Great to have RMweb back after the Christmas downtime - thanks to Andy for seeing it through.
I decided that I needed to redo the control for the lower section of Empire Basin to use servos. This is partly because I was unhappy with my attempts at wire-in-tube and the rather lashed up linkages I'd made which didn't work very well and partly because I wanted to have a test bed to demonstrate the use of servos for point control.
I needed to make a proper drive mechanism for the point blades which allows for some adjustment in the blades when fitting them to the layout, I also wanted to separate the servo component from the TOU to allow for easier fitting. The use of servos makes for much easier fitting as the adjustment can be made on the servo controller rather than having to fiddle with different lengths of linkages.
In the past I've made TOUs out of brass and paxolin but decided to try using plasticard for this project. The Plastruct square tube is pretty robust stuff and when stuck onto layers of 60 thou plasticard make a component which I think will be strong enough for last.
I cut two lengths of plastruct tube which telescope together, the larger is 5/16th inch and the smaller 1/4 inch. These were cut 50mm and 55mm long respectively.
Starting with the larger tube I drilled two pilot holes 16mm apart (for P4, EM or OO would be closer) and then slowly openned the holes up to 6mm in 1mm stages. The larger tube was then stuck on to a base plate made of two pieces of 60thou plasticard which gives a base almost 3mm thick. This base blade was then drilled through the same 6mm holes. At the same time I drilled some 3mm mounting holes in the base plate (only done on the front one in the picture).
The unit connects up to the point blades via lengths of 16th inch tube which have a length of .8mm brass wire fed through them. The wire gets soldered to the point blade and the adjustment in height can be managed by moving the wire up and down in the tube. I drilled the holes in the smaller of the two plastruct tubes 1.5mm and then openned them out to be a force fit for the 16th tube. The brass tube is around 40mm long and is pushed through the smaller section plastruct once it is slid inside the larger tube. For my depth of base board and underlay I ended up with 11mm of tube protruding above the base. It is worth remembering at this stage to put a broach into the end of the brass tube to remove the burr from where it has been cut, this makes pushing the wire through later a lot easier.
To make sure the tube doesn't move I mixed up some aruldite and pushed it into the smaller of the two plastruct tubes and around the brass tube. This should secure things but also avoids the risk of sticking the whole thing up solid which I thought might happen if I tried applying superglue from the outside.
Finally I stuck and shaped an operating wire connection by laminating three little bits of 60thou together and sticking them into the inside of the sliding section of tube. This is drilled with a .8mm hole which will have the operating wire threaded through it. A little bit of tidying up with some sandpaper and we have a pair of complete TOUs ready for installation.
The holder for the servo is even simpler, base plate of two 40mm square pieces of 60thou and then 'walls' of 60thou 12mm high cut to go around the servo. I drilled these walls with a pair of 2mm holes which allow the servo to be securely fastened by simply threading a short length of 2mm rod through the mounting holes on the servo case.
The result is a unit which I can screw to the baseboard but from which the servo can be removed without too much problem. These servos are TowerPro SG-50s which cost about 3 pounds each depending on where you buy them. I got mine from 'Giantcod.co.uk'.
So with these components ready to mount it is time to fasten them to the base board.