Julian Roberts wrote:Hi Tony
Many thanks for the latest instalment on diamonds and slips.
One thing I didn't understand is how to use a check rail gauge when adjusting the fit of the first point rail.
Adjusting the fit of the first point rail using the wrong gauge. The check rail gauge should really be used but this is easier when making the initial fit.
SS9 Adjusting initial fit of point rail using wrong gauge.jpg
(The actual photo doesn't copy over to this)
No, I can never get them to copy over either.
If one thinks about how a Check rail gauge is intended to be used to measure the distance between the check face of a rail and the running face of the opposite rail, it becomes apparent that in this situation there is no rail to measure it from. Hence my comment about using the Crossing Flangeway gauge to help temporarily set the first point rails. I fully expect further adjustments to be required in the alignment of the point rails, indeed the likelihood of getting them spot on first go is rather small.
Do you have any advice on the filed tip of the point rail - should it be slightly rounded as with a crossing V?
I make the Obtuse Crossing point rails the same way as I do for Acute crossing point rails except that for certain circumstances I make the end of the nose thinner than normal to help reduce the unchecked distance. I tend to make the obtuse check bends sharper than they should be for the same reason.
The other thing I'd like to ask is about the S4/P4 difference, as you model in S4. I'm not sure you've said this on the thread, could you confirm that a 1:8 is the flattest angle allowed on the prototype and thus in S4, because of the unchecked length? What is the flattest in P4 to give the same safety margin? Similarly, the table of curving diamonds must be slightly more restrictive in P4?
This is an interesting question and the one I really had to think about.
Yes my own track is all built to S4 standards, but the same methods described are generally related to P4 standards.
Obtuse crossings are perhaps the situation where the difference can make itself most felt.
You are correct in as much as 1:8 fixed obtuse crossings are the flattest that are allowed on the prototype, flatter angles being of the switched type only, but that is only a partial answer as the situations is complicated where curves are concerned as already discussed.
Since S4 dimensions mirror the prototype, there is no reason why S4 track should perform any differently to the full size in any given situation.
The same cannot be said of P4 track however.
The S4 flangeways are 0.58mm and for the maths assuming a sharp end to the fine point, for a 1:8 obtuse crossing, the gap will be 0.58 X 8 = 4.64mm.
For P4 flangeways at 0.68mm the corresponding gap will be 5.44mm. To reduce this gap to 4.64mm requires a crossing angle of 1:6.82, so even a 1:7 crossing does not give the same level of security in P4 as a 1:8 does in S4.
This result was greater than I expected before I did the sums.
Lastly, ideally could that table of curves include a conversion to scale mm (or perhaps a link to John Donnelly's conversion thingy which was on the Forum recently?)
Not sure I can help with a link but I will see what I can do with the my construction thread.
EDIT: 4mm equivalents now added.
Regards
Tony.
Inspiration from the past. Dreams for the future.