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Sunday, February 19, 2017

When is "O" a "0"? - A Lesson Learned!

So here I am.  I've finished my course in aircraft electrical theory, now I have a week off before I start off into hydraulic and pneumatic systems.

My mentor and my nemesis.  The electrical trainer.
The instructor didn't let us use lights so we couldn't cheat! 
That gives me a little time to work  in Fusion 360 again, and I took the opportunity to build 3D model of a spar for a PT-17 Steerman biplane.

A PT-17 Steerman
By USN - scan from Robert L. Lawson (ed.): The History of US Naval Air Power. The Military Press, New York (USA), 1985. ISBN 0-517-414813, p. 72. US Navy cited as source., Public Domain,

The modeling itself isn't very difficult, but my goal was to focus on the process of designing in Fusion 360, instead of focusing on the design itself.

I've purchased a subscription to a set of plans via my membership to AirCorps Library (A great resource BTW),.  They have a digitized copies of the spar, so I have a template to work from

The spar, being made of wood, has metal bushings pressed into it, and there lays the seeds of my error.

The spar modeled in Fusion 360, with the bushings assembled in.

The bushings are listed in tables, your typical "A" "B" columns with numbers.

But written in one of the columns was a suspicious looking "O".   It had to mean the number "0".  You know.  Zed. Naught.

An example of the "O" that got me. 
Confident in my conclusion of the figure's value, I modeled the bushing without a hole in it.

But as I my design progressed, I came to realize something.  That's not the number "0"!  That's the le letter "O".  As in a "letter O drill"!.

A letter "O" drill has a diameter of .316.  You can find it on any Imperial system drill chart.  Like this one here.

Fortunately, Fusion 360 did let me remedy that issue pretty fast.  But it did provide me a few moments of humor of what I saw, versus what the drawing meant.

Some of you may be thrown a mocking laugh, chuckle, or perhaps a guffaw in my general direction.  And I can't say I blame you.  Looking at the plans, the quotes around the letter should have implied it wasn't the number "0".

It also seemed odd to me that there would be a metal pin pressed into the wooden structure, but I shrugged it off as being a reinforcing pin.  It was only when I studied the assembly plans a little closer did I see that the bushing did have a hole in it.

I choose to blame it on the early hour of the morning (it was 5AM), and the coffee having yet to fully clear the fog in my brain.

So what have I learned?  And perhaps, what can you learn from my mistake?

  1. It pays to know how something is built.  Even though I missed the mark initially, it did finally dawn on me that the drawing called out a letter "O" drill.  Had I not drilled my own holes, and known that drills can be called out by letters, I may have shaken my fist and accused the drawing of being wrong. 
  2. Try to use your common sense.  I should have double checked the bushing against the assembly when I had my initial doubt.  It did clearly show the bushing as having a hole.  That might have tipped me off. 
  3. Try to understand how the drawing is meant to be interpreted.  This drawing was originally made in the 1930s.  It's format is much different than the drawings I use at work today.  Things considered obsolete in 2017, like fractions and certain styles of notes, were the standard when this drawing was created. 
  4. If you think that bullet 3 is a bit of a stretch, there are places that maintain drawings for decades.  Even at work we have drawings from the 1970s that look more like the drawing from the 1930s than the 2000s. 
  5. You're going to learn something new everyday.  Embrace it! 
So there are my lessons!  Thankfully, it's a small error, easily corrected, and with a lot to teach!   

Learn from my mistakes, and keep an open mind to the lessons our profession has to teach! 

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