
Matters of Tolerance - 80mph
https://www.nybooks.com/articles/2018/10/25/precision-accuracy-perfectionism/
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tlb
Great essay. Another, parallel essay could be written about designing for
coarse tolerances. For instance, in things made of sheet metal, elongated
boltholes are standard. In house construction, many of the ornaments we take
for granted like baseboards and door flanges exist to cover up sloppy
tolerances, allowing faster construction by less skilled workers.

Some wide tolerances aren't caused by workmanship but by mechanical stress or
thermal expansion. For instance, bridges usually have a set of roller wheels
between the deck and the piers, allowing them to move relatively by several
centimeters.

Mechanical timekeeping is a study in having some kinds of thermal expansion
cancel out other kinds.

The article mentions silicon as a domain of precision, but just as much effort
goes into designing transistors that work with as much lithography error as
possible.

~~~
theoh
In mechanical design, there is the related problem of how to make a system
which has the minimum number of constraints while performing correctly:
[https://ocw.mit.edu/courses/mechanical-
engineering/2-76-mult...](https://ocw.mit.edu/courses/mechanical-
engineering/2-76-multi-scale-system-design-fall-2004/readings/reading_l3.pdf)

A bridge which is supported by a horizontal hinge at one end, and rests on a
roller at the other, is effectively fixed, but it's not over-constrained by
being attached at both ends. This isn't quite what you are talking about:
rather than somehow covering up error due to large tolerances, or cancelling
it out (as in a gridiron pendulum or PAL tv signal) degrees of freedom are
included in the mechanical design so that those errors don't have structural
repercussions.

Kinematic couplings use in mounts for precision instruments use all sorts of
interesting combinations of geometrical contact surfaces, with the goal being
repeatability rather than accuracy: the instrument always ends up at precisely
the same location, and there are no potentially distorting stresses in the
structure (as would inevitably occur if there were multiple over-constrained
points of attachment)

[https://www.precisionballs.com/Micro_Inch_Positioning_with_K...](https://www.precisionballs.com/Micro_Inch_Positioning_with_Kinematic_Components.php)
[https://www.precisionballs.com/basic_kinematic_designs.php](https://www.precisionballs.com/basic_kinematic_designs.php)

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bhritchie
The first paragraph reminds me of the great example by which J.L. Austin
distinguished precision from exactness: a stick could be exactly, but not
precisely, six bananas long.

In the present case we could say that six random bananas could accurately but
not precisely represent the length of a stick.

~~~
bitofhope
My go-to example of explaining accuracy vs. precision is the distance from
Earth to its moon. 400 000 km or 250 000 miles is a fairly accurate, but
imprecise figure. 31 415 926 nanometers is a ludicrously precise, and
incredibly inaccurate one.

Also, accuracy is dependent on appropriate precision. Rounded up to nearest
billion kilometers the moon is zero terameters away from the Earth, which is
only accurate if you're comparing it on a scale where the Moon is
insignificant. Conversely, reporting the exact distance to the nearest meter
will imply variations insignificant on one-meter scale, which is obviously
wrong, making a less precise figure more accurate for use over an arbitrary
period of time.

