Martin Wynne wrote:Hi Julian,
My sketch was intended to show that IF the tip is reduced to zero width at the gauging depth, then the gauge is unaffected and no joggle is needed. And that it can have a finite thickness below that depth in a practical model switch without affecting the measured track gauge.
It wasn't intended to suggest that this is the correct tip depth for any specific prototype because prototypes vary.
Ahh! - the penny drops. I thought you were saying the top of the tip of a practical model switchblade shouldn't be below the gauging line. Thanks for clearing up that misunderstanding.
Please note Paul's point about model rail* not looking much like the prototype section, tending to have a more square top corner radius and thicker web. I have made this point several times. You are not going to be able to replicate prototype switch planing in 4mm scale even with CNC milling, unless you get your own rail section drawn.
Don't think I'm fixated on modelling the prototype to the nearest gnat's whisker! I'm solely interested in what will
work to get ZERO derailments given properly suspended and weighted vehicles. It seems to me the NER pattern of the horizontal profile is eminently practical and sensible, as is Tony's.
Getting accuracy of the height of the tip top and then installing it in such a way that the height is still accurate in relation to the stock rail is pretty tricky. I'd say to be within 0.05mm either way would be good going.
I was looking in vain for your diagram of the prototype wheel/rail interface on another thread. I think you must have deleted it. It was something like my drawing posted a few days ago, though without the switchblade.
Which as you say is all very well in theory but the model actuality is rather different. As if I knew that Paul was going to make his point, last night I tried to photograph this model wheel/rail interface. Even with the posh camera on my daughter''s Huawei phone
it is pretty impossible to photograph without a proper manual control of the focus. I reckon there's horribly little room for a blade to get between the flange and the rail below the gauging line, when the flange is against the rail. That gauging line looks as though it may be lower down the flange than the theoretical roughly half way 0.19mm. The photo of the RH wheel seems to show no gap at all between flange and rail all the way down. More usual however was the rather healthier situation shown of the LH wheel.
If I have worked it out correctly Tony's blade will rise to the gauge height after about 6mm on a B switch, which is pretty much the same distance along the blade as where I said there was room for 0.2mm blade thickness.
Anyway, obviously it can be done, and I've done three such undercut style turnouts myself that work very successfully on our group layout.
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Of course all these very small numbers are just a rough guide when it comes down to it.