Why Not Fabricate and Assemble 3D Parts in Space?

If 3D parts printers can be made to work in zero gravity...

Well that's a pretty big if, isn't it? The ones I know about work using by selectively curing regions within a bath of liquid resin. It's kinda hard to hold down a bath of liquid resin in zero-g. And wouldn't just the cost of putting all that resin in orbit cost more that putting just the finished parts into orbit?

Using a version of the Navy electric rail gun that was discussed recently on CR4, finished parts could be launched into orbit pretty cheaply.

Some of the advantages of low-earth orbit is that you can make a centrifuge that can produce any degree of gravity needed.

However, I just can't see a payoff for all the extra effort and cost unless the end product is something very exotic and of great value (here or outside the confines of Earth).

"Science reporters" should be required to get the decimal point to within at most two places....

There are 3D printers that build solid parts a layer at a time without a liquid vat, but the final parts are usually very weak and brittle, so are not suitable for use as a working part. Materials are constantly improving, though. Also, the energy requirements are significant, and would also result in a great deal of heat that would have to be dealt with.

Like others have said, there will have to be a financial benefit to building on site versus on earth.

The first thing that comes to mind is weight. A collection of finished parts would probably weigh less than the raw material needed to print it. Isn't weight one of the most important considerations in escaping Earth's gravity?

If you could send up or process scrap into powdered aluminum, you have the right vacuum conditions to form and weld aluminum since there is no atmosphere to oxidize the weld. With the powdered aluminum, you might be able to focus the energy to grow the piece slowly rather than needing the instant mega-Btus of a furnace. Just exposing the powder to the sunny side should bring the temprature up to a few hundred degrees right off-the-bat.