THIS ARTICLE DISCUSSES the rationale of tapered boilers; scratch-building them in 7mm; and a complete, simple process of marking out, cutting and rolling, easily and accurately.

We need only two nails and a bit of wood. It does not involve CADCAM, glue, sticking bits of paper to sheets of metal nor complex drawing instruments. Neither does it suggest trying to force a rolled cylinder into a tapered ring.

January 2022 saw quite some discussion in the Guild’s Forum concerning the setting-out and construction of tapered boilers. Much of what was said seemed logical and correct; some I thought to be wrong.

In the to and fro, reference was made to Arthur Wesson’s article in the November 2010 edition (Vol18 No1) of the Gauge O Guild Gazette; readily available from the Guild’s website archive.

The article put forward complex geometry techniques and used exaggerated examples for marking out scratch-build components. The process outlined involved cutting out paper shapes and gluing them to metal sheet to then be formed, but with little hint of how, other than to send a CADCAM file to etchers or forcing a rolled cylinder into a tapered boiler ring.

Perhaps 12 years on, an update might be useful, particularly for newcomers and those looking to expand their scratch-building expertise in a home-based hobby. In particular, by applying some simple techniques and using realistic examples.

The primary concern of George Jackson Churchward as Locomotive, Carriage & Wagon Superintendent to the Great Western Railway 1902–1921, was the generation of steam; maximising water flow around the boiler rear tube plate adjoining the firebox and reducing the formation of steam cavities which can damage heat surfaces.

His study of American locomotive practice and the introduction in the 1890s of the Belpaire firebox led him to apply a taper to the boiler back ring, making the component as a truncated cone, with the lower edge being horizontal. All the ‘taper’ was showing on the top edge, the upper slant offering maximum steam space next to the firebox crown. Churchward avoided steam collection domes in the boiler ring, which he considered to be a source of weakness, using instead the two feet of steam space above the water line over the crown of the (raised) Belpaire fireboxes; thus, also eliminating the problem of priming.

The ‘Type 1’ tapered boiler was introduced by Churchward in March 1903 on No. 98 4-6-0 two-cylinder locomotive; the Saint class. Churchward Locomotives - A Pictorial History by Brian Haresnape gives a good history. See Note (1) Ref (1). For a useful table of (GWR) boiler dimensions see: An Introduction to Great Western Locomotive Development by Jim Champ. See Note (1) Ref (2).

Fig 2 shows the principle Churchward achieved, by widening the rear tube plate and creating extra water space, tapering the boiler from there towards the front. Even a mere 6-7in (150-175mm) expansion of the tube plate created a dramatic increase in the water spaces. A corresponding increase of the front firebox plate enabled a pseudo combustion chamber to be formed, further increasing the thermal efficiency. The author of the 2010 Gazette article stated that the question of avoiding water surging to the front of the boiler was a primary objective in tapering boilers. This does not appear to have been a design factor for Churchward and is not mentioned by him in his 1906 paper to the Mechanical Institute, apart from the general question of ‘priming’. See Note (1) Ref (3).

The shape needed on a flat piece of metal, which can then be rolled to produce the item, is called its ‘development.’ The development of a regular cone is a segment of a disc and the development of a frustum is a segment of a ring:

The development of the regular frustum needs to be adjusted to reflect the oblique frustum through B2 and B4. Fig 7 shows the final developed shape, with the two adjusted arcs, shown in red. Each amended arc has a different pivot point, their loci not being aligned to the axis of the regular cone.

The inner radius through B2 is 192.7mm (R3). The outer radius through B4 is 218.2mm (R4).

Take a piece of board; long enough for the largest radius – here, say 350mm wide and say 150mm deep (14 x 6in). Mark a base line or, better, tape a straight edge across the front edge of the board. A 24in/600mm steel rule is ideal for this. Drill a pin hole at one end of the board on the base line as the main pivot P1, and place a nail/pin in it to mark the angles. Tape the boiler plate in the appropriate place against the base line, ie at the points A2 and A3 in Fig 7 (269 - 305mm from P1).

Now place a nail through the trammel at the left-hand pivot hole and into the board at P2. With the other pin in the trammel hole B2, scribe the inner oblique arc A3B2A3 on the metal. Move the pivot pin to P3 in the board and the outer pin to trammel hole B4. Scribe the outer arc A2B4A2 on the metal. Make only one light score with the pin. Repeated scoring risks moving the trammel and blurring the lines.

Scribe the line A2 A3 at the top edge of the arc by t1. We now have the development of the oblique frustum marked out, directly onto the metal. Note that the inner radius R3, should meet the base line at A2, some 269.4mm from P1; the outer radius should meet the base line at A3, being 305.0mm from P1.

I find tin snips are the easiest tool to use here, especially with thinner gauges (I am not good with a piercing saw) and it is useful to have a pair of curved snips for the inner concave edge. It may be wise to leave about 0.5mm of spare metal along the cuts to allow for fettling to final fit. We are now ready to roll.

Set the rolls so that they are holding (but not squeezing) the outer, convex edge and merely touching the inner, concave edge (Figure 13). You should be just able to move the inner edge from side to side, whilst the longer outer edge stays fixed.

This diagram is, of course, greatly exaggerated, but hopefully shows the principle that one side of the rolls is tight on the workpiece, whilst the other side is barely touching.

Set the forming roll (or back roll) to give only a slight degree of bend. The secret is to run through 5-6 or more light passes of the rolls, slightly increasing the degree of roll at each pass, rather than two or three heavy squeezes. Slowly and gradually feed the longer edge through the rolls; half using the handle to turn and pull the metal through; half lightly pushing with your fingers to feed the metal through. Feed only some 5-6mm at a time and then move the metal sideways to line it up again, with the radial (red) marks parallel with the rollers.

Join the seam with a length of soldered scrap etch (Figure 17). It makes the task easier if the ring is slightly over-rolled - say by about 3mm, so that the edges press against each other without being forced or wired up. Solder from the middle of the seam outwards.

Lightly file the ends or pass them across a sheet of Wet & Dry to result in a perfectly square and accurate tapered boiler ring.

In Fig 20. we have the No. 5 GWR taper ring, with end plates, filled with lead, a pressure valve marked and fitted; ready to be assembled into a half-coned No. 5 GWR boiler and firebox for a GWR 4-4-2T 4600 project.

Shown clockwise from the left are:

• The No. 5 GWR ring with top taper;
• a GWR No. 4 type 11ft long tapered boiler, as fitted to 4-4-0 38xx Counties, 2-6-0 Moguls and larger tank engines;
• an LMS Duchess type with bottom taper;
• an 11in zinc bucket (7mm tall). See Note 3.

All these were made to the same process as outlined above.

1. Churchward Locomotives - A Pictorial History, Brian Haresnape and Alec Swain p.24-25, ISBN 0 7110 0697 0
2. An Introduction to Great Western Locomotive Development, Jim Champ [ Appendix B p.318-323 ], ISBN 978 1 47387 783 2
3. Large Locomotive Boilers, G J Churchward, Proceedings of the Institute of Mechanical Engineers, February 1906, p.165
4. Tapered Boilers with Maths.pdf

Locomotive crews had a bucket. On non-stop expresses there was no stopping for comfort breaks. Footplate crews either went into the coal space or used a bucket. This also served to carry fire when prepping on shed and was reserved for the fireman to wash and brush up in at the end of shift before meeting his date for the evening (no washrooms).

There are two forms of horizontal rolls; ‘Forming’ rolls which are limited to bending operations only and ‘High’ rolls which can (if robust enough) be used for planishing or reducing thickness of the material. With ‘high’ rolls there is always a tag at the start (and perhaps at the end), which is not bent, requiring the piece to be cut off or the material reversed and re-rolled.

G W Models Ltd,11 Croshaw Close, Lancing, Sussex, BN15 9LE, 01903 767231 (6in and 10in fide ‘forming’ rolls)

VEVOR, support@vevor.com, +44-330-175-5511, Vevor (SR 300-600mm J slip roll rolling machine manual solid metal sheet roller bender).

H S Walsh Ltd, Birmingham Branch, 1-2 Warstone Mews, Birmingham, B18 6JB H S Walsh
+44 (0)121 236 9346 birmingham@hswalsh.com (Economy combination rolling mill - 5 rollers - TR214)

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