Design for Assembly: Examining a Child’s Sippy-Cup

When I was in graduate school at Carnegie Mellon University I did my master’s research in Design for Assembly (DFA) using Fitt’s Law as the centerpiece, and Boothroyd Dewhurst as a supplemental methodology.  One of the things I researched was the idea of partial symmetry; essentially, there are often things are almost symmetrical, but not completely symmetrical.  Why is this important?  Because when an object is truly symmetric, it can be assembled in any number of ways (e.g., a square peg into a square hole because there are four orientations that work vs. a peg with a duck profile into a duck profile hole).

Often, however, things can be almost symmetrical visually but not quite enough to be assembled in multiple ways.  When assembling such a component, that asymmetry must be detected so that the assembly can be done correctly; sometimes that asymmetry can be very difficult to detect.  And in the worst case, there might a situation where something could be assembled multiple ways, but only function in one.  My research on partial symmetry – part of my overall research – resulted in a paper:

Hunt, D.O. & Sturges, R.H., 1994.“Detection and Evaluation of Planes of Partial Symmetry in CAD Models,” ASME Design Automation Conference, Minneapolis, MN, Sept. 1994.

I recognized an application of this at home and thought the practical example might be useful.  Please take a look at the picture, below.  It is the cap of a child’s sippy-cup, which has two bosses inside it where a silicone rubber piece – which prevents leaks if the cup is tipped – is pressed into each of those two bosses (and is removed each time for washing).  So here’s where partial symmetry comes in: the two female bosses on the cap’s inside are different diameters, which means the male parts of the silicone piece also need to be different diameters in order to fit snugly to prevent leaks.  The picture shows my attempt to assemble them the wrong way – it doesn’t assemble. (Click on image for bigger picture.)


Each time I put this together I need to squint at the silicone piece to figure out which end is which; it takes time, and is getting on my nerves.  The time to recognize which end is which is especially critical when my daughter is screaming “Juice!  Daddy I want juice now!”

One thing I mention repeatedly when discussing DFA is that DFA is performance-ignorant.  So I will grant you that there might be some functional reason why these two bosses might need to be different sizes.  But it’s a mystery what that reason might be.  I see no significant differences between the two sides of the insert, which has the stopping-leaks function.

During a design review, the question should have arisen: Why does this asymmetry exist?  If there is no functional reason why it needs to be there – and, again, I see no reason why it does – the two bosses inside the cap and the silicone insert should have been redesigned so that the insert was functional in either orientation.  However, if there truly was a need for the asymmetry, then the design should have been altered to make the asymmetry more distinctive.

Lesson One: When components are almost symmetrical, during design reviews ask whether the asymmetry exists for a functional reason.  If there is a functional need for the asymmetry, design the parts to be less symmetrical so that the difference between orientations is clearer – even to the point of adding cosmetic-only features to help people differentiate which orientation is which.  If there is no need for the asymmetry, then design the parts to be truly reversible.

A second lesson exists.  This is not based on actual results, but merely my own observations and thoughts on symmetries and asymmetries.  (If I ever did a PhD, I’d love to study this further.)

Lesson Two: If you need to put in an asymmetry, don’t just try to distort the shape or alter the size of something, as was done in this example of merely different diameters.  Instead, add or subtract a feature so there is a feature-based difference between the orientations, not just a size-based difference.  This, I believe – without empirical proof though it “makes sense” – will make that orientation requirement far more noticeable. 

And a final observation about partial symmetries:

Lesson Three: If a component needs to be asymmetric, do your best to make sure that it cannot be assembled the wrong way. (I concede that was done here.)

Let me give an example of where this didn’t happen; this story is in my ASME paper, actually.  A small felt wick was assembled into a high-speed bearing; its function was to wick oil from a reservoir to the bearing for lubrication.  First, the wick was partially symmetric as it had a slight taper on one end, but the difference was small and complicated by the wick being a small part to begin with.  Second, it could be installed in either orientation, and the people didn’t know.  Needless to say, after a few months on the market, bearings began to fail left-right-and-center.  Which leads to:

A corollary to Lesson Three – make sure your line operators know too.  During my grad school research I came across an example of a yoke arm (link is an example only) whose mounting holes on either arm were partially symmetric.  These two holes were just slightly different diameters to ensure proper orientation of the mating piece – this was done deliberately.  However, since the operators didn’t know there was a difference, and there were no written work instructions detailing that there was a difference, the operators would attempt to assemble the unit and then find one hole was “too small”.  They would then blame the machinists (doubtless with colorful language), and enlarge the smaller hole until things went together.

Lastly, I would be a poor criticizer if I didn’t attempt to show what I think might be a good redesign.  So let me assume that there is, in fact, a functional reason why the two sides need to be different – please look at the other picture, of a CAD model I whipped up. (Click on image for bigger picture.)


  1. One side has small tabs, which should match with new notched cut-outs in the cap’s boss (following Lesson Two but also Lesson Three).
  2. The tabs add material, as do different sized fillets between the bosses and the center connecting piece; this is to attempt to balance material between the two sides so the gate can be in the center.
  3. The two bosses are different diameters, to ensure Lesson Three applies.


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