Stretcher bar material for Ambis etches?

Discuss the prototype and how to model it.
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jon price
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Stretcher bar material for Ambis etches?

Postby jon price » Wed Feb 09, 2022 10:49 am

The material used for Ambis stretcher bars in Howard Boltons article in Snooze 181 of 2013 is .5mm epoxy glass laminate. The source he mentions still supplies this but at £8 plus £6 delivery for an A3 sheet this seems an expensive way to achieve the required end.

In view of the fact that styrene tube is used by many people for TOUs would .5mm flat section Plastruct strip be a suitable alternative?

Any (useful) thoughts?

Jon Price
Connah's Quay Workshop threads: viewforum.php?f=125

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John Bateson
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Re: Stretcher bar material for Ambis etches?

Postby John Bateson » Wed Feb 09, 2022 11:21 am

I have an A4 size double sided PCB sheet 0.8 mm glass PCB.
Plus smaller sizes at 0.6 mm which is 5cm by 9 cm and a sheet of 0.8 mm which is 5 cm by 20 cm.
How much do you need?
Are any of these any use.

Minimal cost, these are number 497 on my to do list so will still be here when the house clearance is done. :?

John
Slaving away still on GCR stuff ...

Porcy Mane
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Re: Stretcher bar material for Ambis etches?

Postby Porcy Mane » Wed Feb 09, 2022 11:47 am

Eileens do a sheet of double sided 2.5" x 2" x 0.25mm for £1.00 (+ p & P) that looks like it could be used. Part no is PCB025.

P

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stephenfreeman
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Re: Stretcher bar material for Ambis etches?

Postby stephenfreeman » Wed Feb 09, 2022 1:02 pm

I usually get mine from Ambis Engineering.

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jon price
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Re: Stretcher bar material for Ambis etches?

Postby jon price » Wed Feb 09, 2022 1:06 pm

Thanks for this info

The epoxy glass laminate recommended in the article isn't copper coated, it is just non conductive laminate. The suggestion is that a weak point is the bond between the copper and the laminate, though I'm not sure how much this is a counsel of perfection rather than a real issue. The recommended glue is Loctite 435 cyano which is pretty expensive at £30 for a 20ml bottle. I would be inclined to go for something cheaper as the stresses will be shear so not be creating much peel load.

The 2.5mm board might be more accurate with regard to thickness, but since the main dimension will be vertical a thinner edge might not be what you immediastely see.

I will message you John.
Connah's Quay Workshop threads: viewforum.php?f=125

JFS
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Re: Stretcher bar material for Ambis etches?

Postby JFS » Wed Feb 09, 2022 1:59 pm

Hello Jon,

It is a pity others do not read the original post - I would never be guilty of such a failing :lol: and you are quite right that double sided PCB is unsuitable for his job as it will be too stiff where the copper is present causing a stress raiser where it is missing. Mike Norris's article in the same mag relates his (very extensive!) experience.

Somewhere, I must have the remains of the sheet I originally purchased and would be happy to let you have some assuming I can find it! Feel free to drop me a line.

Regarding Plastruct, the problem is that bonding the stuff to metal is not easy and most paints, glues (such as epoxy cyano etc) tend to cause it to go brittle over time (though less so than styrene I think). For that reason I am never keen to use thermo plastics structurally,

Regarding Loctite products - they are unduly expensive but there are direct equivalents available at much lower prices. Where I doing that job again, I might prefer one of the black versions as they contain a rubber filler which improves strength. It is possible that epoxy might do the job - given that is what the material is made of - and it fills gaps better.

The loading on these things is largely due to the fact that the switches themselves do not share a common pivot point with the result that the stretcher takes on quite a distinct S-shaped bend:-

Flex -1.jpg


the example above is straight in the other position:-

Flex -2.jpg


- it would of course, be much worse in coarser standards than P4 or if you choose to have an over-scale switch throw.

EDIT:- I should have said that there is a reason why the Real Thing calls these Flexible Stretchers! (as distinct from older designs which were pin-jointed)

I fully accept of course that there are many ways of doing this job and do not claim my way to be the best way. But I DO claim that it was designed to best meet my particular reliability/appearance/cost equation after looking long and hard at many alternatives! Of course, many others will have differing priorities. and where appearance is less of a priority I whole heartedly commend Mike's approach - but don't tinker with it - it is a properly optimized design just as it is!

Hope that helps,

Howard Bolton
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jon price
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Re: Stretcher bar material for Ambis etches?

Postby jon price » Wed Feb 09, 2022 4:16 pm

Howard: Message sent
Connah's Quay Workshop threads: viewforum.php?f=125


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