This is a temporary re-posting from bits of the old "real track" web page. I have added some additional diagrams to the original 2001 text.
I was not really ready to post this and I feel I'm being pressured into posting stuff before I'm ready. I'm sorry about the formatting and thrown-together look of it.
All this old stuff will be reappearing in the Templot Companion in due course, properly updated for Templot2 with new diagrams and videos and explanations. But not yet.
p.s. The hobby was massively ill-served by an article in MRJ about 20 years ago which mixed up "straightcut" switch planing with the geometry of straight switches. All switches can be either straightcut or undercut planed (or chamfered or other) regardless of whether they are straight, semi-curved, or curved switches.
These notes apply to UK-pattern bullhead track only.
Setting the Curved Stock Rail
The diagram below shows the position of the set
(bend) in the turnout-side (TS) stock rail, which matches the planing angle on the switch blades and is needed to ensure a correct track gauge through the turnout road of the switch. Templot puts a mark across the curved stock rail at the position of the set, as shown below (for a left-hand switch).
In Templot the set is slightly in advance of the tip of the opposite blade, as shown, and in order to ensure that the geometry works correctly the TP peg position
) is in advance of the blade tip by half this amount.
The diagram shows the set mark at the minimum
amount of set advance
. There is no harm in having a little more in models to help protect the mating blade tip from wheel damage. However, an excessive set advance should be avoided as this can cause a knock to the wheels of trailing traffic. Often for facing traffic, and on all GWR switches, the set is replaced with a joggle.
Undercut Switch Blades:
The first diagram below shows the left-hand stock rail and switch blade for a left-hand undercut
pattern switch with a plain unjoggled stock rail. This is the type of blade planing most often modelled because it requires only a plain set in one stock rail.
(bend) is put in the turnout-side (TS) stock rail at the toe of the switch (blade tip) to match the planing angle on the switch blade.
The switch blade is planed (machined) to a sharp tip, and profiled down below the top of the stock rail to fit under the head of the stock rail (undercut). This is in order to have sufficient strength at the tip.
Such a blade acts only as a guide to the wheel flanges for the first part of its length and does not actually support the weight of wheels running onto it until they have reached some way along it. Undercut switches can often be identified in photographs because the shiny top of the open blade stops some way short of the tip.
Here are some photos of an undercut switch. You can see from the shiny marks on the top of the blade that it starts to take some part of the wheel load at about the position of the second stretcher bar. Up until that point it is simply guiding the wheel flange, and is dull on top:
You can see that the tip is very thin, and if continued up to the top of the stock rail it would be a knife edge.
The way to model this is to do the final shaping with fine abrasive paper ("Wet-or-Dry" paper from car shops), with the blade clamped against the stock rail.
Notice also a seldom-modelled but quite prominent feature, the steel sole plate on the toe timber. It is used to hold the toe to gauge, so that the stretcher drive and detection rods can be accurately set. Usually there are strips welded on the ends of the sole plate to bear against the chair bases (just visible under the muck on the lower pic), or the ends of the plate are turned up as a forging for the same purpose. In the photos there is an insulated joint in the sole plate because this track is track-circuited.
Straightcut Switch Blades with Joggled Stock Rails :
When a more robust switch is needed, both of the switch stock rails are joggled outwards by a small amount to create a housing for the switch blade tips and so protect them from wheel damage. This next diagram below shows this alternative straightcut
pattern switch with a joggled stock rail.
Instead of a plain set
in the stock rail at B, it is joggled
sideways between positions A and C, with the maximum deflection from its previous unjoggled alignment being at the blade tip position B. In bullhead track the joggled section is always created by bending the rail rather than by machining a notch in it.
The switch blade is planed to a blunt tip (straightcut), to a thickness corresponding to the amount of joggle, which has been exaggerated in this diagram for clarity. Apart from a rounded corner at the tip the blade is the full height of the stock rail and plays its part in supporting the wheels along its full length. A straightcut switch can often be identified in photographs by showing a shiny top on the open blade all the way to the tip.
In defining a joggled switch, two dimensions are needed. The joggle depth
(sideways deflection) at B, and the joggle return length
in front of the blade tip between positions A and B, in which the rail returns to its normal alignment. (The length between B and C always corresponds to the planing length
for the switch blade). These dimensions need to be entered when creating a custom
joggled switch in Templot.
Having entered joggle dimensions for your custom switch, you can choose whether to actually use the joggles for an individual template by means of the joggled stock rails
Here is a photo of a GWR joggled switch with straightcut planing. Notice the thicker more robust nature of the blade tip, and that it is full height and shiny all the way to the tip. Notice also that the depth of joggle in the stock rail is very small in relation to the rail width. It is not easy to create such a joggle in models.
REA and straight loose-heel switches can be either joggled or not - normally joggled switches are used only in facing positions on running lines. In trailing positions there is a danger of rough running when wheels hit the joggle return length on the open switch blade side, most severely when the main-road is curved. However, all GWR and BR(WR) switches are joggled.
For REA switches the joggle depth is 3/8" (0.375 inches) and the joggle return length is 6 inches. For older pattern straight loose-heel switches these dimensions vary, but are usually similar.
For GWR old-pattern switches the joggle depth is 3/8" (0.375 inches) and the joggle return length is 4 inches. The later GWR and BR(WR) curved flexible switches have a less severe joggle, the joggle depth is 1/4" (0.250 inches) and the joggle return length is 6 inches.
These joggle depths are barely perceptible in the smaller model scales. A 1/4" joggle is only 3 thou (0.003 inches) (0.08 mm) in 4mm/ft scale. Some modellers prefer a more pronounced joggle, and Templot provides for this with the generator > overscale joggles menu option, which creates a joggle depth of 3/4" (scale).
Visitors to the Severn Valley Railway steam heritage line in Kidderminster UK can very easily observe the difference between joggled and unjoggled switches. In platform 1 the engine release turnout at the buffer stops has an REA semi-curved flexible switch with undercut-pattern switch blades (no joggle). In platform 2 the engine release turnout has a GWR curved flexible switch with straightcut-pattern blades and joggled stock rails. The very much more robust nature of the blade tips in the latter case is very evident.
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