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Currently working on the 1PL, 2PL and 1PR, 2PR switch block chairs:
View attachment 3161
These chairs are essentially short slide chairs, the difference from the P chairs being that the stock rail is conventionally keyed instead of bolted, and the switch rail has an outer jaw support.
I have increased the length of the slide table a fraction, to allow for the wider model flangeways and increased switch opening (even P4 flangeways are significantly over scale):
View attachment 3160
The intention for these chairs is that they will be in two parts, separately plugged, so that the rails can be plugged in one at a time. There is a very tiny gap between them, so that the mesh repair tool doesn't merge them into a single chair print (having two plugs).
If that is actually wanted for a more robust chair the gap can be set to zero. That would require horizontal threading of the rails into pre-fixed double chairs of course, rather than vertical plugging of a single chaired rail. Doable for a single turnout, but tricky within more complex formations.
View attachment 3158
There is an issue with these chairs which is not immediately obvious.
Unlike on the prototype the one is not a mirror image of the other. There is a very small difference in the offset between the two rail seats. That's because the switch timbers are square-on instead of being equalized to split the deflection angle. This means that chairs printed for a left-hand switch will not be usable on a right-hand switch, and vice versa. When making plug track it will be important to keep the chair sets carefully labelled for the matching timbering brick and template.
The prototype gets round this by shifting the set in the diverging stock rail forward from its true geometrical position, so that the blade tips are exactly opposite each other. The consequence of this is that the track gauge is fractionally reduced over the diverging part of the switch, although I have never seen this actually marked on a prototype drawing.
Templot puts the set at the true geometrical position (shown as the "set advance" on some Templot diagrams) in order to preserve an accurate gauge through both roads of the switch. I'm not about to change that after 40 years just to make the chairing easier.
View attachment 3159
Now for the jaws and key.
cheers,
Martin.