On the Reverse Engineering of Big Old Machines

Levels and Kinds of Models

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Not ideas about the thing but the thing itself. - Wallace Stevens. Collected Poems.
... a Map of the Empire whose size was that of the Empire, and which coincided point for point with it. ... that vast map was Useless. - Jorge Luis Borges. "On Exactitude in Science".

Any representation we might make of a machine (even just some hasty notes or a sketch) is a model of it, because what we create stands in for the machine but is not the machine itself.

This can be done in many ways, to various degrees of exactitude. All of these ways are (potentially) useful, and all of them (potentially) useless, depending on what we need at some particular time. It's well to review a few of them.

Perhaps the simplest model need not even show the machine itself: the rigger's list of dimensions and the erector's floor plan. If you've only ever dealt with pictures of and words about old machines, you'll be apt to dismiss these. But when you come to move your first 3,000 pound cast iron Behemoth, you'll be desperate for them.

[TO DO: User's manual level drawings / 3D models.]

[TO DO: 3-D scanned parts, and new parts made directly from them.]

[TO DO: Drawings/Models sufficient to manufacture a new part.] A 3-D scan of a part captures the part as it actually is today. A proper engineering drawing (and 3D model, in modern systems) captures the part as it should be.

There are two levels here. The first level, which is entirely adequate, is a modern 3-D model plus GD&T 2-D set of drawings which would enable any modern machine shop to manufacture a brand new part which would fit.

The second level captures the part as the original designer really intended it. This is of course difficult at best, and almost certainly impossible if you desire perfection. In many cases we just don't - and can't - know. But with care and sympathetic imagination we can get close.

See also the discussion of Becoming an Engineer of 1886 in the ../ Intellectual Habits Notebook.


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