Why the cable moved inside the arm
The cable on Model 1 caused more repairs than any other part. Here is how Model 2 hides it, and what that cost.
Of the Model 1 lamps that came back to us for repair, a little over half came back for one reason: the cable. It ran outside the arm in a loose loop at each joint, and every time the arm moved, the loop bent in the same place. After three or four years of daily use the outer braid wore through.
Threading a cable through a moving joint
Putting the cable inside the arm sounds simple until the arm has to turn. A cable passing through a joint gets twisted every time the joint rotates, and twisting is worse for copper than bending. The answer on Model 2 is a hollow brass knuckle with a 9 mm bore. The cable passes through the centre of the pivot, where it turns around its own axis instead of folding.
We limited the knuckle to 270 degrees of rotation, so the cable can never wind up more than three quarters of a turn. A stop pin inside the knuckle does this, and it is the only part of the joint you cannot see.
What it cost
The hollow knuckle takes four times as long to machine as the solid one on Model 1, and threading the cable adds about six minutes to assembly. Those two changes are most of the price difference between the two lamps.
What we got for it
In two years of testing, the arms on our test rig have each moved through 40,000 cycles, which is roughly ten years of being adjusted a dozen times a day. None of the cables has failed.