My progress, since the previous post in this series, has been slower than anticipated, mainly because of the need to build a chassis that deviated from that intended for the BGS kit. I also took a few false turns, which led to a significant amount of re-work.
In the comments following my first post about this coach, I said that I prefer scratch building to kits. I suppose what I really meant is that I like to do things my own way. The down side of the scratch building approach is that it needs lots of research, if one wants to build an authentic replica.
As it happens, the under-gear supplied with the BGS kit represents a later modification of the Mail Coach, with vacuum brakes that were not introduced until around 1878 – ten years later than the time in which my model is set. Hence, I have been forced into carrying out the research needed to design and then build an earlier form of chassis.
Before starting, I made a deviation, to try and understand the operation of the brake system described in the kit instructions. This is the clasp brake with external rods connecting the brake shoes on opposite sides of each wheel. At first, I could not understand how this mechanism worked but, after receiving help from members of the GWR E-LIST, I realised that the reversing action is achieved by rocking levers working in a horizontal rather than a vertical plane, which is the usual method in most brake mechanisms. In the GWR system, which I believe was developed by Dean, a horizontal lever running across the back of each of the outer brake shoes both applies that brake to the wheel and pivots about the back of the shoe to pull the opposite brake shoe via a coupling rod that runs outside the wheel. I made myself a diagram to illustrate how this works:
Diagram of Dean Outside Clasp Brake
After this distraction, I could return to the design of the earlier type of break gear, in use during the 1860s.
As I explained in my previous post, I found drawings that included early types of clasp brakes in Alan Prior’s book of 19th Century Railway Drawings. From these, I was able to produce a sketch of the gear, in general terms. In order to apply the gear to the Mail Coach, I needed to determine several key dimensions and also the locations of the various components, underneath the floor of the carriage.
Fortunately, as a member of the Broad Gauge Society, I have access to their excellent series of data sheets, which provide many important facts about the design and construction of broad gauge vehicles. Although the Mail Coach is illustrated in a later form, the data sheets record that this vehicle was rebuilt from the Revised Standard 2nd class coaches, of which 40 were built in 1854. Carriages of this type appear in the photo that was used by Frith as a ‘model’ for his paintings of GWR trains. I have based my own coloured illustration, below, on this photograph:
Note circular Normanville axle boxes
According to the BGS data sheet, these coaches were fitted with clasp brakes, operating on all three wheels on one side only. The outside rods that operated these brakes are visible on the carriage in the centre of the photograph above.
Six of these coaches were modified to meet Post Office requirements in 1858. The data sheet states that luggage rails were added to the roofs of the 2nd class carriages from 1859 onwards, with added tarpaulin sheets, but it is not clear whether this applied to the Post Office conversions. The photo evidence shown in my earlier post suggests that these rails and tarpaulins were fitted on the lower section of the roof.
For further details and dimensions, I consulted the drawings of the original 2nd class carriages, as shown in the BGS data sheet, which formed the basis for my scale drawing of the chassis below:
Comparing this scale drawing with the sketch I made for my previous post (repeated here, for convenience), I could now place the various components in their ‘correct’ positions under the floor of my Mail Coach.
The components I needed to make comprised three pairs of brackets, to carry the cross shafts at the two ends of the carriage and at the location of the brake handle, the various levers mounted on these shafts, and the various lengths of rodding. In order to allow the wheels to be removed for maintenance I decided to omit the inner pair of brake pull rods, since these would not be visible in normal operation.
It was now time to start actually assembling the chassis and I soon hit a problem with interpreting the instructions. The instructions started with a note that “these instructions were written after building a 7mm kit”. I started to realise that things were going to be different in 4mm, when I read that “the inside hornguides (xx) are fitted in the slots in the floor”. My floor had no slots but there seemed to be a secondary ‘sub-chassis’, labelled on the fret as Part , which is not mentioned in the instructions. I assumed (wrongly, as it turned out) that this was to be a complete separate chassis, to be fixed under the plain floor of the coach body.
Hence, I started to look at how to fit the solebars to the sides of this sub-chassis but there seemed to be no provision for any proper support. Then, after reading a number of apparently irrelevant instructions about a sprung chassis (for which no parts were supplied), I found that the solebars were meant to be soldered into grooves in the coach floor. Perhaps this would work in 7mm but the solebars in 4mm scale are very thin, flat strips of brass, which I felt needed more lateral support than this very shallow groove. In addition, because I had assumed that the chassis would be a separate item, I had already spray-painted the body, so did not wish to start doing more soldering there!
After a pause for thought, I decided to make two brackets, running across the under-side of the sub-chassis , to support the solebars along the outer edges of this component. The brackets for the brake cross-shafts, which I made in a similar manner, provided additional support near the ends of the coach. All the material for these brackets was cut from spare areas of the etches supplied for the kit, using my jewellers’ snips.
With these modifications, the underside of the sub-chassis was as shown below:
I soldered the axle-guards to the solebars before soldering the assemblies to the brackets on the chassis. I find that those small dressing stones, often included in accessory kits for mini-drills, are very handy for removing the small burrs from small parts, where they have been attached to the fret. I find that I can remove the burr without damaging the main profile of the part, as shown below:
These stones are also good for rubbing down any excess solder or working on white metal, to avoid clogging up good-quality needle files.
The springs supplied in the kit are brass castings but I decided to use super-glue (cyanoacrylate) rather than solder, to avoid the possibility of melting other joints around the axle guards. I find that, providing the surfaces are thoroughly cleaned, super-glue is very effective for attaching these small components. I have also discovered (initially by accident) that iso-propyl alcohol acts as an excellent accelerator when applied to super-glued components, making the glue polymerise almost instantly. Any surplus glue turns into a white residue that can be scraped off.
So, after a few twists and turns, I now have a ‘rolling’ coach, which needs various bits of brake rodding to be added, to complete the appearance of the underframe. I shall look for some different axleboxes from those supplied with the kit, since the early photographs show the characteristic circular pattern of Normanville’s patent design.
I have darkened the roofs and will probably paint the skylights white, before weathering them a little, for a more ‘realistic’ appearance. There are still several details to add but I am pleased to have solved the chassis problems.
Edited by MikeOxon