I spent a few hours today fitting the TOUs and servos which I built yesterday. I'm hoping a step by step guide might be useful to some people.
The first step is to drill the holes which the TOU will go through to connect to the switch blades. If you are sensible and have planned ahead you will of course have drilled these before you stuck the point down or at least before you fitted the rails! However all is not lost as you can carefully drill a couple of pilot holes in the right place about 5mm from the end of the switch blade. Having drilled these I then VERYCAREFULLY opened the holes out to 8mm in several stages, making sure that the drill bit didn't go to far and touch the rails! A decent electric drill helped here (good to know I got something useful for working with my current employer for 5 years!). Designing the bracing on the board so it won't foul the TOU is obviously a must (spot the deliberate mistake).
The TOU and servo mount were positioned under the board, It is a bit tricky to see both sides of the board at once and I found that putting some double sided tape on the units helped me stick them to the board while then allowing me to adjust a bit. It provides enough hold to them allow me to get a pilot hole drilled prior to putting the screws in.
Then the length of .8mm brass wire can be bent up. I usually go for a short right angle bend and then bend that slightly to one side so that the short section will sit into the web of the bull-head rail. The wire can then be threaded down through the brass tube and soldered to the rail. Once both sides are done the TOU should move the blades smoothly. There is space for a little bit of tweaking if the blades aren't quite the right distance apart, the tube will take a little bend if you need 1/2mm of extra or less movement.
Next job is to link the TOU to the servo, and this is where the real advantage of the servo comes into play. Because you can adjust the throw so easily you don't have to fiddle about making the linkage exactly the right length. I simply made a short length of steel wire by eye the distance from the hole in the TOU operating arm to the centre of the servo drive. Then I fitted one of the smallest servo arms (the servos have a variety of shapes and sizes supplied) and threaded the wire linkage between the two units.
The downside of servos of course is that they need some electronics to make them work, but the 4 servo unit produced by the MERG (called a Servo4) is very easy to build and cheap at GBP 5.50. This unit has 4 double-pole-double-throw switches connected to it, one pole switching the frog polarity and the other switching the input on the Servo4. To set the board up you can either build and use the purpose built programmer which the MERG sell (probably a good idea to get one to take to exhibitions if you need to do any adjustment) or use a serial cable to a PC running the ServoSet application.
The Servoset application runs on windows (there is a java based version available I believe for linux users). The left hand side of the display shows which servo you are programming and where it is connected on the Servo4 board. In the centre are the sliders which you move up and down to set the on and off positions and the speed you want the servo to move at. Once happy you just write the data to the board and you can disconnect the PC.
The servos make a noise if you push them too hard so it is easy to get a feel for where to set the end points, you shouldn't need so much torque that the servo buzzes. The servo controller switches off the power to the servo after 5 seconds so that they are completely silent once changed (unlike some of the alternatives which stall a motor and can hum gentle to themselves).
If you are offended by a brief plug for the MERG please stop reading now!
Just to finish, a few thoughts about cost. A quick internet trawl shows a typical Tortoise point motor at about GBP 14.50, a Cobalt point motor at GBP 17.50 and a typical Servo at under GBP 3 and the MERG's Servo4 board at GBP 5.50. So, four points driven by Tortoises would cost GBP 58.00 while four driven by servos comes out at GBP 17.50. I think the difference pays for the MERG membership fee several times over.