Thursday, June 11, 2009

It's All In the Units

I am excited about working on a cool project this summer - I am designing a new machine. I'll post more on the project in a later post, but right now I am just in the earliest concept and planning stages. I've done some pencil and paper sketches, and shopped around for the parts I think I'll need.

I sat down today to start putting this stuff in CAD, and crank out some pretty 3D models to show my advisor at our next meeting. I immediately ran up against the very first design decision.

There it was, staring at me in a little pop up box. "You are creating a new assembly" it said, "Would you like mm or in?"

Well gees. I hadn't thought about it.

You know, shuttles have exploded due to picking the wrong units. This is important stuff.

There is always a debate in engineering on which units to use. The rest of the world has settled on metric units, but here in the US everyone think in English units. In academia we can get away with using metric for classwork (because who wants to deal with the stupid g_c factor anyway?), but in the real manufacturing industry (which our lab interfaces with, since we are a production and manufacturing lab) everybody still uses English.

Most professors compromise by requiring students to do problems in both sets of units, and so I've become used to converting over the years. We used to joke about the "kip" (which is a thousand pounds) as being, in the words of one of my more colorful professors, "the bastard child of the English and metric systems."

This units business is especially an issue in my lab because we make very tiny things. And everybody knows tiny things must mean micro-(insert your favorite buzzword here) or nano-(insert buzzword here). Most of my data is measured in microns, which is short for micro-meters. Some of my data is in nanometers. Nobody works in nanoinches.

But the machine I built for my Master's is in nice English units - 1" by 3" platens, 1/4-20 bolts. I use drill bits by the thousandths of a inch, and the machine shop tools are incremented in tick marks every five or ten thousandths. We have more standard than metric wrenches and hex keys and hand tools.

So I have to make a choice. I will have to use many shiny toys to build this machine - our new micromill included. Should I order the 1/64" drill bits, or the .5 mm end mills?

In short, "Would you like mm or in?"

I picked inches.

Because in the end, I think better in English units. I have a better sense of scale for English units. I will still have to convert between units I am sure for various reasons, but I am less likely to make a mistake in English.

And in the end, possibility of less mistakes and more convenient resources trumps international standards.

What do you work in? For class and/or for research?


  1. I'm having the same issue with the giantnormous mill in my lab. I'm in Canada though, so I've grown up with the metric system. When I started my first lab job building equipment and stuff, I quickly learned that the machine shop exists in a world where there's only one measurement system, imperial. In the end it's just easier for me to do everything in imperial units, too, even though I miss my beautiful base 10 metric system.

    Please encourage your country to abandon imperial units and force machine shops to switch to metric so I can use base 10 again :)

  2. milliliters!
    the only unit i can eyeball :)
    then again, i'm a chemist...

  3. I can't handle the English system anymore. I only think in metric... which is quite complicated when it comes to cooking. I need a quart of something? What the hell is a quart?! Do I have enough? GIVE ME UNITS IN METRIC!!