Blooming Belpaire 'Boxes (forming fireboxes)

Discussion in 'Techniques' started by Steph Dale, 20 December 2018.

  1. Steph Dale

    Steph Dale Western Thunderer

    In the past couple of months we've hit a minor snag at Finney7 towers, which was associated with the construction of belpaire fireboxes. The problem was that although they're relatively common things to have to make, they're complex shapes and as we went through the process of developing our new instructions we felt that we should offer more complete guidance on how to do it, just in case the builder was not familiar with the techniques.

    So, I had a look around; there’s some stuff in a couple of old Model Railway News and books by Guy Williams and others; some go back to the fifties. Here then is the official Finney7 recipe for constructing a Belpaire firebox. This applies equally to those that were designed by Martin Finney and other designers such as Malcolm Mitchell or David Andrews; it's not even scale-specific as the components required can be selected on size for the kit and scale in question. I've also included a note further down the page on how to mark out a wrapper from scratch if you're not building a kit. The example given here is for a 7mm 47xx, from the Martin Finney/Finney7 stable:

    The 47xx kit comes with double front firebox spacers and a single rear one, all out of the 0.7mm (28thou") nickel-silver fret and a wrapper, mostly half-etched from the 0.45mm (18thou") brass fret. This gives a good combination of surface detail and solidity; the double front thickness helping with forming the corners of the firebox. The spacers are provided with pairs of nominally 3.5mm holes, so a length of 4BA studding was purchased as well as both brass and stainless steel nuts; it has to be a good fit in any provided holes and with 4BA being specified as slightly larger than 3.5mm it's worked well:

    First job is to work out how long the sections of studding need to be, the answer is 'a bit longer than the wrapper':

    The front spacers are then laminated and all spacers cleaned of their cusps. The brass nuts are threaded on to the studs:

    Using the small dimples provided on each part the centre lines are marked on the outside...:

    ...and the inside of each part:

    With all the parts cleaned up and marked out, construction can commence by setting the two spacers on to the studs, retaining them with the stainless steel nuts and ensuring the length of the assembly matches that quoted in the instructions. In the case of the 47xx, that's given as 60.9mm. Always measure the distance from the bottom of the firebox; even using a steel rule and eyeglass you can get pretty close to this sort of dimension with care. Take your time, measure and check it a few times:

    It's much easier to use a vernier or similar gauge as not only can you be sure the measurement is pretty precise, it will also be a good check that the spacers are parallel to each other. Assuming the nuts and studs have reasonably well-formed threads, they should be...:

    I'll be along with part 2 shortly, I'm just going for a cuppa...

    Last edited: 22 December 2018
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  2. Steph Dale

    Steph Dale Western Thunderer

    Right, tea's a-brewin', so let's crack on:

    It's worth quickly checking that the spacers are square, both front:

    And rear - of course this was done on a decent flat surface...:

    This is the 'a-ha' step; do the stainless steel nuts up tightly and then solder the brass nuts to the spacers. A good blobby tack, as here, will do fine:

    And here, still properly blobby:

    Just as a short aside - once you're at this stage the spacers form a pretty strong assembly. Any attempt to twist the assembly results in one stud tightening as the other slackens; it doesn't want to move far. As a little experiment, you can actually slacken off the stainless steel nuts at this stage, and turn the studs. Re-tightening the nuts will result in the assembly forming itself back square and level. Just make sure the nuts are tightened up and you've checked the assembly is square again before moving on to the next stage!!!

    For those that may be scratchbuilding a belpaire firebox, the wrapper shape can be worked out now. I would normally wrap the firebox former assembly in cartridge paper and mark round it with a pen - don't forget to mark the centre line:

    Which gives a wrapper marked out like this, which can be cut out and used as a template. It's slightly over-size, so will need trimming as assembly nears completion:

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  3. Steph Dale

    Steph Dale Western Thunderer

    So we're now on to forming the wrapper. The aim here is to get it to closely approximate the shape of the spacers. So the aim is to form it for the spacers, not with the spacers.

    Aligning the centre line marks, the top can be formed to a gentle radius. This is a simple rolling job, using a length of dowel and finger pressure. In this shot, I've started forming the shoulders around an old 12" round file - the tapered section is useful on GWR fireboxes that don't have a consistent radius: DSCF4265v1.jpg

    Once the second shoulder is formed, it's tricky to re-establish the centre lines if they're not maintained, so make sure the wrapper forming remains as symmetrical as possible and that the centre lines remain in synch:

    On waisted fireboxes, such as this one, I then tend to start forming the concave sections:

    Of course, when comparing the spacer shapes with the wrapper, you don't have to always do it with the wrapper around the spacers...:

    The final job to form the shape is to pull in the waisted section, by putting a gentle curve on the sides of the firebox; again this is dowelling and finger pressure:

    As can be seen, it's not a perfect match to the spacers, but gentle finger pressure is enough to get the wrapper to meet the spacers without distortion:

    So we're now ready to start soldering...

    Last edited: 20 December 2018
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  4. Heather Kay

    Heather Kay Western Thunderer

    Steph, I note you haven’t annealed the wrapper. I assume this is because the etched material in the F7 kits is thin enough to be formed readily as it is. Is that right?
  5. Steph Dale

    Steph Dale Western Thunderer

    Start by tacking on the outside of the firebox at the centre and corners, both front and rear. Again, take care and check that the centres retain the alignment that we've worked so hard to achieve:

    We can then work our way down the spacers alternating tacks left/right and front/rear to even out any expansion of the wrapper. In this way, I got 7 tacks in on the front:

    And 9 tacks at the rear:

    Before running the seams round at both ends:

    Last edited: 20 December 2018
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  6. Steph Dale

    Steph Dale Western Thunderer

    In general, I wouldn't anneal it anyway, because of the risk of a thin part distorting when heated. But, on kits where the material is thick enough to warrant it then it's a fair technique. In either Mitchell or Finney kits annealing is really unnecessary and generally, if I was scratchbuilding, I'd use 10thou" to form the wrapper, which should also be thin and soft enough.

    If I was faced with a thick firebox wrapper without any bend relieving lines I'd almost certainly make a new wrapper using the old one as a template. Hacking through 10thou" brass with a scrawker is just a couple of minutes work after all, and probably quicker and easier than wrestling with something unsuitable.

    Last edited: 20 December 2018
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  7. Steph Dale

    Steph Dale Western Thunderer

    Nearly there!

    With the wrapper now firmly attached to the spacers, the stainless steel nuts can be undone and the studs spun out:

    I've then run an extra fillet of solder into the front shoulders of the firebox to support the area which will be filed back. The brass nuts have also been heated and flicked off with a scriber:

    At the rear, the scrap section of the spacer is cut out and the top nut removed as before:

    The base, front and rear are now rubbed down on a sanding board to keep them flat, this will remove the cusps from the wrapper at the same time and leave the firebox ready for the final shaping and fitting to the rest of the loco:
    DSCF4319v1.jpg DSCF4316v1.jpg

    That's it! ;)

    Last edited: 20 December 2018
  8. simond

    simond Western Thunderer

    Nicely explained, Steph.

    Pretty much exactly how I did the firebox for my Finney 47xx some 21 years ago, my first brass kit, so if I could do it...

    (Although I have a very vague recollection of it having 4 threaded rods - or was that someone else’s kit?)

    I concur about not annealing the wrapper, and I vote twice for not having etched bend relief lines on the inside of the sheet. They really don’t help IMO.

    Season’s Greetings
  9. SimonT

    SimonT Western Thunderer

    I'll need at least two more A3 pages to get this lot in;)
  10. Dikitriki

    Dikitriki Flying Squad

    I knew there was I reason I liked Fowler engines!

    Can I add one thing - I always add copper wire (around 1mm), carefully formed to be a snug fit, on the inside of the front wrapper/former joint using plenty of solder to minimise the risk of filing through when rounding, and to provide a stronger bond.

  11. Steph Dale

    Steph Dale Western Thunderer

    Simon (D),
    Thank you! If you used four studs to hold the spacers it wasn't one of ours, I think all the kits Martin designed use just two studs...

    Simon (T),
    Yeah, but only two key strokes :D (Ctrl+c, Ctrl+v)....

    That's a good point. Reinforcing wire for the front fillet is probably unnecessary in this case as the front is something like 1.4mm thick already due to the laminations, but for other kits or when scratchbuilding it's almost certainly required.

  12. Yorkshire Dave

    Yorkshire Dave Western Thunderer

    Something you would have to consider for Belpaire fireboxes with large rounded corners e.g. SECR E and L fireboxes. Or would they be castings?
  13. eastsidepilot

    eastsidepilot Western Thunderer

    I've never annealed a wrapper on fireboxes, boilers or smoke boxes as it can distort them. If your scratch building or junking a kit part I've found 10.00" is best thickness.

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  14. Steph Dale

    Steph Dale Western Thunderer

    I think I'd cross that particular bridge if I got to it. There are advantages and disadvantages with each approach and it would have to be figured into the overall design.

  15. Arun

    Arun Western Thunderer

    David Andrews' kits had four studs I believe
    Last edited: 21 December 2018
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  16. Steph Dale

    Steph Dale Western Thunderer


    One of m'colleagues contacted me off-forum earlier today wondering why a bit of Finney7-specific info was appearing here. In order to clarify that this is a generic technique, I've added another half a paragraph to my first post which I hope is useful in adding some more context.

  17. Arun

    Arun Western Thunderer

    I rather like this thread - It will be seriously useful when I get around to reaching the bottom of the cupboard where there is a Mega Kits 47xx hiding waiting to be started!
  18. Steph Dale

    Steph Dale Western Thunderer

    Thanks Arun,
    I suspect you'll find plenty of other things in that kit to 'entertain' you, not just the firebox... :D

  19. Arun

    Arun Western Thunderer

    Steph - I did think about asking your Finney7 gang at Bristol whether they would swap it for one of their 47xx but suspected that they might not!
  20. P A D

    P A D Western Thunderer

    Hi Steph,
    Very useful technique. It works equally well on cylinders, as here on those of the Gladiator L1. In this case I was lucky enough to find some metric bolts just the right size for the piston gland casting holes on the front and rear plates.