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Method 4. Do-it-yourself return curve (using the slew functions) :

In this final part of this tutorial we shall create a return curve by means of a completely different method, which is to slew the end of the loop track into line with the turnout.

This involves pushing the slew functions somewhat beyond their intended purpose as a means of "tweaking" alignments, but if done carefully can produce an acceptable return curve. The advantages over the other methods are that the length of the return curve can easily be set initially to fit the available space, and the return curve can be combined in one template with the loop track, which is useful when you want to insert a turnout which bridges the end of the return curve, as shown above.


screen 26:

In screen 26 I have again started with a left-hand B-6 turnout on a negative curve of 4000 mm (F6 curving mouse action), in S4/P4 gauge (control > gauge and scale > menu items), and extended the overall length to approximately 1200 mm (F4 overall length mouse action).

Then I clicked the geometry > adjacent centres... menu item,

screen 27:

and I again set the adjacent track spacing to 100 mm (screen 27).


screen 28:

With negative curving a curved crossing is often appropriate, and will be useful here as I want a fairly short return curve. Click the track > crossing... menu item to bring up the V-crossing selector window (screen 28), and then select curved crossing in the list.

For more information about curved crossings and the different types of crossings, click the ? help info button (or see screens 36a and 37 in the track plan tutorial).


screen 29:

Then tools > make double-track on the turnout-side (TS), as before (screen 29).


screen 30:

Now in screen 30 we have our adjacent track, with its CTRL-0 datum end at the left (arrowed). It will be much easier to adjust the slewing if the datum is at the opposite end, so click the tools > swap current end-for-end menu item.

  Note carefully that this is not the same thing as simply moving the peg to the other end of the existing template in the usual way by clicking the peg indicator or pressing CTRL-1. We want instead to create a completely new template on the same alignment as before, but with the CTRL-0 datum end at the opposite end, as shown (arrowed) in screen 31. Note that in this process the template also swaps to the opposite hand.

screen 31:

Now we are ready to apply a slew to this template. There are two slewing modes, using different maths to calculate the alignment within the slewing zone. We will first try slew mode 1, so click the geometry > slew (nudge) > slew using mode 1 menu item (screen 31).


screen 32:

Screen 32 shows the result. If what you are seeing is unfamiliar, please click slewing and read the notes about the slew functions before proceeding. (Note that compared with the diagram in the slewing notes, our slewed template here is facing in the opposite direction, with the unslewed section on the right.)

(The length of the slewing zone and the amount of slew may initially be different on your computer. This is of no consequence, because we are about to change these settings.)

Now we can use the SHIFT+CTRL-F5 slew start mouse action to position the slew start marker (arrowed 1) at the desired position for the end of our return curve. Because we are not changing the slewing length while we do this, the slew end marker (arrowed 2) moves also.


screen 33:

I settled on the position shown (screen 33), with an unslewed length from the datum end of the template on the right of approximately 350 mm. Notice that the slew end marker (arrowed) may now be outside the visible part of the template, which no longer contains the whole of the slewing zone.


screen 34:

Now we use the CTRL-F7 slew amount mouse action to shift the slewed track sideways until it is approximately over the turnout road exit on the the turnout (screen 34).


screen 35:

Then in screen 35 we zoom in close on this area so that we can set the alignment by eye with greater accuracy. This is much easier to do if the timbering is temporarily switched off for the current template (geometry > timbering > no timbering menu item).

The procedure now is to alternate between the SHIFT+CTRL-F6 slew length and the CTRL-F7 slew amount mouse actions to achieve the required alignment. This will need some practice at first. It is helpful to first adjust the slew length to get the rail-edges and track centre-lines approximately parallel at the join, as shown (yellow arrows), and then adjust slew amount to bring them into line. You will probably need to alternate between the two adjustments a few times to get an exact alignment. Notice that it is not necessary to have the slew markers actually visible while adjusting their position.

 It is convenient when swapping back and forth between two mouse actions to press the BACKSPACE key (for repeat last action), or to click one of the hollow-triangle repeat last action symbols (arrowed blue in screen 35). For the repeat last action functions to work as intended the mouse action panel should remain visible, i.e. don't explicitly cancel the mouse action which is in force before swapping to the previous one.  

 Or use  instead the single-key alternative shortcuts, which in this case are the # (hash) key for slew length and the 7 key (on the main keyboard) for slew amount.


screen 36:

Screen 36 shows the alignment I achieved. The current template rail-edges (showing in black) are aligned over those on the background turnout template (showing in blue) (arrowed 1), and likewise for the track centre-lines (arrowed 2).

Notice that it is not likely that you will achieve an exact match over a long length (arrowed 3), because the slewing mathematics do not generate circular arcs for the curves (but see also later for mode 2 slews). It is sufficient to achieve a reasonable match over a length approximately equal to the track-gauge as shown (arrowed 4).

The information panel shows the dimensions which I finally used. It is likely that your figures will be similar, but unlikely that they will be exactly the same. If you are struggling, you could try directly setting these dimensions initially as a starting point (geometry > slew (nudge) > slewing data... menu item).

If it proves difficult to find the alignment, it can be helpful to temporarily zoom back out, and possibly to move the slewing start position a little (screen 32).  

Remember that this view is several times larger than full-size. There is little to be gained by striving for greater precision than that to which you can build the finished track.


screen 37:

Reinstating the timbering and zooming back out (SHIFT-F11 or the . full-stop key) shows the complete return curve (screen 37).


screen 38:

Then in screen 38 we shorten the template back to the join (F4 overall length mouse action). A slight gap or overlap at the join is of no consequence.


screen 39:

Screen 39 shows our finished slew-method return curve. Note that the slew end marker remains visible on the left, even though the track does not reach that far.

If you might want this template again, now is the time to put a copy of it in the storage box (control > store this) because we are going to change it shortly.

Compare this curve with the transition-method return curve which we created earlier (screen 25).


screen 40:

One important difference between the two methods is that the slew-method return curve can be combined with the loop track in one single template. This is useful when we want to insert a turnout which bridges the end of the return curve, which is quite often needed. I have shown an example in screen 40.

To do this on a transition-method return curve, or one created using tools > make return curve, would require half-turnouts to be duplicated and blanked off as necessary on two separate templates. (And it is not possible at all for a parallel crossing return curve.) Don't forget that the tools > make crossover function is also available when the loop lines are more closely spaced than we have here.


screen 41:

In the final part of this tutorial we shall experiment with the alternative mode 2 slew functions. Screen 41 shows the effect which can be achieved, showing our previous return curve (mode 1) in blue, and an alternative mode 2 return curve in black.

Generally for a mode 2 slew the curvature at each end of the slew is eased (arrowed 1), at the expense of a more prominent reverse curve at the centre (arrowed 2). But the extent of this effect can be varied by means of the adjustable mode 2 slew factor. (There is no slew factor for mode 1).  


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© revised 11-12-00.


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