Templot Club Archive 2007-2020
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posted: 5 Feb 2011 22:47 from: polybear click the date to link to this post click member name to view archived images |
Hi all, Having just built a copperclad turnout using the Masokits etched brass chair system (I've still to add the check rails) I thought I'd enquire about switchblade clearances. I've used the "Norman Solomon" method of tiebar construction as described in MRJ (basically a length of fibreglass board with two lacemaking pins soldered to the inside faces of the switchblades). In order to ensure a reasonable minimum clearance along the open switch rail to ensure free movement of wheelsets (I measure 0.85mm minimum) this means that the resulting points opening gap at the blade tips is in the order of 2.2mm. I uderstand that (from a posting by Martin): "For nearly all UK switches, the points opening at the tip is 4.1/4", and the minimum clearance anywhere along the open switch rail (point) is 2". - This scales (in 4mm) at 1.42mm at the tip, with a minimum clearance along the switch rail of 0.67mm. Whilst I'm happy with the clearance figure of 0.85mm I've achieved (I'm using 00/EM profile wheelsets, including modern RTR wheelsets) I'm a little concerned about the resulting tip clearance of 2.2mm. The curvature of the blades matches the Templot template, so am I doing anything wrong or are my figures to be considered "the norm" please? Is the figure of 1.42mm (or close to) actually achievable whilst at the same time maintaining minimum clearance along the blade? Many thanks. Brian |
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posted: 6 Feb 2011 00:00 from: Jim Guthrie
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polybear wrote: - This scales (in 4mm) at 1.42mm at the tip, with a minimum clearance along the switch rail of 0.67mm.Brian, A lot will depend on how your blade is mounted and how flexible it is. For a start, bullhead rail is more flexible than flat bottomed rail. And the flexibility of the tips of the point blades can depend on how you have filed them. So if your blades have a fixed mount at the heel end, then there are some variabels which can effect the flexibility at the blade tips. You might find that if you just go for the 0.67mm clearance, that you are only moving the tips about 1.4mm, but trying to achieve the 0.85mm clearance requires a greater force which, with the blade flexibility, requires the 2.2mm movement. You could decrease this movement by having a stiffer blade and that would depend on how you had filed up the original blade. The recommended way is to file the planing on the outer side of the point blade to half the width of the rail, then file the tip to a point by filing only the top of the inner side of the rail so that the foot of the rail is left at half width to retain as much stiffness as possible. If you have sharpened the point blade by filing both top and bottom of the rail, then the tip end will be very flexible. Another thing you might consider is hingeing the point blade rather than having it fixed at the heel so that the flexibility of the blade doesn't become a factor in the operation of the turnout. Jim. |
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| Last edited on 6 Feb 2011 00:00 by Jim Guthrie |
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posted: 6 Feb 2011 09:01 from: polybear click the date to link to this post click member name to view archived images |
Hi Jim, Thanks for the reply. I'm using Code 75 Bullhead rail, and the blades were filed using the EMGS/Portsdown Models Filing Jig (which helps create blades in the same manner as you have described). My tiebar has the holes for the dressmakers pins spaced at 13mm apart (for 00-sf). I tried increasing this to approximately 13.5mm apart in order to reduce the tip gap by a similar amount. However, the resulting "minimum clearance" gap (I don't have the actual measurement but it was in the order of 0.6mm) meant the wheel flanges were being gripped by the blades and acting as brakes. I could try a tiebar with 13.25mm hole spacings but drilling accuracies become a pretty hit and miss affair... Hinging the heel end of the blade may not be very practical bearing in mind the method of construction used (etched brass chair construction soldered onto copperclad sleepers). I did wonder about using a second tiebar moved "one sleeper space" closer to the heel, and change the existing one to a cosmetic tiebar (Mike Clarke actually includes two detailed tiebar etches in the pack, but I have yet to try them). Then the turnout blades could be driven from this "inner" tiebar, where the blade tips are stiffer and consequently the blade tip gap should (or rather I hope!) reduce. Kind Regards, Brian |
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posted: 6 Feb 2011 10:05 from: Martin Wynne
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Hi Brian, I think for EM and 00-SF with overscale 1mm flangeways you have to accept that the points opening will be overscale too. Ideally for smooth running through the switch the open clearance should not be anywhere less than the 1mm EM flangeway. The traditional method for EM pointwork from the olden days was to use a 3/32" drill shank as a points opening gauge, which is 2.4mm. A trick you can use to find a suitable figure is to pretend you are modelling the prototype to a larger scale -- taking the prototype dimensions for a flexible switch of 2" clearance and 4.25" opening, the opening is greater than the clearance by a factor of 2.125. Using the same factor for your model, a clearance of 1mm gives an opening of 2.125 x 1mm = 2.125mm. So your 2.2mm opening is close to that and probably the best you can hope for.  You may be able to reduce it a bit by leaving the switch rail flexing over a longer length than the prototype, and/or by thickening the blade tip. You can do that by leaving more metal on the back of the blade and filing the front planing at more of a top angle. But if you have a dead-scale opening of only 1.42mm with a clearance of 1mm, the curvature of the open blade will be less pronounced than the prototype and it would probably look less prototypical than having a wider opening. regards, Martin. |
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posted: 6 Feb 2011 19:44 from: polybear click the date to link to this post click member name to view archived images |
Hi Martin, Many thanks for the reply; it's reassuring to know I'm not too far off the mark after all. I'll perhaps have a play with the next turnout in order to see if I can reduce the tip gap a little, but I won't lose sleep over it Kind Regards, Brian |
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