Adam, I am nervous about getting deep into the answer of what kind of cable and why. Here is the plain English version:
Mostly USB needs to be three conductors and a shield, or four and an shield. The data + and - should be a twisted pair within that shield. Beyond that it mostly needs a strong enough jacket to withstand whatever physical things you do to it.
The various versions of the USB spec call for magic limits on length, capacitance and other parameters. Most likely any short extension you add will not effect the operation.
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Electrical engineering rational behind the previous advice- the usb is designed for a maximum cable delay of 1500ns. Signals require just under 2ns per foot (1.8 is the text book value) to travel down the wires. So that allows a physical limit of just over 700 feet. Capacitance effects will come into play before that. And transmission line effects wont happen until approximately 1/10 of that which is around 70 feet. The specifications prohibit cables longer than 16 feet, which is designed to be sure that no-matter-what it will work. WIth a little care it can probably be made to work at distances well over 100 feet.
The cable has resistance, so the power capacity of USB will degrade with distance. That is directly computable from the wire gage you use and the amount of current needed. The shopbot has its own power, so I suspect there is none being carried on the power line of the USB, so it should not be a factor.
The twisted pair for the data does matter, though it is not super critical. I have worked with LVDS signals in other applications where they were not twisted for "equipment cabinet distances" and everything still worked ok. The length of the + and - should be very close to the same, and twisted cable helps there.
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Feel free to add some wire if you need to, or replace the original. There is nothing really magical about it. Try it, save the original cable, you can always go back.
D
"The best thing about building something new is either you succeed or learn something. Its a win-win situation."
--Greg Westbrook
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