Tolerance and Precision


Lufkin…Starrett…Brown & Sharpe…The Holy Trinity

Skeat:                                                                                                                                           precision: (L>OF) precis, strict. (L) praecidere, to cut off near the end.                   tolerance: (L>MF) tolerantia, endurance, sufferance.

precision: accuracy, degree of refinement in measurement, etc.
tolerance: permissible variation in dimension or weight; forbearance.

In particular, I am obliged to Mr. H. Wedgewood for his publication entitled ‘Contested Etymologies in the Dictionary of the Rev. W. W. Skeat’. I have carefully read this book, and have taken from it several useful hints. In reconsidering the etymologies of the words which he treats, I have, in some cases, adopted his views either wholly or in part. In a few instances, he does not really contest what I have said, but notices something that I have left unsaid…Hence the number of points on which we differ is now considerably reduced; and I think a further reduction might have been made if he could have seen his way, in like manner, to adopting views from me.
from the Preface to the Third Edition, Skeat’s Etymological Dictionary of the English Language (tolerance and precision, in one brief paragraph…)

I have encountered a lot of different attitudes about precision in woodworking, as in “How close do you want this?” House framers are usually satisfied when the difference is no greater than ⅛” (praecidere: cut off near the end), finish carpenters and cabinetmakers measure to the finer side of 1/32″, hand plane enthusiasts can shave as fine as .0003″ off a surface. As I see it, if you can cut to the shop standard, your work should be acceptable. Tolerance: permissible variation in dimension.

Precision, expressed in the tools of the machinist: accurately divided rulers, micrometer calipers, dial indicators, etc., is thus subject to tolerance, “How closely does it need to fit?” Tolerance can also accomodate changes in dimension of parts, the space that allows a drawer to operate during a humid season, for instance.

On a summer job in my teens, I assisted an older black man, Edward Perkins, a master millwright and a Methodist minister. Perk was an exacting teacher. We spent a lot of time fitting the bearings and sprockets on the massive shafts that pulled the conveyor chains that lumber rides along its way through the sawmill. The shafts: several hundred pounds of cold-rolled steel, three inches in diameter, twenty-some feet long. Once they were on horses, my job was to make a bearing race slide easily from one end to the other. My tools were a large mill file (clerk, “Do you want one of these little bastards?” customer, “Nope, I’m gonna need one a’ them big mother f***ers!”), emery cloth, and a ball-pein hammer.

The other men joked about my teacher, that he was so slow you had to make a mark beside him to see whether he was moving. The fastest way, it turned out, was to do things right the first time. Drive something into place with the ball-pein, and two men and a sledgehammer can’t get it apart. One part out of sequence, and the entire assembly has to be taken apart and done over. All the little empirical lessons of mechanicking…precision, tolerance, patience, sequence of assembly…never force anything. The effort required to move a jammed piece is exponentially greater than the effort required to put it there. Simple in theory, difficult in practice.

So, all measurements are precise within tolerance, OK?

The other meaning of tolerance seems to have been largely forgotten in our society: forbearance, endurance, sufferance. If we add the Latin precedent precis as an adjective, strict tolerance would mean to my mind an imperturbable patience. No one has ever suffered my ignorance any more gracefully than did Edward Perkins during that long, hot Mississippi summer of 1969. In particular, I am obliged

n.b.  The little tool at lower left is (to my reckoning) a patternmaker’s draft gage.

About michaellangford2012

Timber framer, boatbuilder, dreamer, writer, musician; collector of books, tools, aphorisms. "There is nothing, absolutely nothing…half so much worth doing…as simply messing about in boats."
This entry was posted in architecture, boatbuilding, carpentry, traditional building, woodworking. Bookmark the permalink.

3 Responses to Tolerance and Precision

  1. Pingback: Tolerance and Precision | michaellangforddotorg

  2. Lenard Burgess says:

    Excellent post, as a diesel mechanic of 30 years + it is amazing to me how many of my coworkers over the years could not understand this simple, yet incomprehensible idea “The fastest way, it turned out, was to do things right the first time”

  3. Pingback: Lie-Nielsen #102 | michaellangforddotorg

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