1) I am very aware that machining and casting makes higher quality parts. My point is that for all mechanisms there comes a point where the quality is "good enough".

2) The printed gears you linked to have abysmal quality. My 3 year old printer has made vastly superior parts. See this thingiverse part, with many high quality print examples from home printers: http://www.thingiverse.com/make:17409

I also used to be of the opinion that CNC machining was so superior as to render 3D printing useless. I looked at those stringy prints and said it would never be useful. But then, I slogged for years making CNC machined parts (see some examples of my parts here: http://www.tlalexander.com/files/portfolio.pdf ) and eventually decided to buy a 3D printer. I make more stuff now than I ever did when I had daily access to a CNC.

But as far as quality - notice how that stringy gearbox you linked to still seems to work? Sure, one is more photogenic, but the goal of a gearbox is to transmit power not win a beauty contents. Meanwhile I have been using a printed gear on my 3D printer's extrusion system for over 2 years with no sign of failure. The application is low speed but medium torque, and I see no wear or signs of weakening. The printed gear satisfied the requirements of the mechanism.

When I think of 3D printing, it reminds me of manufacturing in the 1940's. Parts were clunkier then, like the seat hinge on an old Volkswagen Beetle. But clunky works - often better. Newer techniques might make smaller hinges, but the one from the old beetle still works well. Sometimes higher quality isn't necessary to make something. A designer may choose to make something clunkier so that it can be made at home on a basic printer.

Another example- I use this coat rack and lamp mount every day: http://www.thingiverse.com/thing:16485

My long rant about CNC machining was meant to highlight the huge amount of labor involved in machining things. It takes a lot of human skill to write a CNC program (I do expect that to eventually change though) and more labor to set up the machine to get the first part made. In the time it would take me to CNC something I can print a part while I eat a sandwich or run some errands.

Of course 3D prints are slow to produce, but that is counteracted by the fact that it takes less of my involvement to get the parts made. It takes less overall effort to produce a part by print than by machining.

I mentioned metal but the entire story I wrote applies to plastic machining as well - just replace "Campbell Metal" with "Professional Plastics". They're both across from the Fry's off Brokaw.

I totally don't expect most people to have "run into" a time where home 3D printing is best, but you can still do thought experiments. 3D printing is far more accessible to most people than CNC machining is. As home machines get better and we gain more critical mass of users, we will see more designers who take advantage of the stuff that can be made at home. That's what I'm doing with 3D printable robots, and luckily I have a good intersection between people who want to make robots and people who own a 3D printer.

I would say the two key differences between 3D printing and machining is that 3D printing takes zero skill to operate and zero or near-zero labor. Not all home machines have delivered on the ease of use promise of course, but that is an issue with the machines, not the process.

My point is, no other manufacturing technique is well-suited for use at home aside from perhaps laser cutting. The value in printing is of course not the quality of the parts, but the fact that it is a type of manufacturing accessible to consumers. Imagine if Dyson provided printable replacement parts for example, and you can begin to see the value.

Every manufacturing technique has trade offs. A single print takes longer than machining a single part, but with less set-up. Time to first part can often be lower on a 3D printer.