Given two or more ways…

Most people know Murphy’s Law as the cynical maxim “Anything that can go wrong, will go wrong.” A more engineering-oriented form, which some claim to be closer to Murphy’s original cry of exasperation, goes like this: “Given two or more ways of hooking something up, if one of them will result in disaster, then someone will do it that way.”

For example, the Jargon File says this about it:

…you don’t make a two-pin plug symmetrical and then label it “THIS WAY UP”; if it matters which way it is plugged in, then you make the design asymmetrical.

I bring this up because I solved an experimental problem on Friday that has been torturing me for months. I was using an aspheric lens to focus a laser beam. For people who don’t know what that is, it’s a small piece of glass with a flat side and a curved side, and it matters which way round you put it in. Take a look at the lens and the tube mount that came along with it. There is external thread on the lens holder, and internal thread on the tube mount:

Picture of the lens and its tube

How would YOU put it together?

“Now that’s an example of good design,” I thought to myself on what must have been October 26, 2009 according to my lab journal, when I first started using the lens. “There’s only one way of screwing it into the tube, so it’s impossible to get backwards!” And I thought no more of it.

And so, between that time and now, I tried all sorts of tricks to correct the strange deviations I saw in my experiment. But for some reason, this lens occupied a mental blind spot, until last Friday, when I was talking to one of my advisors about the experiment.

He asked if I’d put the asphere in backwards, and I said, “No, that couldn’t be it — it only fits into the tube mount one way, and if I was supposed to use the tube the other way around, then the focus would be inside the tube.” But I checked anyway, and, well, what do you know.

It turns out that the correct way to mount the lens in the tube is to drop it into the unthreaded end, and then use a special tool to tighten it.

The special tool

The special tool

This has to be some of the dumbest engineering I’ve ever seen on the part of Thorlabs, the manufacturer. Not only are there two ways of hooking something up here, but the only obvious way is the wrong way. And the right way is not only not obvious, but even requires a special tool which is guaranteed to get borrowed and never returned or misplaced in some toolbox somewhere. And worst of all, the whole issue could easily have been avoided by putting the threads on the other end of the lens holder.

At least I got the satisfaction of solving all my problems at once by flipping over a centimeter-sized piece of glass. Shame on you, Thorlabs.

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