Building an 'invisible' mechanism

[Vanders]

I've just finished building and laying the track for my layout, and thought I'd share the details of how I built the stretcher mechanism for the turnouts.

 

My criteria were:

 

1. No visible mechanism. I didn't want to have a "sliding timber" attached to the point blades. Timbers don't move in real life, and the stress created by a rigid mechanism soldered to the blades can cause failures.
2. It had to work with my baseboards. I'm using solid 50mm extruded polystyrene for the super structure, which means access to the underside is restricted.
3. It had to work with wire-in-tube. I intend to use servos to control the turnouts, which combined with the solid board construction meant that I'd have to transmit the movement of the servo to the turnout via. wire-in-tube, with the servos mounted on the top of the board.

On top of all that, I'm building my turnouts for N gauge with code 40 bullhead rail, so everything is quite small!

 

What I settled on after a little bit of experimentation has basically evolved out of a method used by 2FS modellers. I doubt this is entirely new or novel but I haven't seen it show anywhere else, so if this is of interest to anyone then here we go!

 

 

First of all we start with a bog-standard N gauge turnout. Copper-clad sleepers, code 40 rail, with the droppers soldered to the underside of the rails already.

 

 

I take a short length (about 30mm) of 1mm wide Phosphor Bronze strip, the sort of thing you'd use to create wiper pick-ups on a loco. Hold each end between a pair of pliers and twist it through 90 degrees. I want the twist to be about 1/3rd of the way along the strip, and then I trim off the short end so it's about 3-4mm long. Do this twice, one for each blade.

 

I then solder the short end of each "wiper" to the inside of each blade. Because the phosphor bronze strip is so thin, this doesn't interfere with the point blades at all. It also means that because I'm soldering two flat surfaces together I get a (relatively) large contact area between the wiper and the blade. This should mean a stronger joint compared to methods where something is point-soldered to the bottom of the blades, which has a very small contact surface.

 

 

I then add a sub-base below the toe of the point. Im using 1.6mm balsa sheet, cut into a 40mmx40mm square. The thickness of the sub-base is chosen to match the thickness of the track base (cork sheet) that I'm using. In my specific case I also need to pack each turnout with a piece of card so that the rail height will match the fiNetrax I'm using for the non-turnouts section, which you can see here glued to the bottom of the turnout, and the balsa sub-base is then glued to the bottom of the card. If you don't need to pack your points then obviously you can skip that and just glue the sub-base straight to the bottom of the turnout!

 

The stretcher is then soldered into place. This is just any old piece of spare PCB sleeper strip, suitably gapped. I've also drilled a 0.5mm hole through one end to take the steel wire from the wire-in-tube. I've soldered the stretcher close to the balsa, but not so close that the mechanism binds when it moves.

 

 

Here's the neat part: I then bend the remaining length of phosphor bronze back over itself and down onto the sub-base. This acts like a spring, pulling the point blades down. I add a couple of pieces of card after I've bent the phosphor bronze, which puts a bit of tension into the mechanism and keeps things nice and tight. This is self-tensioning; it's tight enough to keep the blades down, and stops them from moving on their own, but it's free enough to allow the blades to move without much force. I can move the mechanisms by hand using the wire-in-tube and they'll hold themselves in position even when a loco passes over them.

 

 

The finished turnout being fitted. I've drilled a hole straight through the board using a 32mm hole cutter; this gives space for the mechanism to operate and also gives me a way to access the bottom of the turnout should I need to (although I hope I won't ever need to!) I've cut away the cork to give room for the sub-base on the turnout to fit in neatly; this is why I've used the same thickness material for the sub-base as the cork! Doing it this way means I can have a gaping hole in the board below the turnout, but the sub-base covers it neatly and ensures that the turnout is still firmly fixed down.

 

That's it, in a nutshell. Time will tell how successful this is, but fingers crossed it'll continue to work well for years to come!