Securing plastic roofs

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Guy Rixon
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Securing plastic roofs

Postby Guy Rixon » Sun Feb 07, 2021 12:53 pm

In the past, I've had problems with plastic arc roofs on coaches and vans, specifically those provided pre-shaped in kits. The given curvature is rarely exactly right, and the spring in the plastic makes it hard to fix to the etched body. Bad attachment at the cantrails gives a wavy edge to the roof and looks awful. Lack of support in the centre of the roof lets it sag. The wrong support in the centre can make it "tent". Above all, gluing plastic to brass, with sub-millimetre alignment, while the joint is trying to spring apart, is rather hard.

I'm currently making a GER D34 van from a D&S kit. The roof is close enough to be useable, and this time I want to avoid the headaches.

20210207_122149.jpg

My new approach is to add polystyrene surfaces to which the roof can eventually be welded. I'm gambling that a solvent weld will let me align the roof accurately and will set quickly enough that I can just hold the roof in place with my fingers. I'm trying to avoid the "rubber bands and random heavy objects" approach to clamping the joint, which invites creep and local separation of the joining surfaces.

The central beam to support the roof is broad to deter pleating. It's three lengths of 4 mm x 1.5 mm strip, preassembled and fixed to the brass with epoxy glue. The voids at the van ends accommodate the tails of the soldered handrails.

The side supports are built up from two steps of 0.5 mm sheet on the brass top-flanges of the sides. CA glue is sufficient to secure the bottom strip as the bond area is huge.

All the supports were faired in to the roof profile, with files, after the glue had set.

I'll find out later if this approach works; I need to get the body decorated before fitting the roof and it may be a few months before the transfers become available. If the solvent-welding strategy fails and I have to roll a brass roof, then at least I have done no great harm to the body.

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Will L
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Re: Securing plastic roofs

Postby Will L » Sun Feb 07, 2021 3:40 pm

That certainly looks like a way of tackling the problem. However I've done a number of D&S coach kits with plastic roofs over the years and I came up with a design which works, keeps the lids down tight and leaves them removable if you want, which I think is worth sharing.

This is all based around the top flange that Danny always put on his coach sides. Yes I know these vans an not strictly carriages but the same principles apply. But before that, where there are no cross coach partitions, even with the top and bottom flanges on the side, the apparently rigid sides can be inclined to bow inwards slightly. The picture suggest yours have done this. I always found it wise to put a cross member between the top flange at the centre of the coach to stop this happening.

The basic plot for the roofs goes something like this.
  1. You start by fitting a flat plasticard sheet "ceiling" sitting on the top flanges and a good fit between the end. This need to be slightly less than the full carriage width by twice the thickness of the roof.
  2. Select a couple of lines along the flat roof near and equally distant from the centre line, their exact position isn't critical but you should avoid then being directly under and any roof furniture like vent and lamps.
  3. From the end profiles work out how high the roof profile is above flat sheet above these lines.
  4. Cut plastic strips to this height and glue these strait along along your flat sheet so they just match the roof profile at the ends.
  5. Once set hard, your push the ceiling firmly down into the pre-curverd plastic roof which should be (needs to be) slightly more curved than the finished roof should be, run solvent round the joins between roof and this strips. Keep the pressure on till the joints set.
  6. The result is remarkable rigid and done right will match the profile of your carriage, and is much less susceptible to damage than single thickness plastic roofs tend to be.
  7. Once done you trim the curved roof sides back flush with the underside of the flat sheet, and the ends to slightly overfly the coach ends.
  8. Fit the roof to your coach (see below) and cut and fit plasticard cantrail strips so they cover the joint.

This approach works I've done it often and conceived the guy who made the other 11 other brass coaches we ran on Knutsford East. You can glue the roof down if you wish, or you can fit a couple of captive nuts above the flat roof section and bolt it down so that it is removable. All mine are. I fit further U section cross pieces between the and bot the roof to that. Other run long blots up fro the floor.

Having got that far I've also applied the same idea to sheet brass roofs where they are provided.

Winander
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Re: Securing plastic roofs

Postby Winander » Sun Feb 07, 2021 4:06 pm

Will L wrote:[*]Cut plastic strips to this height and glue these strait along along your flat sheet so they just match the roof profile at the ends.


Will,

Do you shape the top edge of this strip to match the curvature of the roof? Otherwise, if you use a thin adhesive, you are merely gluing the very corner edge of the flat strip to the roof.

Guy suggested the alternative is to replace the roof with a brass one - what's discouraging you from doing that?

cheers
Richard Hodgson

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Guy Rixon
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Re: Securing plastic roofs

Postby Guy Rixon » Sun Feb 07, 2021 4:29 pm

Will, you said "You start by fitting a flat plasticard sheet "ceiling" sitting on the top flanges and a good fit between the end. This need to be slightly less than the full carriage width by twice the thickness of the roof." Is that for a half-elliptical roof by any chance? If so, I entirely agree with your approach, and mine would not work so well.

My current roof is an arc roof, a fairly flat one.

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Will L
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Re: Securing plastic roofs

Postby Will L » Sun Feb 07, 2021 4:32 pm

Winander wrote:
Will L wrote:[*]Cut plastic strips to this height and glue these strait along along your flat sheet so they just match the roof profile at the ends.


Will,

Do you shape the top edge of this strip to match the curvature of the roof? Otherwise, if you use a thin adhesive, you are merely gluing the very corner edge of the flat strip to the roof.
If that's true it isn't a problem. I've got a number of roofs made this way, and know of more (all Knutsford East Coaches were roofed this way) it has never been a problem.
Guy suggested the alternative is to replace the roof with a brass one - what's discouraging you from doing that?

Didn't see the point when I had the plastic roof and the results were good. Where I had a brass roof I used it but it was harder to do and I think you would have to inspect them carefully to tell the difference on the coach. You must remember I'm producing a removable roof so I can furnishing the inside, so just soldering on a rolled brass roof doesn't work for me.
Last edited by Will L on Mon Feb 08, 2021 10:22 am, edited 1 time in total.

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Will L
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Re: Securing plastic roofs

Postby Will L » Sun Feb 07, 2021 4:46 pm

Guy Rixon wrote:Will, you said "You start by fitting a flat plasticard sheet "ceiling" sitting on the top flanges and a good fit between the end. This need to be slightly less than the full carriage width by twice the thickness of the roof." Is that for a half-elliptical roof by any chance? If so, I entirely agree with your approach, and mine would not work so well.

My current roof is an arc roof, a fairly flat one.


That's a good point I hadn't noted quite how shallow the arc was. I like the solidity of the roofs I produce but to make it work you would have to chamfer the ceiling edges to suit the roof profile. I've got a couple of these in my to do pile so I could well find out the truth in due course.

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iak
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Re: Securing plastic roofs

Postby iak » Mon Feb 08, 2021 9:46 am

An interesting way forward is this. :thumb
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