Plywood disaster

bobwallison
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Re: Plywood disaster

Postby bobwallison » Tue Nov 26, 2019 3:22 pm

Tim,
Many thanks for sharing your technical knowledge with us, very useful.

The reason why I was exploring a new method is mainly weight. After my wife had helped me heave the last 8 x 4 sheet of 12mm MDF onto the workshop trestles, she vowed 'never again'. Plus, I really like the aesthetic appeal of birch ply.

Talking of weight, do you have any experience of lightweight MDF, such as Norbord's Caberwood Light? From their technical brochure, it appears to be half as strong and half as stiff as the normal density MDF, so my initial reaction is to steer well clear.

Regards,
Bob

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Le Corbusier
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Re: Plywood disaster

Postby Le Corbusier » Tue Nov 26, 2019 4:19 pm

Sorry ... its not a material I have had put forward to me as of yet by any of the joinery shops we work with. As with all of these things I suspect it is 'horses for courses' and as a track base I would have my doubts.

Others will have much more hands on experience than me, but it seems to me that for a track bed, stability and rigidity is of the essence. I might be wrong but this suggests using a material which requires either very regular support and bracing or one which has a cross section sufficient to ensure rigidity. Quality plywood certainly has the right characteristics, but is pretty dense. MDf if anything is more stable, though has a greater tendency to sag. I have always been impressed with Chris Gough's monocoque approach ... but even here the track base is quite hefty .... though the rest of the construction is light weight.
https://www.modellayoutservices.co.uk/services/baseboards/contour-baseboard/

Gough Monocoque.jpg


One of the reasons I want to have a play with styrofoam is that it is very rigid, strong and stable when 4" deep yet at the same time very light .... I wonder if you might bond 4mm or 6mm ply directly to it .... or perhaps even bond cork directly to form the track base. There may of course be many reasons why this would be a silly idea :?

However, if you are building a permanent installation surely the installed weight is incidental? I sympathise with the weight of full size boards, but most suppliers are now happy to cut them down into more manageable sections if you supply a simple cutting schedule .... wouldn't that be a better approach whatever material you settle on?
Tim Lee

bobwallison
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Re: Plywood disaster

Postby bobwallison » Tue Nov 26, 2019 4:59 pm

Unfortunately, hardly any of the trackbed I need is straight, so cutting large boards into, say, 8 x 2 would probably lead to a lot of wasteage. But I'll cut out some templates at 1:10 scale and play around with them.

I'm not sure about about a cork/foam sandwich, there is nothing rigid to fix things like point motors and signals to. But a foam/timber sandwhich could work, and with the foam supplying most of the strength, the timber could be fairly thin.

I like the monocoque construction from thin ply.

Decisions, decisions!

Regards,
Bob

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Le Corbusier
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Re: Plywood disaster

Postby Le Corbusier » Tue Nov 26, 2019 5:59 pm

bobwallison wrote:I'm not sure about about a cork/foam sandwich, there is nothing rigid to fix things like point motors and signals to.


Not a suggestion for you or indeed most. I am playing with it because I am going down the battery controlled route and looking to have point actuation etc above board and mechanical .... so no wiring to speak of and no motors etc. Could all end in tears, but the rodding on the test track is still working fine nearly 18 months in - so fingers crossed ;)

https://www.scalefour.org/forum/viewtopic.php?p=62727#p62727

I was interested in Neil Berrington's use of 'smooth on' to create a strong shell to the foam ....

https://www.scalefour.org/forum/viewtopic.php?p=70803#p70803
Tim Lee

BrockleyAndrew
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Re: Plywood disaster

Postby BrockleyAndrew » Tue Nov 26, 2019 10:18 pm

I've just been thinking about materials.... surely in any layout the trackbed is a small part of the overall scene. I've been considering an inner city location where all track is on viaduct. Thinking aloud....the viaduct bed could be solid flat metal construction without becoming too heavy and sit above not so perfectly flat baseboards? I suppose then the scenic elements would have to hang off the viaducts, or sit next to the track......what if the track plan is built on one structure and the scenic surroundings slotted in?

Andrew

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Tim V
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Re: Plywood disaster

Postby Tim V » Wed Nov 27, 2019 12:27 am

Have you looked in 'Model railway Layout Design' (WSP 19990) by Iain Rice at his 'Jigsaw' layout? Track on one board, scenery on others.
Tim V
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Terry Bendall
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Re: Plywood disaster

Postby Terry Bendall » Wed Nov 27, 2019 8:20 am

BrockleyAndrew wrote:I've been considering an inner city location where all track is on viaduct.


Brighton Road, built by the mid Sussex area group and now scrapped, had the main line on a viaduct and the freight yard at a lower level in front. Boards were all 12mm plywood, including the upper track support. There was a little sagging of the upper track base but not a lot. The embankment on Pulborough also has a 12mm plywood track base which after 34 years is still acceptable.

If metal is used care would need to be taken with the design. Something thick, e.g 10mm would probably have to be aluminium so as not to be too heavy and would be quite expensive. Thinner metal with angle sections supports would probably work but would be fairly complex to design and make.

Properly supported, with a deep longitudinal support, all in 12 mm ply would do the job. A batten of something like 20mm x 30mm cross section, screwed and glued could be used to strengthen the joint between the vertical beam and the horizontal track bed.

Terry Bendall

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Le Corbusier
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Re: Plywood disaster

Postby Le Corbusier » Wed Nov 27, 2019 9:39 am

Terry Bendall wrote:If metal is used care would need to be taken with the design. Something thick, e.g 10mm would probably have to be aluminium so as not to be too heavy and would be quite expensive. Thinner metal with angle sections supports would probably work but would be fairly complex to design and make.

Terry Bendall


One thing that you should I think experiment with if metal is contemplated is levels of expansion under different conditions. If the layout was subject to quite significant degrees of variation, this might become something for which allowances should be made within the design.
Tim Lee

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John Donnelly
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Re: Plywood disaster

Postby John Donnelly » Wed Nov 27, 2019 10:24 am

I went against all conventional wisdom when I built the board for my Pelton Level layout and used 6mm MDF throughout. I did, however, make sure that the trackbed had full support along it's length as well as cross braces. In 12 months (part of which involved it sitting in an unheated garage) it didn't move or warp at all even after using the diluted PVA method to ballast the track, perhaps I just got lucky?

Image

Image

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Le Corbusier
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Re: Plywood disaster

Postby Le Corbusier » Wed Nov 27, 2019 10:53 am

John,

That is pretty much what I would have expected given the pictures of the construction you have posted. :thumb
Tim Lee

Philip Hall
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Re: Plywood disaster

Postby Philip Hall » Wed Nov 27, 2019 3:47 pm

Most of my railway boards are large flat things and as such there is, I think, no chance of distortion as the surface is firmly screwed to the longitudinal and cross supports which are themselves quite hefty. At least for the last two years or so there has been no sign of movement. They also have the advantage of being equally firmly screwed to the (very substantial) framing of the building.

However, I am having built a 6 inch wide curved section, 4 ft long or so, which is a sort of swing bridge across the doorway. The surface will again be 15mm Contiboard, but braced with ply side members and cross pieces. As it's thin and could possibly be bendy I am hopeful this has been a good way to go. The intention is that it will be firmly attached at each end when in use. I do think that where any baseboard surface has no bracing along the line of the trackbed there is potential for distortion, so here that kind of bracing is useful. I am also planning a viaduct/embankment along one wall which won't have that kind of support, but with a 15mm trackbase supported at 6 inch intervals, itself on a flat topped board as detailed above I think I'm OK. The cosmetic stone cladding to represent the viaduct will go around this. You will all understand why this railway is going nowhere!

I do agree with Bob that the considerations of a permanent layout are different to a portable one. If you do find something has caused a level problem with the track, it's not the end of the world to be able to lift the track, shave off the hump or whatever or fill it in and relay the track. Doing that on a portable line with all the baseboard joints getting in the way isn't anywhere as easy. I always liked Peter Denny's approach to extensions - just screw on a few more bits of baseboard and away you go. And if you think that means I haven't quite worked out where all the track is going to go you're quite right; I've got a plan but each time I look at it I fancy easing the curves a bit more so more woodwork to be added on goes into the mix!

Philip

Alan Turner
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Re: Plywood disaster

Postby Alan Turner » Wed Nov 27, 2019 4:32 pm

John Donnelly wrote:I went against all conventional wisdom when I built the board for my Pelton Level layout and used 6mm MDF throughout. I did, however, make sure that the trackbed had full support along it's length as well as cross braces. In 12 months (part of which involved it sitting in an unheated garage) it didn't move or warp at all even after using the diluted PVA method to ballast the track, perhaps I just got lucky?

Image

Image



At last someone who understands structural engineering.

regards

Alan

bobwallison
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Re: Plywood disaster

Postby bobwallison » Wed Nov 27, 2019 5:34 pm

Alan Turner wrote:

At last someone who understands structural engineering.

regards

Alan

Ermmm ... I was a chartered structural engineer for nearly thirty years before I retired! To paraphrase the Institution's own definition, structural engineering is about designing, with economy and elegance, structures that can safely resist the forces to which they may be subjected (my bold). Hence my aversion to providing complex bracing for a layout which will only be subjected to purely vertical loads.

The principle of a flat slab, without vertical stiffening ribs, supported at centres of about 20 to 25 times the slab's depth, is very well established: just consider the thousands of concrete motorway bridges up and down the country.

Regards,
Bob

trustytrev
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Re: Plywood disaster

Postby trustytrev » Wed Nov 27, 2019 6:17 pm

Hello,
I would suggest any timber based material suspended in the air and inadequately secured is likely to warp due to variations in temperature and humidity above and below.
trustytrev :)

Phil O
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Re: Plywood disaster

Postby Phil O » Sat Dec 07, 2019 2:54 pm

The last Tavistock club layout was open framed construction, using 6mm ply with vertical stiffners under the track bed and all glued. After the club merged with the Plymouth club the layout spent 3 yearsin a dampish store room and around 2 years in a steel container due to no permanent club room and during its 12 years of its existence it never showed any signs of twisting or sagging and the boards were 6 ft by 2ft 9 ins. They were not treated in any way.

The track was lightly pinned whilst ballasting took place, dilute pva was used and once dry the pins were removed.

Phil.


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