Dial Face Mold Design

The dial face is a complex part with many raised and cut features. The finger holes cut through a spline surface, while the small spring holes meet a cutout half-way through the mold. These two features caused us the most trouble in the production of the mold.

The holes in the dial need to be solid parts on the mold. One idea was to mill out the core side such that the remaining material had raised cylinders from the surface. This would have required milling a very tall feature with a very small end-mill in order to fit between the holes. Instead of doing this, we drilled holes in the core and then pressed in aluminum pins. This was easier to perform, but still required a lot of time. We had to turn down and part off 10 small ~0.434" diameter pins about 0.85" long and then press them into milled holes. After the pins were installed, they had to be turned down to match the smooth surface of the cavity mold.

The steps required to make the core mold are shown below. A .pdf of the machining steps used for both the Dial Core and Dial Cavity can be found here:

-Rough and finish turn the smooth surface and the center boss:

-Drill and mill all ejector pin holes and pin-holes, spot drill locations for small pins.

-Turn and part all pins on the lathe

-Drill, ream, and press in small (1/16" OD)dowel pins

-Press-fit large aluminum pins:

-Turn down mold with pins to match the cavity:

We then started to use the injection molding machine to make our first round of test parts. We started with the settings left on the machine for the previous run and then iterated to get closer to our final desired properties. The first part was a short shot with only 15mm of stroke:

The two sides of the mold on the BOY injection molding machine in the LMP:

Initial speed profile for injection:

Cavity mold with shims and ejector pins on the mount:

We slowly increased the shot size until the parts were complete, then increased it further to ensure that the plastic was being packed into the mold to reduce shrinkage. We did a run of about 15 parts while varying the stroke length. We found that at 22mm of stroke we could get reliable parts, but at 24 the machine couldn't full finish the stroke and would sometimes fail to enter the packing step.

Travis came back the next day to make a run of about 20 parts to be used to adjust the bearing diameter:

The parts were designed to have a 0.020" oversized bore when accounting for 2% shrinkage of the plastic. This would give us the option to remove material around the boss later to adjust the bearing fit. As predicted, the bearing was very loose and wouldn't stick into the part on the first try.

I measured 11 parts in 3 places each to get 33 values for the bearing pocket ID on the plastic parts. The average value was 0.647" ID while the bearing was 0.629"OD. However, the average value of the plastic parts doesn't clearly represent how the bearing and plastic will fit together. The parts varied significantly in ID with a standard deviation of 0.0021". This may seem small, but this means that 95% of all parts lie within +/- 0.0042" of the mean. This is a huge variation, because if the means of the ID and OD were the same, about half the parts could be 0.004" oversized, and the other half could be that much undersized.

One idea we had was to offset the mean value of the ID to be 0.004" below the bearing OD, this mean that 95% of all parts would interfere with the bearing OD, but by as much as 0.008"! 

We were very confused, but then we remembered that the injection molding machine takes a while to settle in response to a change in inputs (stroke length, temperature, pressure, etc). We measured the last 10 parts produced from the run of 20 and found a much tighter variation, with an average of 0.646" and a 95% confidence interval of +/- 0.0013" This was much closer to what was expected of injection molded tolerances. We re-machined the center boss to have a diameter of 0.630" which suggests that 95% of all new parts will still be oversized from the bearing OD. We decided not to shave off too much material because we wanted to get the fit right the first time, and if we removed too much material it would be permanently too tight.

We are planning to mold more parts this week, re-measure them, and then shave the boss down to the final value, where we think that 95% of parts will have a good press-fit with the bearing of at least 0.001" of interference.

First Yo-Yo test!

The dial, along with all of the other parts of of YoYo, not only had to be optimized for production (with injection molding settings), but had to be optimized for fit. In our YoYo this meant after we machined our molds and injected the first run of parts, we had to test it with the other injection molded (and thermoformed) components. This lead us to make a few changes on a number of our molds. While this was expected (and the molds were made large in order to account for these differences) this made the optimization phase of production much longer and more labor intensive. For example, the Finger Stop on our YoYo did not come out with the spline curve necessary to fit over the dial. Although we had finished the base mold we needed to re-machine it in order for the Finger Stop to work. 

Through this process we learned that it is very important for each part of the team with interfering components (ie. plastic press-fits and free-fits) to communicate well on part tolerance and geometry. Although we learned this is important we also think we did a good job communicating the fits for each of our parts -- we were able to catch the mis-fits (some of which were made on purpose) during the optimization phase and before production. Communicating and organizing our designs around each-other's models is something we have learned is important and will take from 2.008 into future classes and jobs.

3D Printed Prototype(s)

Over the past two weeks we've 3D printed a number of iterations of our Yo Yo to test out a number of different features. These features included:

1. The press-fit for the finger-stop on our Yo-Yo (refer to this post for more information)
2. The retaining ring to hold down the thermo-formed part
3. The sizing for the bearing and bearing post
4. The torsion spring to Yo-Yo connections

Below are a number of our 3D printed parts:

Figure 1: Version 1 - FDM 3D Manufacturing

Figure 2: Version 2 - FDM 3D Manufacturing

Figure 3: Version 3: SLA 3D Manufacturing

Through out prototyping process we moved from using FDM printing to test the rough fit of the first iterations of our parts to the higher accuracy SLA printing for our most recent iterations. As part of the prototyping we also took images comparing the FDM and SLA finish on our Dial.

Figure 3: FDM Resolution (layer height & thickness)

Figure 4: SLA Resolution (layer height & thickness)

And a video of our most recent SLA printed version of the Yo-Yo. This version includes the bearing and spring setup we'll have in the final design, but does not have out thermoformed part or the finger-stop.

We were able to test each of these to the point where we feel comfortable moving forward with our design. During this process we changed a number of things including the size and form of the finger-stop press fit, the thickness of the retaining ring, the geometry of the bearing post, and the attachment points for our torsion spring. 3D printing was invaluable in our design process because it allowed us to quickly test our ideas without putting tons of effort and time into testing out our ideas.

Overview of the Call Me Maybe Yo-Yo

Yo-Yo Design:

The vision of team Call Me Maybe is to design a working Yo-Yo based off of the aesthetic of an old-style rotatory phone – you can see the full assembly in Figure 1. The Yo-Yo will be comprised of four injection-molded parts (finger-stop, dial, retaining ring, base), one thermoformed part (number pad), and other hardware including a dowel pin, torsion spring, and bearing (see Figure 3). The bearing, dowel pin, and spring will be used to simulate the spinning dial on a real rotary phone. The parts will be assembled almost entirely using press fits. The retaining ring will press into the base to hold the number pad in place, the bearing will press into the underside of the dial, and the dowel pin will press into the bearing.  The torsional spring will then be integrated on the dowel pin, with its two points of contact being on the underside of the dial and the inside of the base. Finally, the dowel pin will press into the base, completing the half assembly.  The two halves will be connected using a set screw to complete the full assembly.

Figure 1: Yo-Yo Assembly

Figure 2: Exploded Yo-Yo Assembly Isometric

Figure 3: Exploded Yo-Yo with Labeled Parts

Design for Manufacturing Considerations:

The major considerations for our Yo-Yo were deciding how many and what parts to injection mold or thermoform. Eventually we settled on making the 2D shapes in our Yo-Yo out of thermoformed plastic and the dense 3D features from injection molded parts.

When designing using the thermoformed part and the injection molded parts we had to make sure to give each of our parts the correct sizing to allow for a nice interference fit between the parts. This is the most important design consideration – without these interference fits our Yo-Yo would not fit together. Examples of this though process can be seen in out retaining ring – this interference fit is used to hold the thermoformed part in the Yo-Yo. Similarly when designing the “Finger Stop” on the face of our Yo-Yo we needed to make sure that the interference fit was dimensioned correctly for the size of the small part.

Finally when designing parts we tried to use as many circular features as possible – this allows us to use the lathe to create the tooling for injection molding which will greatly reduce our time machining/fixing our molds. 

Specification Sheet:

Specifications for each Yo-Yo part and the full and half  Yo-Yo assemblies are given in the following chart.

Team Gantt Chart (w/ link):

https://www.dropbox.com/s/9l5harj9kx8vs9f/GanttChart.xlsx?dl=0

Thermoform Part Testing

Over the long weekend, we tried making a test thermoform piece with raised numbers on it. In the overall yoyo design, this piece sits behind the dial. The numbers have to be really small be seen within the holes of the dial (5/8" diameter), so we were concerned they might not form well and wanted to see if it was feasible before we committed to a design.

Here's a screenshot of the mold design that we printed:

We 3D printed the test mold on the Form2. Here's a video of the print (huge thank you to Dave D. for shooting it, and for dispensing the best advice for mold design and 3D printing!).

Here's a final photo of the test thermoformed piece.  

The numbers came really sharp! We're hoping to use 3D printed molds for the rest of the thermoforms during our actual production as well, to preserve the sharpness of the corners (as opposed to machining our molds). The next step is to figure out how we're applying ink to the raised numbers. 

Introducing Team Call Me Maybe!

                Team Call Me Maybe is here for the Fall 2016 2.008 season and all reports seem to indicate that this team of six will surely be turning heads at the upcoming 2.008 yo-yo exposition.  Team members Elliot Owen, Travis Libsack, Kerri Wu, Kyle Pina, Lindsay Epstein, and Revanth Damerla have worked together to develop a revolutionary rotary phone yo-yo design that is sure to create childhood nostalgia.  Consisting of four injection molded parts, one thermoformed part, and also including newly discovered “spring” technology, the design is intricate yet simple.  Stay tuned for developments as team Call Me Maybe makes its way through this year’s 2.008 season.