How to Build Junction Mechanisms

Here are step-by-step instructions for building and installing the original type of junction mechanisms that I used before I changed to servos. As with most things, there are many ways of achieving the desired result - so do‘t feel that you have to do it this way.

The materials consist of:

20 thou (0.25mm) plastic sheet
80 thou (1mm) plastic sheet
2mm dia steel shaft
31mm dia plastic gear x 2
9mm plastic gear x 2
Small electric motor

Important note: None of the quoted measurements are critical.

Click any picture to enlarge it.

Step 1:
Cut strips of 80 thou plastic card approx 17 mm wide for the top and bottom, 12 mm wide for the side and ends.

Step 2:
Cut top, bottom and side pieces from the strips approx 65mm long.

Step 3:
Drill a 2mm diameter hole in the bottom piece, roughly in the middle lengthways and about 7mm in from one side. Open the hole out slightly by running the drill through it so that the steel shaft is a loose, but not sloppy, fit. The plastic should spin easily around the shaft if you flick it.

Drill a hole in the same location on the top piece. You can do this by putting the two pieces together and marking the hole location on the top one through the bottom one.

Step 4:
Place a 31mm plastic gear on the shaft and rest it in the hole. Position a 9mm plastic gear on the end of the drill and locate it to mesh with the 31mm gear. The drill should be about 7mm in from the same side as the first hole. When you can rotate the gears without them slipping or binding then mark the location for the hole with the drill. Remove the gears and drill the second hole (which should be about 20 mm away from the first one).

If you intend to make a number of junction mechanisms then make up a jig consisting of a bottom with the two 2mm diameter holes in position. You can then use this to consistently mark the position of the holes on top and bottom pieces.

Step 5:
The motor has a small shoulder next to the shaft. On the motors shown this is 6.5mm in diameter. For these motors open out the second hole with an 8mm diameter drill.

Step 6:
Glue the bottom to the side. The side piece should be on the side that is 10mm from the centre of the holes, not 7mm.

Step 7:
Cut an end from the 12mm wide strip to fit at the end nearest to the larger hole and glue in place.

Step 8:
Glue the top in place, using a shaft fed through the 2mm diameter holes in the top and bottom pieces to establish the correct location. The shaft should be at right angles to the top piece.

Step 9:
Cut one of the 31mm gears as shown. The arc A should be a little larger than the angle between the diverging guide wires. Arc B should rest against the upright wall of the mechanism when arc A is at its furthest travel from the upright but still in contact with the 9mm gear. It may be necessary to cut a slot in the rear upright to allow arc A to reach the other end of its travel. The exact amount of travel will be fixed later.

Temporarily mount the cut down gear by pushing a shaft through the bottom hole, then through the gear and into the top hole. You can now rest the top of the mechanism on your workbench and check that the cut down gear reaches both ends of its travel when you rotate the 9mm gear. It is vital that the cut down gear does not run off the 9mm gear at eiter end of its travel.

Step 10:

Using some superglue gel between the motor and the bottom of the mechanism you can now fix the motor in place, adjusting to get the best mesh of the two gears.

Step 11:
Remove the shaft and trim it to length (height of the mechanism plus thickness of the baseboard surface plus height of 9mm gear - unless the baseboard is less than xmm thick, in which case see step 15). File the slightly raised centre on a 31mm gear flat and then fix it to one end of the shaft. Pass the shaft through the top hole, the cut down gear and then through the bottom hole. Push a 9mm gear onto the other end of the shaft so that the shaft can rotate but not move vertically.

Step 12:
Cut two 31mm diameter circles from 20 thou plastic card. For this I use an Olfa circle cutter which I purchased from Squires. A similar item is available from Rapid Electronics called a compass cutter at just under two pounds.

Glue one circle to the top of the 31mm gear.

Step 13:
You will need a circular hole in the baseboard for the mechanism. Do not be tempted to use a drill bit like this

as they are hard to control and often result in over-sized and mis-shapen holes.
I use a cutter like this

which is set to drill a 34mm hole. This type of bit is available from DIY stores and by mail order from Squires.

The hole needs to be positioned with the outer edge running through the point where the two routes diverge and with the centre midway between the two diverging routes.

Step 14:
If your baseboard surface is less than 0.25”/6mm thick you will need to add some packing pieces to the top of the mechanism to allow the top of the mechanism to lie flush with the top of the baseboard. With the second circle in position on top of the first, but not fixed, adjust the shaft up or down so that the top of the second circle is flush with the surface when the mechanism is flush with the underside.

Step 15:
Temporarily mount the second circle on top of the first using double-sided adhesive tape or blu-tack. Hold the mechanism in place and, using the 9mm gear at the bottom, rotate the shaft to the clockwise end of its travel. Make a pencil mark on the top circle in line with the right-hand route. Rotate the shaft to the anti-clockwise end of its travel and see where the mark points.

If the mark lines up with the left-hand route then you can continue to the next step.

If the mark overshoots the left-hand route you will need to pack thin styrene scraps behind arc B to reduce the travel. Repeat the test until the mark lines up with the two routes. Glue the packing pieces in place before you move the mechanism to avoid any accidental movement.

If the mark does not reach the left-hand route then you will need to create or enlarge a slot for arc A. You can create the slot using the 2mm drill to make two holes and then opening them up to make a slot with a file or craft knife. For testing the alignment you now need to use the anti-clockwise travel/left-hand route as your baseline and extend the slot towards the centre of the mechanism to increase the travel until the pencil mark lines up with a route at each end of the travel.

Step 16:
Line the mark up with the right hand route and draw a line (straight or curved as appropriate) from the mark to the entrance wire. Now line the mark up with the left hand route and do the same.

Step 17:
Go back to the workbench and take the top circle off. Cut the circle along the outside of the lines giving you three pieces with room for the guide wire between them. Glue the two side pieces of the second circle to the first one. Cut two lengths of guide wire to fit along them and glue them in place. Check the fit of the centre section and then glue that in place.

Add filler in any gaps and along the wires and then smooth it down to give a flush surface.

Add a second end piece at the end away from the motor the add to the strength of the mechanism.

Step 18:
Solder wires to the motor and then glue the mechanism underneath the baseboard.

Once the glue has set align the guide wires on the top circle with the diverging routes and adjust the vertical alignment by pulling or pushing the shaft. Connect the motor to an electrical supply and check that everything performs correctly.

Step 19:
The mechanism is now ready for use but will need further attention when the road is surfaced. The road surface itself is self-adhesive plastic film which would stop the mechanism working. To prevent this cover the disc with a 4 or 5cm square of thin film. You can use the self-adhesive film sticky side up, parcel tape sticky side up or any very thin film. It doesn't need to be transparent, but this can be very useful in the early stages before the road is painted.

Parts List

Rapid Electronics:
37-0350 Steel shaft 2mm diameter (Pack of 50 x 75mm) - enough for 100 or more! 91p
37-0215 31mm miniature gear (Pack of 50) enough for 25 - 212p
37-0200 9mm miniature gear (Pack of 50) enough for 25 - 88p
37-0142 3V 5240 rpm miniature motor 45p each (36p if ordering 25 or more)
37-31562 Plast.Sheet 1x457x305mm White (Pack of 10) £5.95 - enough for 100 or more!
405p postage on orders under £29.37 in value.

Mainly Trains:
SL0120 Slaters 20 Thou Styrene Sheet White 105p (enough for 12 or more)
SL0180 Slaters 80 Thou Styrene Sheet White 294p (enough for 12 or more)
225p postage on orders under £30 in value.

Guide Prices

Building 5 mechanisms, cost each is £3.29
Building 10 mechanisms, cost each is: £1.87
Building 20 mechanisms, cost each is: £1.36
Building 30 mechanisms, cost each is: £1.20