[clfrye]

Why does Taig mills use small plastic tubes to connect the couplings between the stepper motors and the lead screws and not some other form of metal to metal connector?

[awetmore]

You need something with a minor amount of play to handle any mis-alignment of the stepper and leadscrew.

Their coupler puts a easily replaceable failure point in the system that is a lot cheaper to replace than a broken stepper or leadscrew.

Besides replacing the plastic bits every few hundred hours of milling it is a system that works great. If you want to use something else you could get an Oldham coupler. I wouldn't recommend just putting metal in there instead of the plastic, you'll either end up with couplers that can handle no misalignment or they'll have some play that will turn into serious backlash.

[cameraguy]

exactly. an oldham coupler does the same thing as four bits cut from a roll of tube, but one costs $15 per each axis and the other $15 for a 100' roll.
finding unique ways to cut $50 off of the machines price here and there, in ways that don't ultimately affect or degrade performance, is why the Taig is a bit unorthodox in places. However, its also why it is such an amazing value for its price.
the gentleman who designed the mill had very detailed and coherent answers for why he did everything the way he did on that machine. Cliff was an amazing machinist and tinkerer, and he did a few odd things in managing to design it to still hold that price point without gimping the machines capability. it very easily could have turned into a $2k machine instead of a $1k one. so, in some ways its construction is a bit odd compared to commercial machines, and considering why they were done i am seriously ok with that.