Sunday 1 October 2023

The "Ultimate" fiddle yard - Latching mechanism

The next job in the fiddle yard rebuild was to create a latching mechanism that aligns the traverser tracks to the entry track, and provides power to the aligned track. This uses the same sprung notch and pin arrangement as proven on Loctern Quay and the home fiddle yard for Hexworthy. 

First holes were drilled in the PCB near the end of the traverser deck centrally between each set of rails. Normally I'd fix the track after putting the pins in and line them up to the entry track, but here the tracks are already in place. I cut a jig from brass sheet to slide tight between the rails with a hole dead centre, then used the Dremel in it's stand to drill a vertical hole, first as a pilot then opened up to the size of the brass tube. The holes go through the PCB strip, the 5mm PVC foam deck, then the second layer of PVC foam that forms the protruding lip, this makes sure the brass pins are held very securely. 

The alignment pins are made from brass tube chopped into lengths of about 20mm to go through both layers of PVC foam and protrude about 5mm below. This tube was 3/32" or ~2.4mm diameter, not that the diameter matters that much. 

The pins are a tight fit through the hole so were tapped into place with a hammer, the top just protruding above the PCB, and are then soldered to the PCB. The PCB was gapped to one side of the pin so that it is electrically connected to one rail. The pins are all connected electrically to the right-hand rail looking at the end of the traverser, so those at opposite ends of the traverser are connected to different rails. The protruding lip has been covered in graphite to ensure smooth travel below the fixed end deck.

The components of the latch mechanism:
  • A spring taken from a soap dispenser as shown
  • A length of aluminium - in this case T-profile - with a pivot hole at one end, and a curve cut and filed at the other, with a V-notch at its centre. There are also 3 holes through the vertical web
  • A long screw which passes through a hole in the aluminium, through the spring, and into the block of wood which is fixed to the board.
In this way the screw acts as a limit stop, spring retainer, and tensioner, the three locating holes allow the spring/screw to be located at varying tension and leverage locations. Note that this fiddle yard has TWO sets of alignment mechanism, one for each end of the traverser.

From the underside the aluminium arm is bolted in place with the wire to one rail (labelled "front") connected to it via a washer, it is free to pivot in the horizontal plane. The wood block is fixed to the board too, with the spring between it and the arm pushing the arm away. The long screw both locates the spring and acts as an end-stop for the arm, in this view the arm is at its furthest travel with the end between two of the five brass locating pins. 

When the traverser slides the pins push the arm back until they drop into the V-notch, being a round pin in a vee they locate the track, and being metal they conduct the power to the rail.

This just shows one end, an identical mirror-image latch is provided at the other end of the fiddle yard with the arm connected to the red wire. 

From above with the traverser pushed back the arm can be seen pushed fully out by the spring. The shape of the face of the arm and the notch are adjusted so the pin slides easily into place. Note how the lower lip of the traverser slides under the fixed deck helping to ensure accurate vertical alignment.

The entry track was trimmed and fixed in place lined up with the layout tracks, and also with the traverser tracks when they are aligned with the pin/notch (including both ends of the rotating traverser). A couple of the traverser tracks needed realignment with a soldering iron, but despite the less than ideal order of construction this seems to work well. The fiddle yard connects to the main board with an audio (RCA) lead, the other end of which connects into the choc-block connector under the board.

The extra depth of the rebuilt fiddle yard allowed the pivot bolt to be inserted with the nuts below (Awngate had a very shallow baseboard), this meant only the shallow mushroom head of the bolt protruded. This meant I could make the centre track a through road rather than having two dead-ends, both making it more useful and making the electricals easier, so a short length of track was cut to fit the gap between the existing PCB strips. However, the bolt head caused a hump in the track, and cutting away the sleepers meant the rails touched the bolt head. I carved away the lower part of the rail with the dremel so they sit neatly over the bolt head, and the red insulating tape should prevent any shorts. There seems to be adequate clearance for wheel flanges. 

An overview of the assembled fiddle yard. It can be seen how the two "handles"  lock the rotating upper deck to the traversing lower deck, they are unlocked when the handles are folded down to act as end-stops preventing stock runaways when rotating. Rotation is free and smooth, the latching seems to work well, and sliding the traverser forwards is smooth. However... when pushing the traverser back the latch is difficult to overcome and movement can start with a jolt, which is likely to derail trains.

I realised that the previous latching fiddle yards I have built were sector plates, the curvature of which allows the latching arm to be pivoted at a point in line with the track join, and so the arc of the movement of the arm is perpendicular to the movement of the pin - that is, the v-notch moves directly away from the pin and resistance is equal in either direction (see here). In this case the arms have had to be pivoted some way back from the track join, and despite making the arms as long as possible they must be at about 20 degrees to the line of travel of the traverser. Pulling the traverser forward the pin has an easy job to slide out of the v-notch, but when pushing the angle of movement of the arm effectively steepens the "slope" the pin has to push against to free itself. 


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