A Tender for 'Rob Roy' - 2
Entry posted by MikeOxon
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Having suitably ‘girded my loins’, I decided to make a start on the under-frame for my Broad Gauge tender, initially described in my previous post.
Drawing of Gooch 6-wheel Tender
The section of the Instructions relating to ‘Frames and Spring Hangers’ starts by stating “Decide the type of brake gear to be modelled”. Not knowing much about such things, I set off in search of more information and the instructions helpfully referred me to ‘A Study of GWR Tenders, Broadsheet 18, White, Roger & Jolly, Mike.’ The Broad Gauge Society (BGS) has produced a memory stick containing PDFs of all ‘Broadsheet’ back-issues, so I was soon able to extend my knowledge.
From that source, I learned that this type of tender first appeared in the late 1840's (1847-48) with plate frame horn-guides, and outside springs below the footplate. There were two types of suspension, with associated differences in the break (brake) gear. The most obvious visible difference was that of the drop of the 'J' shaped spring hangers. The photo in the Instruction Sheet (shown in my previous post) shows long spring hangers but it appears that short 'J' hangers were the earlier pattern and were associated with one-sided brakes, a small plate front-step, or a bar type of step.
Early-type tender attached to ‘Antiquary’ (Waverley class)
The record states that rebuilding to two-sided break-gear was accompanied by adopting the longer. 'J' hangers with added rubber secondary suspension. (perhaps to limit the amount of coal falling off at the sides?) The dates of re-building are not stated but photo evidence suggests it was after 1870 and, therefore, after the period I am modelling.
As a result of this research, I decided to build my model in the early form, for which the instructions tell me to “remove the triangular brake hanger brackets from the frames (37), (38).” For a start, those are the wrong part numbers – should be (29),(30) – and there is another trap for the unwary. At first glance, these parts appear to be attached to the fret by a number of tabs along one long side but do not cut these. Only cut two tabs, one at each end, as all the others are actually for fixing the frames to the floor of the tender.
Somehow, I knew this was not going to be plain sailing! Those little triangular brackets were not very easy to remove. I scored lines with a knife between the main frames and the triangles and then, after clamping each frame in my simple bending jig, I broke off the unwanted parts by fatiguing them along the lines, by wriggling with fine-nosed pliers. This produced a nice clean edge that needed minimal cleaning up afterwards.
The parts that are actually numbered (37), (38) are described in the parts list as ‘Frame and Spring Hanger’. The kit also contains some cast parts that are not mentioned in the Instructions but these include springs and axle boxes. Sadly, the castings do not include the J hangers at the ends of the springs. On another issue, it would have been nice if they could have included a casting for that tank filler!
Frame components in kit
From the instructions, the J hangers have to be fabricated by soldering together four layers of tiny brass etched parts. The method is described as follows: “Make up a jig by drilling through the spring hangers into a suitable material such as hard wood or paxolin. Wire passing through these holes will need to stand vertically. Stack three solid and one half-etch overlays (18) on each and solder.” I looked at the parts (very tiny in 4mm scale) and thought about the amount of work needed to build up 12 spring hangers and decided “There has to be a better way”
J hanger components on fret
One thought was to search the bazaars from some cast springs that include J hangers. There are some from Dart Castings that might be suitable. Then, I did some lateral thinking. After all, 3D-printing is all about building up parts in layers, so why not print these components as overlays to mount onto the brass main frames? Purists might object to the use of mixed materials but there are times when one has to place personal sanity first.
It was actually quite easy to 3D-print the overlays. I selected the relevant parts from a drawing of the tender, shown above, converted my drawing to DXF format and opened it in ‘Fusion 360’. I then used the ‘Push/pull’ tool on selected parts of the drawing to make the overlay. The base is very thin (0.5 mm) and the springs, J hangers, axle boxes, etc. were all extruded by appropriate amounts from the surface of this base.
3-D Drawing in ‘Fusion 360’
It was an easy drawing to create, as I only needed to draw one spring and axle box assembly and then copy it for the other axles. The opposite side of the tender frame is a mirror image, which is created by a click of the mouse! The actual printing time for each of these thin overlays was 10 minutes, so I had a complete set of overlays within half and hour.
3-D Printed Overlays
I have written before about combining 3D-printing with the use of my Silhouette cutter, in order to produce coloured panelled sides for carriages. Now, I have shown how 3D-printing can be combined with traditional sheet-brass modelling, to add additional surface detail.
After surmounting this major hurdle in the build, it’s back to routine fitting and soldering. The brass frames fitted neatly into their slots in the base plate and feel nice and firm. The instructions start to get a bit meditative again, with questions about which type of drag bar to use. I’ve looked at the fret and, at the moment, cannot see the difference, so I shall take a pause and open another bottle. A nice cold Sauvignon Blanc this time, I think.
I think the 3D printed frames look rather nice underneath the tender body. In my excitement, I forgot to provide openings for the pin-point axles but that’s a simple job for a small drill.
Two views with the printed overlays loosely attached so I can continue to solder other parts later.
Mike
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