Wrapping up

We used Ashby's Model to estimate cost and assumed that all manufacturing was done in the United States with labor cost $13/hour. The material used was ABS. We assumed that the total cost of the equipment was $100000 and that the life time of the machine was estimated to be 10 years with a load factor of 0.9. The waste fraction was assumed to be 0.3. We approximated overhead costs by considering wages of plant workers and energy consumption by the plant.

For prototyping a run of 100 yo-yos, we calculated a price of $12.81 per yo-yo. A production run of 100,000 would bring that price down to $4.41 per yo-yo.

Ashby cost model
part mass	0.0384	kg
mtl cost	3.902	$/kg
waste frac	0.3
C1 (matl)	0.21	$/part
prod rate	0.23566	parts/min
equip cost	100,000	$
load factor	0.9
lifetime	10	yr
	5256000	min
C3 (equip)	0.09	$/part
*Assume need to buy but will be used again
Ct	840	$/tool
nt	1,000,000	tool lifetime (parts)
n	10,000,000	total run (parts)
C2 (tooling)	0.0008401	$/part
	8,401	total
Coh	58	$/hr
prod rate	14.1396	parts/hr
C4 (overhead)	4.10	$/part
Total cost($/hour)	4.41	$/part
for 100 yoyos
n	100
C2	8.4014
total cost	12.81
for 100000 yoyos
n	100000
C2	0.0084014
total cost	4.41

2.008 shop design constraints

The first iterations of our design involved high levels of detail in areas that would have to be pocketed by the milling machine. This was a poor design decision since the capabilities of the milling machine only allowed a depth of 1/2” pockets for 1/32” radial precision. To improve our design from this stage we decreased the width our yoyo by decreasing the depth our pockets, such that the depth of our pockets maximized the capabilities of the milling machine.

To make the yo-yo more suitable for manufacturing, we would alter the design of our thermoform core. The current pattern is difficult to line up with the die for cutting, slowing down production. A better set of alignment holes would improve the process.

Improving the course

Here are some suggestions from various members of our group.

“Although we all gained familiarity with MasterCam towards the end of the semester, the process could have been a little less painful if we would have had to complete a guided tutorial by ourselves, with Dave roaming around, answering questions about why exactly we are modifying the settings and changing parameters described. Dave’s tutorials were good, but most of the times I just followed along cluelessly, leading to very little awareness. Lectures were also good but not engaging. The first few ones had demos and were exciting (especially the guest lecturers from Stroud), but towards the end, energy dwindled tremendously.”

“I think that it would have been better if the homeworks, lectures, and labs were more related to each other”

"Overall, I loved the class and deeply appreciated how much I learnt by the end, but I think that it would have been better if the homeworks, lectures, and labs were more aligned. Specially I feel that the first Psets were not related to what we were doing in the lab, and this made me a little bit confused. Also, I feel like I would have learnt much more if we were allowed to discuss Psets with other people. "

“I think the lectures should’ve had a quiz component at the end of every lecture. I really appreciate the synthesis that this provides. I also think that expectations for psets were unclear and that the lecture material was very unrelated to the psets. Though I appreciate the learning that happened during the psets, part of what makes learning fun is going to lecture and applying the fundamentals learned there to a word problem. I also think turnaround time could’ve been better. We didn’t know what level of detail was expected for psets until the third pset. I thought the lab component of the class was fantastic though an extra staff member would’ve been useful.”

“I was unable to make it to office hours for many of the homeworks, and found myself struggling as a result. I wish that the lectures had covered the homework material in more depth.”

Putting everything together

1.

The back shell of our yoyo in the two images above is a relatively simple part.  The distinguishing features are the text on the outside, and the bevel on the inner diameter.  This allows the holder part to be press fit into the back shell.  We didn’t encounter much difficulty with this part, as it is very thick and did not experience much shrinkage.

Front and back views of the holder part.  Each yoyo has a pair of these, one on each half.  The nut in the center holds the shaft that connects the two halves.  This part went through a number of design changes.  It has to be able to press fit into two different parts, the back and front pieces of the yoyo, and must be very precisely sized.  In our first iterations, the ring around the edge was too thin, and was unable to hold a tight press fit.

The front filler piece is the front part that attaches to the holder.  The inner cavity is very similar to the corresponding back shell, as it has to fit the same piece.  The front surface is patterned with Mike’s eye, horns, and teeth.  The horns and teeth poke out through holes in the front outer shell.  This part didn’t see a lot of re-designs, as long as we kept it consistent with the back piece.

The gap between Mike’s teeth.  This part press fits in between the two rows of teeth.  Despite its unusual geometry, this was one of the easiest parts to design – our first mold design worked perfectly, once we tweaked the cooling time to make it fit properly.

The front shell is Mike’s face.  You can see the holes for the mouth, eye, and horns.  The inner diameter is sized to press fit around the inner fill.  The most notable feature is visible in the second picture – the thermoformed eye fits in the center hole, where it is held snugly in place by the pressure of the press fit and the ring that surrounds it.

This bit of transparent thermoformed plastic is Mike’s eye.  It is sandwiched inside the front shell and front fill pieces, held in place by the pressure between them.

Here he is, all put together.  This isn’t quite the final version – it’s the wrong color, and our fully assembled Mike has a sticker inside his eye for the pupil and iris – but the dimensions and parts are all the same.

2.

General Parameters	Parameters of Interest	Specifications	Measurements
Length Dimensions	Diameter	2.3 in	2.289 in
	Width	1.26 in	1.266 in
	Gap Width	0.15 in	0.16 in
	Height of Oscillations	2.5 ft	2.5 ft
Mass Dimensions	Mass	0.12 lbs	0.12 lbs

Our specifications were generally on-target.  Due to material shrinkage, the diameter is slightly smaller than the one specified.

3.

We manufactured 112 holder parts during our production run, with an average diameter of 1.7187 inches. As shown in figures 1 and 2, the process was well within control limits, and displays no trends that indicate a problem with the process. With a standard deviation of 0.000369 inches and a tolerance of +/- 0.002 inches, the process capability is measured to be 1.807. This value is high enough to be satisfactory.

R-Bar control chart for the Holder part

X-Bar control chart for the Holder part