This is what manufacturing will be like in the future. Ask a
factory today to make you a single hammer to your own design and
you will be presented with a bill for thousands of dollars. The
makers would have to produce a mould, cast the head, machine it
to a suitable finish, turn a wooden handle and then assemble the
parts. To do that for one hammer would be prohibitively
expensive. If you are producing thousands of hammers, each one
of them will be much cheaper, thanks to economies of scale. For
a 3D printer, though, economies of scale matter much less. Its
software can be endlessly tweaked and it can make just about
anything. The cost of setting up the machine is the same whether
it makes one thing or as many things as can fit inside the
machine; like a two-dimensional office printer that pushes out
one letter or many different ones until the ink cartridge and
paper need replacing, it will keep going, at about the same cost
for each item.