Query: how to develop a Gresley wide firebox from scratch?

[Horsetan]

This question has been bugging me for a while, but things have hastened since the arrival of a Judith Edge scratchbuilders' etch for a Gresley V4. It has patterns for rolling the boiler, but the firebox development is not catered for. I have an Isinglass drawing for the V4 but obviously that's all very two-dimensional.

The Finney A1/A3 and V2 kits have the firebox all nicely set up with an internal cradle to help form the bends, but how do you lay out a plan for a Gresley wide firebox from scratch? What calculations are required to get the shape of the firebox outer wrapper right?

Would be grateful to know if anyone out there has done this type of thing.

[Atlantic 3279]

Maths aside, can you create, off the loco, a set of accurate formers for the curves and slopes, all connected together to create an internal rigid frame for the shape, and then "simply" roll a piece of sheet metal of excess size to fit exactly over those formers and finally trim to size?

If you want a lasting record of the final shape and size you can re-flatten the sheet and mark out an archive copy before you form it up again for use on the model.

[john coffin]

First question is do you have an accurate drawing of the loco?

Second I would consider making it in 3 pieces, the top of the firebox round the boiler to the centre line and
then down to the footplate where the edges are "straight. Then make the front in two pieces which you can
shape and make more easy to install.
Whilst Graeme's idea is correct, see what Sawdust has done with the beaver tail and he posted over the weekend,LNER News
what you should do is check the etch thickness, and then try to find some cardboard that thickness, or glue up a
couple of pieces of paper to get the thickness, these you can fiddle with and cut, and then layout flat for the cutting
of the metai. The former should of course be smaller by one thickness at the top, and two at the sides.

As those trying to fit the vestibule on a Gresley carriage will know, this nowadays is more complex than in olden times
when designers laid things out by hand. Very few available computer programmes allow you to design flat metal work
from a 3D image.
Paul

[Hatfield Shed]

I would start

... I would consider making it in 3 pieces, the top of the firebox round the boiler to the centre line and then down to the footplate where the edges are "straight". Then make the front in two pieces which you can shape and make more easy to install...

Me too! Which method somewhat corresponds to how the Crimpsall formed the boiler cladding.

The conic surface of the firebox top, down to 'just above' boiler centreline: I would make the internal frame with the two radii, and then finish a piece of wood to fit between the two, take that wooden former out and shape the sheet metal over it, and offer up to the frame, marking the cuts once the fit is 'perfect'. Worth taking plenty of time over this, and committing yourself to multiple 'cut and try' efforts, as our eyes detect imperfections in this kind of surface very readily...

[Pebbles]

Here we go. Firstly do you know how to develop a tapered boiler section? This is rather an issue as the classic Churchward conic boiler is from a side view only tapered from the top whereas the Gresley boilers are equally tapered - I hope that makes sense. This point is where Roche's LNER drawings fail! For years I have developed the Gresley tapered boilers by extending the taper of the drawing to a complete cone. Within the cone fit in the two dimensional boiler then using the point of intersection draw two arcs one being the intersection point to the larger diameter the second to the smaller diameter. On the larger arc space out the developed circumference of the larger diameter part of the boiler and from the two resultant points draw again to point of intersection. You should now have the developed coned boiler.
Turning to the firebox problem as has been pointed out the upper part is part of a cone so using the same principle this can be developed but only using half of the developed section. Tack onto this either side the lower flared part of the firebox. You may have to fiddle about but you should be within a fighting chance of success!

[greenglade]

Just to add that there are free programs on the web that will work out tapered cones for you which you can then print off to size. try google..

Pete

edit: I can't recall which program I used for my 3 1/2 gauge Great Northern but a quick search found this one which looks just as good.

https://www.templatemaker.nl/en/cone/

[Atlantic 3279]

It did occur to me after posting my initial suggestion that the first attempt to create a wrapper over the formers might as well be performed using thin card rather than sheet metal, as that is a great deal easier to curve to shape, mark, and trim. It can then be used as a template for a second (and hopefully final) attempt using sheet metal.

[greenglade]

It did occur to me after posting my initial suggestion that the first attempt to create a wrapper over the formers might as well be performed using thin card rather than sheet metal, as that is a great deal easier to curve to shape, mark, and trim. It can then be used as a template for a second (and hopefully final) attempt using sheet metal.

That's how I did the cleading on 4470 using the free cone program... printed the drawings off, transferred them to card, in my case a cereal box, cut out and they all fitted first time. Then transferred those to brass sheet, rolled in the rollers and hey presto, the cleading was completed. I'll do the same thing when I get to that stage on my 5" 4472.

Pete

[Horsetan]

Maths aside, can you create, off the loco, a set of accurate formers for the curves and slopes, all connected together to create an internal rigid frame for the shape, and then "simply" roll a piece of sheet metal of excess size to fit exactly over those formers and finally trim to size?

Yes and no. The Isinglass drawing is not entirely clear where the bottom of the front former should be, and of course the apex of the rear former is lower than the apex of the front one, as the firebox top slopes downtothe cab front.

If you want a lasting record of the final shape and size you can re-flatten the sheet and mark out an archive copy before you form it up again for use on the model.

This is a must, if only to get Mike Edge to add it to the templates he includes in the instructions.

First question is do you have an accurate drawing of the loco?

Only the Isinglass drawing at the moment. This has the usual side and top plan views, but doesn't pretend to touch cross-sections.

I think the NRM holds some V4 drawings, but it's unclear from their list as to whether those drawings include firebox cross-sectional views, which would really help. They would take several weeks to retrieve, given the current restrictions.

Second I would consider making it in 3 pieces, the top of the firebox round the boiler to the centre line and
then down to the footplate where the edges are "straight. Then make the front in two pieces which you can
shape and make more easy to install.
Whilst Graeme's idea is correct, see what Sawdust has done with the beaver tail and he posted over the weekend,LNER News
what you should do is check the etch thickness, and then try to find some cardboard that thickness, or glue up a
couple of pieces of paper to get the thickness, these you can fiddle with and cut, and then layout flat for the cutting
of the metai. The former should of course be smaller by one thickness at the top, and two at the sides.....

I'll have a look at the beaver tail, thanks.

Here we go. Firstly do you know how to develop a tapered boiler section? This is rather an issue as the classic Churchward conic boiler is from a side view only tapered from the top whereas the Gresley boilers are equally tapered - I hope that makes sense. This point is where Roche's LNER drawings fail! For years I have developed the Gresley tapered boilers by extending the taper of the drawing to a complete cone. Within the cone fit in the two dimensional boiler then using the point of intersection draw two arcs one being the intersection point to the larger diameter the second to the smaller diameter. On the larger arc space out the developed circumference of the larger diameter part of the boiler and from the two resultant points draw again to point of intersection. You should now have the developed coned boiler.

I'm familiar with tapered boiler development, although it took me several goes years ago to understand how it works!

Turning to the firebox problem as has been pointed out the upper part is part of a cone so using the same principle this can be developed but only using half of the developed section. Tack onto this either side the lower flared part of the firebox. You may have to fiddle about but you should be within a fighting chance of success!

This is the hard part, because the firebox not only flares towards the base, but the front former also slants backwards.