In commenting on an earlier post, Michael Edge pointed out that the visible diameter of a locomotive boiler is greater than the diameter of the metal boiler itself and he suggested that I should have bought a larger diameter tube for my model of 'Tantalus'.
I had already been thinking about how to add the visible boiler rings and then had the idea “why not 3D-print the boiler cladding?” After all, the prototype boiler was metal, whereas the cladding was initially of wooden planks, then with an iron covering over the wood, and later, a range of different materials, such as asbestos, were used. In all cases, the cladding was an insulator and, therefore, non-metallic. So, I would only be following prototypical practice if I added a PLA cladding as the insulation on my own model boiler.
A few years ago, I measured the cladding thickness on one of the boilers at Didcot Railway Centre, using my ever-ready measuring stick. (Sadly, they wouldn’t let me strip some cladding off ‘Iron Duke’, to measure that one). Two inches thickness translates into 0.67 mm in 4mm-scale. All I needed to do was to print a cylindrical sleeve of 0.67mm thickness and add the visible bands in the appropriate places. This last requirement gave me some food for thought but I eventually came up with a very simple method.
The Gooch Boiler had three intermediate bands, dividing the length of the boiler into four equal segments, with an additional band at each end, adjacent to firebox and smokebox respectively. My easy solution was to design one section, with a band at one end and then copy the additional segments and join them all together, to create the complete cladding. That left one additional ring to be added at the final end. I illustrate the steps, when using ‘Fusion 360’ below:
After extruding the first segment, with its ring, I copied it to make 2 rings and copied again to make 4. For the final ring, I split the ‘body’ 0.5mm from the end and extruded the top part to make the final ring. When printed, the cladding sleeve looks as shown below (taken on the printer bed):
The printed tube slid smoothly but firmly over the brass-tube boiler:
The difference in diameter is not that obvious but another advantage of this method was that I could include the boiler bands in the 3D print, so solving a secondary problem in completing the model.
My model of 'Tantalus' now looks as shown below, the measured outside diameter of the clad boiler being 19.3mm. There is, of course, quite a lot of work still to be done on my model, in terms of ‘fixtures and fittings’