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A Tender for 'Rob Roy' - 2




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.












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I have just spent 10 minutes tracking down the origins of the term 'to gird your loins'. I shouldn't have done that because now every time I read your posts I will think of you as a biblical figure with a 3D printer.


Those axle boes are superb. The days of components made from layered etches and whitemetal castings are surely numbered.

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I'm pleased to add a few idioms to your vocabulary, Mikkel.  Kit-building can sometimes feel like preparing for battle.


I had to pinch myself when I found that I had written "It was actually quite easy to 3D-print the overlays."  A year ago, it would have seemed an almost impossible and daunting task.  There were a few tricky bits to making the 2D drawing but the 3D part was no more than a few mouse clicks on the 'Push/pull' tool :)

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Wonderful work. When you say you selected parts from the tender drawing and converted them to dxf, what were you doing?  Was the drawing an imported jpeg or pdf and then converted by software trickery? Is a special tool to use?  What drawing software did you use?





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18 hours ago, drduncan said:

... When you say you selected parts from the tender drawing and converted them to dxf, what were you doing?  Was the drawing an imported jpeg or pdf and then converted by software trickery? Is a special tool to use?  What drawing software did you use?


I described my methods many years ago in this post


If I have a good side-on photo or sketch in JPG format, I copy and paste this into 'Autosketch' and make it a 'background layer', so it can't be moved or changed.  If possible, I adjust the scale of the original in 'Photoshop', so that it is the correct size when I paste it into 'Autosketch'.


I then draw the main construction lines over this original and, when this stage is completed, I move my whole drawing to a clear space on the 'page'.  Now I work on each element of the drawing to make sure all the lengths and angles are correct and that lines meet up where they should.  In 'Autosketch', this involves use of the 'Trim' tools but other packages have their own ways of doing things. 


Once I am content with the 2D drawing, I save it into DXF format (which is an option in 'Autosketch') but other formats, such as SVG, could be used from other packages.  For some reason, though, I find scale errors, when I copy SVG drawings from 'Inkscape' into 'Fusion 360', so I have stuck with DXF.


You have to choose the plane on which you want the drawing to appear in 'Fusion 360' and you then turn it into a solid body by using the 'Push/pull' tool on individual closed areas of the drawing.


'Fusion 360' is very particular about when a section is 'closed', so it is important to examine the 2D drawing very carefully for even minute breaks.  'Fusion 360' has its own drawing tools but I find them a bit temperamental when used on imported drawings.


ADDENDUM  There's more about my way with 'Autosketch' in a post about making carriage sides



Edited by MikeOxon
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Brilliant work. An excellent example of problem solving and there's no doubt the 'mixed media' approach produces the best results. The combination of cast, etched and printed components seems to me to be the ideal since each process has its limitations and where one falls down, another technique can produce better results. I'm still not convinced that 3D printing can give as nice a finish on broad, flat surfaces like tender sides as well as etched sheet, and a cast whitemetal or lost-wax brass detail can add a little extra weight if required. 3D printing comes into its own for making masters and solving the issue of layering etches which as you rightly say is an absolute drag.


Looking forward to seeing the next bit of ingenuity! 

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