Author Topic: Enginehouse Doors  (Read 190 times)

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    • David's Modeling Journey
Enginehouse Doors
« on: September 20, 2020, 07:18:52 AM »
I'd done this effect on the White River & Northern IV: that one required I animate four sets of doors; thankfully, this time around it's just one, although I wasn't going to bother doing it owing to a number of technical issues. Plus, if I couldn't make the doors move smoothly and naturally, then I wouldn't bother. (Ultimately, it worked better than I expected.)

I faced a number of challenges with this project. The most daunting was that the mechanism had to be tiny so it would fit under the roof of the building, because there was zero room underneath the building. (Fortunately I'd just completed the train order signals, and the super-compact mechanism I'd built gave me the confidence to attempt this bit of insanity.) The mechanism also had to fit within the existing rafters, without obscuring any of the interior lighting LEDs. Then, I had to drive two rotating objects, but only through 90-degree arcs, in opposite directions no less, and there was no room for bell cranks or other such linkage. Basically it had to operate by magic.

The first thing I tackled was the doors, for which I made six tiny working hinges. Why? Because I'm a stickler for natural visual effects, and pivoting the door within its dimension, as one would be tempted to do for simplicty, would produce odd-looking movement, similar to concealed cabinet door hinges. So I made my own hinges from brass flat stock, stainless steel capillary tube, and 0.010" brass wire.


I soldered bits of the flat stock to the capillary tube (above left), then ground away the excess metal with a Dremel, making sure to remove half of the tube at the end of each part. I slid two hinge parts onto 0.010" brass rod (below left), then bent and trimmed the wire around the hinge parts to lock them together (below right). Granted, the hinges are much too big for N Scale, but making them any smaller wasn't practical for me, and besides, they look fine at a normal viewing distance.


Then I had to make new doors, because the existing ones weren't the right size (they didn't close, so it didn't matter). I made the new ones just like the last ones: with individual boards. After attaching the hinges to the new doors, I glued them to a new door frame, then painted everything.


For the mechanism, I recycled the micro-drive (double-worm on a pager motor) I'd originally slated for the barber pole, but ultimately replaced because it was too noisy. Since this won't be running continuously, I wasn't as concerned by noise. The business end of the mechanism comprises a gear with two index pins, one each for the open and closed positions, that engage a microswitch to keep the motor running for a full cycle. One of the pins is actually tubing, into which a pair of 0.005" wires are inserted. As the gear rotates, it pushes and pulls the wires to move the doors. (The notches in the ends are to clear the interior lighting LEDs.)

One oddity is that the roof peak does not align with the middle of the doors, so the mechanism is actually off-center. This was both a problem and a happy accident. The problem was that the wires moved slightly different amounts, but had to be the same length to get the same range of motion, so one of them simply flexes when the door reaches its travel limit (indeed, this made the mechanism very robust: accidentally moving the open doors causes no harm). On the plus side, the doors do not move simultaneously, which looks more natural and less mechanical. See for yourself: