Accuracy

During a conversation about craftsmanship, a carpenter friend of mine told me about his first day working for one of the more successful framing companies in our valley.  The boss put him scrambling on the trusses of a house roof system they were sheathing.  He noticed that, each time he called down with his measurements for cuts on the wafer-weld sheets, the fellow sawing the sheets on the ground below would scowl when I called out something like “thirty-two inches and three sixteenths.”   Finally, after he yelled down from the roof for a new cut at forty-five and nine sixteenths, the man on the ground bellowed: “We don’t do teenths!  We are framing.  This is not finish work!  Give me a half or give me a quarter!”

They were very fast and did not see a necessity for undue accuracy in framing.

My friend quickly moved on to doing his own work and developed a finicky framing style that I greatly admire; I hire him without ever asking for a price when I need help with something.  He told me regarding his “persnickety” concern with sixteenths in framing that “a sixteenth in the basement can blowout to being a half-inch out by the time you get to the roof.”  He added. “You will fight it the whole way there.”

At least the framing crew was using the same measurement scale on their tape measures.

Perhaps you recall the famous miscalculation on 1999 Mars Climate Orbiter where the guys on the ground (Lockheed) calculated thruster force in terms of pound-seconds while the guys on the roof (the Jet Propulsion Laboratory) thought the craft thruster stuff was in the metric units of newton-seconds.  I don’t know the first thing about newtons, but this sounds pretty bad.

Near as I can tell, one pound is equivalent to 4.4ish newton.  The miscalculation translated into the craft pushing itself about 60 miles off course after a 416 million mile journey on a trajectory that looped the craft all the way around the sun to intercept Mars on the opposite side of Earth.  Certainly not close enough for guys nailing together a home—a lot of caulking would be required to seal that gap.  The Climate Orbiter stopped communicating while attempting to insert into orbit around Mars and is thought to have either disintegrated or skipped hard against the Martian atmosphere and then tumbled off toward the sun, disabled.

By the way, technically, Mars is ever changing in direct distance from Earth.  Though the two planets orbit the Sun in the same direction, Earth is on the inside track and nearer to the Sun.  We have a much shorter year and essentially lap Mars on the inside every 780 days.  When the two planets are in opposition (on opposite sides of the sun) they are about 249 million miles apart (measured in a straight line).  This translates into 225 million kilometers for those of us interested in further complicating this matter.  Additionally, if you measure from Mars to where my cat (20 pounds worth) lies on the floor of my sunken living room, the distance would be 249 million miles and roughly 1½ feet.   In 2003, while on the same side of the sun, Mars and Earth found themselves a mere 56 million miles apart.

In my years of construction, I have seen all manner of measurement errors.  I once worked on a nursing home where the plumber stubbed all of his pipes up in the poured-concrete floor of the center hallway—missing the walls by almost a foot.  Some of his pipes landed in doorways.  A laborer spent about five weeks on a jackhammer busting-out concrete so the new plumber they hired could fix that one.  I also know of a gymnasium in a small town on Montana’s Hi-Line (Highway 2) that was constructed a full two feet longer than drawn on the blueprints due to a measurement error that started with the concrete cast in the initial hole in the ground.

In horseshoes a few inches can be close enough to win the game.  A bullet zipped anywhere within arm’s reach of you is far too close.  On the other hand, the remote for your television, if only six feet away, is much too far.  I once worked on a 10 megawatt generator that rotated on a 14-inch shaft and weighed dozens of ton.  The tolerances on that required accuracy within 2 or 3 thousandths of an inch.   

Accuracy is relative. 

Just the same, should you ever find yourself needing either a framing carpenter or rocket scientist to help you with a project, you might establish early in the undertaking that the craftsman you hire is interested in teenths.       

--Mitchell Hegman

Click here to see Mars and Earth orbiting the Sun:  http://www.windows2universe.org/mars/mars_orbit.html