype 2

Prototype 2 had a shorter distance between the hook and the circle, so the circle landed in the middle of the bottle cap. We lined this foam core prototype up with our sample bottle and double-checked that it fit correctly, and then we moved on to building our model in SolidWorks, the program that we use to send designs to the laser cutter.

We had a bit of trouble with SolidWorks, mostly just because it has an insane learning curve. I'd only used it a little bit and not in a long time and Angel had never used it, so we were basically starting from the beginning. 

After we drew the part in SolidWorks, we had to transfer it from one computer in the engineering lab to another computer across the lab. This was inordinately difficult. We were the first group to finish building our design in SolidWorks and thus the first to try to transfer our files. We had to be able to open this file on multiple programs on the other computer, notably Corel Draw. We originally tried to use a DXF file, but this wouldn't open correctly on Corel Draw. After about 45 minutes and 10 different file types, we finally got the file to open.

Next, we learned how to set up the laser cutter, including how to load materials into the laser cutter and how to adjust the laser so that the focus is on the surface material. We set the material type, power, velocity, and number of passes, then sent the file to the laser cutter. The laser cutter's first cut didn't cut all the way through the Delrin, so we adjusted the settings and tried again. After a couple of tries, the laser finally cut through the material and our bottle opener was finished!

Final Delrin Bottle Opener

We tested out our bottle opener on a sample bottle and found that it easily worked without degrading at all, indicating that it could be used multiple times, so we were finished!

Reflection

I think this project went relatively well, especially considering our distinct lack of prowess with SolidWorks at the beginning of the project, and I'm pleased with our final product. That being said, I wish we would've made our bottle opener a little bit more aesthetically appealing. It isn't exactly displeasing to the eye, just utilitarian. However, I worry that cutting shapes into the middle of it for decoration would have weakened its overall structural integrity. Anyway, it kind of looks like a slug or a one-eyed monster, which is pretty awesome, and it works really well!

Thanks so much for reading! See you next time, when we'll be designing well windlasses!

Introduction to the Life of Julie Chase, Engineering Student (Probably)

Hello dear readers, and welcome to my blog.

My name is Julie Chase and, while I come from foggy Santa Cruz, CA, I am now a sophomore at Wellesley College in MA, a place where seasons actually exist. I am currently a neuroscience major and further plan on pursuing a Certificate in Engineering Studies from Olin College of Engineering or possibly participating in the Olin-Wellesley 4+1 Program. To this end, I am taking Wellesley College's ENGR 160: Fundamentals of Engineering, which serves as a gateway course to all of the Olin-Wellesley cross-registration engineering courses. Last year I took the first-year seminar ENGR 111: Product Creation for All, and discovered my interest in engineering. I guess my hobby of designing and building structures and products before my time at Wellesley should have been a clue to me that engineering was in my future, but apparently recognizing things that are right under my nose is not one of my talents.

I've decided that taking ENGR 160 will help me to better determine if engineering is something that I really want to continue pursuing and, if so, which specializations I should consider.Furthermore, I think that studying engineering is actually quite helpful even if I end up working primarily in other scientific disciplines, as I suspect will happen. Engineering is basically the application of scientific knowledge, which makes it invaluable to consider when deciding research topics and goals, and then again when interpreting the results of studies or experiments. Without considering the application of their discoveries, scientific researchers are seriously limiting themselves in terms of how their information is then used. I'm not saying that all scientists should follow a project from experiment to discovery to application to revision and on, but it is helpful if everyone involved along the way knows the basics of what happened and why at other stages. This would allow a research and development team to work together as a far more cohesive unit and thus produce far superior results.

Outside of taking far too many science courses in multiple disciplines and analyzing their various intersections, I also enjoy studying Spanish, music (I'm a classically trained singer), and theater (when I have time). I'm a member of the Wellesley College Choir, multiple activist organizations on the Wellesley campus, and an emergency medical services club (again, when I have time). Last year I also participated in WCircus, the Wellesley College circus club, which is currently on hiatus, and so I know how to juggle and do basic solo and partner acrobatics.

While I did have to run a blog for my ENGR 111 class last year, it has been a while, so I'm hoping that I'll remember relatively quickly how to balance the informal, conversational style of a typical blog and the academic style required by the more technical information that will be discussed on this particular blog.

Well, that's all for now. My next blog post should be covering designing, testing, and revising 2D bottle openers. See you next time for some actual science!