I think it is different from e-mailed instructions on how to assemble Lego blocks, in the sense that 3D printing lets you manufacture your own Lego blocks. So 3D printing is really a logical "next step."
As to the material, again, why limit ourselves to plastic? Plastic is merely the "next step," beyond paper. Plastic happens to be easier to mold that other raw materials are, but there's no conceptual reason to think that 3D printing can't produce steel gearsets.
Ultimately, one shouldn't even be dissuaded by the idea that feeding iron to a 3D printer is ridiculous. No reason to assume the 3D printer has to be at home. We can also take our instruction file to a 3D printing facility at the closest shopping mall and have it spit out the finished product. Just as we might do to get professional quality photographic prints made, by taking of JPEG files to a photofinisher.
I agree we should not limit our imagination and the emailable machine/object is a great idea.
But does it differ conceptually from an emailable text file that tells you which lego bricks to put where...or an emailable map which shows the shop on your local high street to go buy a conventionally-made machine/object from.
The point of my article I guess was that everyone seems to think because the CONCEPT is extremely fascinating - be it self-replicating or emailable machines -- they also think that 3D printed manufacturing is juat around the corner and that the self-replicating or emailable machine delivered by 3D printing will be just as good as one made using conventional manufacturing.
There are a lot of engineering challenges between the concept and the reality
For example would you want a 3D printed part made today to be used as a replacement part inside your bicycle gears or automobile engine? Even if it met the dimensional specification perfectly it would not have been approved for use and would expect that its plastic construction would not be mechanically strong enough.
3D printing is perhaps a misnomer, but I don't think we should let that limit our thinking.
Hasn't everyone wondered why you can send only certain types of "product" over the Internet? Used to be just text, then it became graphics and essentially finished documents (minus the paper), then sound and motion pictures. So why the limitation? Why do I need to depend on parcel post to have a vacuum cleaner sent to me from a web site? Why can't the web site e-mail me the vacuum cleaner as it sends me a finished document that I can print out at home?
Conceptually, it's because we have technology that feeds paper in to a printer, but we don't yet have the next step that feeds ANY raw materials into a "printer," to manufacture any product, based on instructions received from anywhere else. So while I can order a book from Amazon, I can also, at least in principle, "manufacture" that book at home, by feeding raw material (paper) and instructions (say, an Acrobat file) into a printer, and create the entire product that way.
For now, the "raw material" is limited to paper (2D printing) and plastic (3D printing). Conceptually, it doesn't need to be so limited. Computer aided manufacturing is used to build all manner of products, and I think of 3D printing as merely the first step to doing CAM in the home. For Star Trek fans, 3D printing is another step toward that "replicator" device, where you place your order and out comes the product.
I did an interview last year for EE Times Japan with former Wired editor Chris Anderson who arguably did more than anyone to hype up 3-D printing.
Perhaps I caught him on an off day but he noted:
--There are still big limitations on what kinds of materials can be used by 3-D printers
--Volume manufacturing is still very much the game of big companies because it involves many kinds of machines and processes that are not networked and not available to the average person with a desktop 3-D printer.