Friday, July 10, 2009

What's in a plug? A challenging fabrication

Interurbans such as the SN 1005 frequently ran together in trains. The motorman worked the controls in the first car, and all the cars with motors responded in sync to his actions. This is done through "multiple-unit control", or "M.U." A control cable runs the length of the train, and jumps from car to car via plugs.

Ah, the lowly plug. We use plugs and sockets for so many things in our society, but who ever thinks about them? You need four to charge a cell phone. Nowadays plastic insulates the various pins. But what do you suppose they did in 1910?

There's a surprising answer. Maple.

The Western Railway Museum plans to M.U. the 1005 with other units, such as the 1020. The M.U. plugs on 1005 needed work. The copper pins were shot (fabricating them is another story!) and the maple insulators were in poor shape. How do you make one of these?

It's not as simple as you'd think. All the holes must have a very particular relationship to the index key. If any of them are off, the plug won't mate. And we had several to make. (they are four inches in diameter.)

Two approaches were tried. Rather than try to machine each plug individually, one of our machinists made a steel "cap" which could fit over the insulator. In this cap, he carefully drilled holes in the right places. A tedious job, but done once, it provided a jig that properly located the holes for any piece. A second approach was explored in the digital realm: software that would tell a CNC router how to cut the part out of a block of wood. Software incompatibilities slowed that project, and the traditional machinist won. Jawn Henry would approve.

The insulators were drilled, shouldered and tapped 1/4-20 for the pins, which screw in. Maple is a hardwood which accepts threads well. These pieces are almost 2 inches thick, and tapping them required special nut taps.

Maple is already an excellent insulator, but only when dry. One way to solve that problem is to boil the maple in paraffin, driving water out and wax in. However, in this case, the pieces were painted with Glyptal paint designed for high insulation value. The pieces really "drank up" the Glyptal. Bolts were screwed into the threaded holes to "mask" them from the Glyptal. The bolts came out easily.

Soldering wires onto the pins was the next step. For each plug location, the crew measured the distance to the junction box, and they cut 12 wires of this length. They tinned 12 pins then cleaned the threads of each pin, screwed the pin into the maple block, and soldered a wire to it. It wasn't necessary to mark which wire went to which pin; the wiring crew will ring those out later. No ribbon cable here!

The twelve wires were threaded through a cast iron rear cap, then inserted into the back of the cast iron housing and the rear cap was installed. The result is the picture you see at the top.

Click on any picture to zoom in. This work was completed on June 30.

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