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: 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.
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