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From the website https://plan28.org/ and blog http://blog.plan28.org/, it seems that the original documents are scattered and hard to decipher. It might be that the plans are not even close to be finished.

So, I wonder, why not, at first, build machine, made of 3D printed parts, that would closely resemble the original mechanics and have the same programing language? Wouldn't it at least help to understand the original schemes?

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For the same reason you don't build a bridge by grabbing some metal and stone, dragging it to the shore of a river, and start stacking it and bolting it together.

Trying just to build and run the Analytical Engine from incomplete plans is about as likely to work or be useful as typing in an incomplete computer program and "just running it."

But worse, since these are physical parts you're talking about, fabricating them without without sufficient planning is would inevitably lead to massive amounts of expensive rework as you find out that pieces you've built don't fit together and/or work properly because you didn't bother to check the design first. It's much cheaper, especially in this age of easy 3-D modelling, to build a computer model and test it before doing the real thing. (And by the way, it's unlikely that much of it could be built from 3D printed parts; the stresses many of the components have to deal with will require fabrication in metal.)

Since there's currently no plan in complete, ready-to-build form, that needs to be made first, and that's made by carefully studying all of Babbage's work over the decades he did it. Even then, we'll need to understand the plan and the techniques used in it, and that's also what we get through study. (Imagine you were given detailed plans for a modern bridge. Would you be able to supervise the construction workers to get it built safely and economically?)

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    To focus on the OP's actual question, how would you produce 3D printed parts, without first building a virtual computer model of them? And once you have computer models of the parts, why not do the obvious thing and model how they work together before you make physical printed parts that maybe you can't even assemble. – alephzero Oct 1 at 16:00
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    A virtual model would require many polygons and the collision boxes would have to be very precise. If you have thousands of elements, I think it would be very slow. What I think is that you are not building a bridge here, but a small bridge, as a proof of concept. There are guns built from with printed plastic, so I am not so sure about the stresses. I am also not so sure of whythere would be no planning. – user36088 Oct 1 at 16:11
  • @user36088 Well are you suggesting that they start building something now, or that they continue on their current path of studying the documents and trying to make plans based on them? – Curt J. Sampson Oct 1 at 16:14
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    @user36088 The slowness of a computer simulation of a candidate design will seem lightning-quick compared to dismantling and re-assembling a physical machine over and over again as design variants are tried out. – John Dallman Oct 1 at 21:37
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    @user36088 It's my opinion, rather than something tested in practice, but mathematical modelling of solid objects is what I do for a living. With real CAD software, there's no need to do it with polygons: you can work with accurate shapes. – John Dallman Oct 2 at 6:53

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