“Ah, well, I sort of measure it off the prototype drawing, convert it into O scale, and work from that.” “You don't have any proper drawings? How do you make sure it all fits together?”

Good question. The easy answer is that if I scale everything correctly, it should all fit. Except that it is not that easy. One obvious example is the wheel back to back measurement, which in fine scale is a couple of millimetres below true scale (be quiet, Mr ScaleSeven, I'll deal with you later!). That means that all the frame stretchers have to be smaller than scale and all the other stuff between the frames like cranks and inside valve gear may have to be moved sideways. Lots of parts are bolted together on the prototype, and even a 16 BA bolt is enormous when scaled up to full size, so I have to figure out another way to hold them together.

It would be very neat to finish this story by saying the light dawned, I saw the error of my ways and resolved thereafter to follow the true path. Actually, I just prattled on about scaling and adjusting as I went along, and after a while he got up and wandered off.

I did not really think about it for some time. Over the years I had built quite a few models from scratch and usually got a decent result one way or another. Measure from the drawing, decide if the scale dimension works or if it needs adjusting, if it does note it down because otherwise I will forget. If a part has to fit between two existing parts, measure the space on the model, not on the drawing, and work from there. Mostly it works, but sometimes a part does not quite fit or I then realise there is a better way to do it, or I decide I need a hole in an existing part to secure the new part and have to strip down what I have done to cut it. When I did think about it, I probably wasted significant time filing and fiddling, and even remaking parts.

I could witter on about this a lot longer, but you are not my shrink being paid by the minute, so let's cut to the chase. Solid modelling is the tool that I have adopted that allows me to plan the model better before I start cutting metal. Solid modelling is otherwise known as 3D CAD, but I think that title is less descriptive of what it actually does. Step back a minute. I'm of an age, and you probably are, when technical drawing meant drawing boards and tee squares. Those of us who learnt it, learned about plans and elevations, projections and sections, and it was up to the person doing the drawing to make sure all the views were consistent and related correctly to one another. Come the digital revolution, 2D CAD was basically the same thing done on the computer screen. Some things were made easier, but it was still essentially a manually driven process.

Solid modelling is different. So different that people often say the first thing you have to do is to forget everything you knew about 2D CAD. That is not entirely true, but it is not that far off. In solid modelling we are working with objects, not views of objects but the objects themselves. Views can be created, and they come from the object. In 2D CAD it is the other way around – the object comes from the views.

This opens the door to many interesting features, a lot of which are not terribly important to model locomotives (as opposed to, say, nuclear reactors or aircraft gas turbine engines), but some of them are. Components can be linked together in various ways, either rigidly, or so that they can rotate or slide relative to another. In that way, for a live steam loco I made a solid model of a complete Stevenson valve gear that was animated and used to verify that the cylinder valves opened and closed at the correct moments. Coming closer to home, components in a solid model are numbers in the computer, not real lumps of metal, and the computer happily allows them to be moved into and through one another. If something does not fit properly, it is immediately apparent. Potential clashes and conflicts between components can be seen and dealt with before any metal is cut.

Here is an example of how I used solid modelling to figure out in advance how to tackle a recent project. It is an 1860s vintage single driver loco, which, as was almost universal at the time, had very large diameter driving wheels and a boiler sitting between them. Of course, the designer fitted the largest possible boiler in the space which meant that there was next to no clearance behind the wheels. You have probably guessed what is coming. A boiler of scale diameter won't fit between the wheels in fine scale. Either the boiler must be made smaller (and the smokebox, firebox and other things scaled down with it), or if it is to be made to scale diameter, clearances for the driving wheels must be cut out. I should add that the wheels do have splashers so the cut outs are not blazingly obvious, but it means the splashers will dive into the boiler at some point rather than standing in splendid isolation. So neither approach is ‘right’, the question is, which one will look better?

My answer was to explore both with solid models. My digital fireman lit the computer's firebox, and when Microsoft reported a good head of steam, we opened the regulator (all right, clicked the mouse a few times, in fact, many times) to create the models you can see in Figures 1 and 2.

Before getting into the details, let me make a few comments. The models are far from complete and only have enough detail to check on the boiler clearance and a few other concerns that I had. So, for example, I have only drawn the wheel tyres and not the centres. That would have been a lot more work and to no point since I was going to use a commercial wheel. Had it been a very special wheel that I was going to be made as a custom job, that would be a different matter, but also one that the software can handle. I did not bother to add fittings like the chimney even though I was going to make them myself because I knew that my method of scaling off the original drawing would work fine. I know that these fittings will not clash with anything else.

Figure 1. Small diameter boiler that fits between the driving wheels.

Figure 1 shows the loco with a small boiler that fits between the wheels. I scaled down the smokebox in the same proportion because otherwise it would look very odd. The splashers are clearly quite separate from the boiler, as they should be. Figure 2 shows the boiler and smokebox to scale dimensions, and you can see how the splashers merge into the boiler. Figure 3 is another picture of the larger boilered variant, looking inside the boiler and with the splashers removed from the model. The clashes between the boiler and the wheels are immediately obvious.

Figure 2. Scale diameter boiler, for comparison.

Figure 3. Scale diameter boiler. Clashes with the driving wheels and cranks are highlighted.

This picture also shows another clash that I was not expecting between the driving axle crank and the boiler. That happened because I was intending to use a set of commercial cranks that have a slightly longer throw than this prototype did. Of course that would be a disaster for a steam powered loco, but this one is electrically powered so I thought I could get away with it, but as it proved not so easily. At least I discovered it before the crank axle and boiler were made.

Figure 4 is the same view, now with cut-outs for the driving wheels and cranks added. The wheel clearances look enormous but remember that they are hidden by the splashers. I later decided that the cut-outs for the cranks were larger than required, and I reduced them to the necessary minimum.

Figure 4. Avoiding the clashes by making cutouts in the boiler.

So, which one did I choose? The one with the small but unsullied boiler, or the one with the scale boiler that has an unrealistic merging of boiler and splasher? It is a subjective decision and your choice may not be the same as mine. Which is not to say either of us is right or wrong. For what it is worth, I chose the larger boiler. By the standards of the time, this was a large locomotive with a boiler whose dimensions were made more apparent because so much of it was visible. The smaller boiler just did not look as imposing.

Some people talk about capturing the character of the prototype, which is a phrase I avoid because it is may be deployed to cover up something sloppy, as in “It's not quite scale but it captures the character”. But if, as here, a large boiler is so obvious and eye-catching, compromising it somehow spoils the illusion. If you compare a photo of the prototype and the model, something looks wrong long before you have worked out what it is. Whereas the relationship between the splasher and the boiler is much less apparent and more easily overlooked. If that is what ‘character’ means to you, so be it. Didn't I say it was all quite subjective?

Right, ScaleSeven, you've been sniggering at the back all the while. Of course, you use a scale back to back dimension and you think you can fit a scale boiler without compromising anything. When I said there was next to no clearance on the prototype, I was not exaggerating. A small gap divided by 43 is a very tiny gap indeed. If your driving axle has no side play at all, and if your wheels are absolutely dead square on the axle without the least wobble, you just might manage it. I would not bet on it.

Just to run the story on a bit more, I added the drive train to the model. I wanted to use an ABC Gears product and added it to the solid model to check that it would fit: see Figures 5 and 6. The motor is the vertical cylinder sticking up into the firebox, and the drive goes to the rear axle because the driving axle is too cluttered with cranks and eccentrics to include a gearbox. It fitted without any problems, and I was able to assure myself of that before handing over any hard cash.

Figure 5. A view of the model with motor and gearbox. The ability to make components appear semi-transparent to see inside them is a very useful feature.

So, there you are. Admittedly it is a tricky prototype, but the decision about the boiler was key and I was very pleased to spend a few hours on the computer rather than make a boiler and then decide it was wrong. The crank issue took me by surprise and just shows you can't be too careful. Either that or I'm not as clever as I sometimes pretend. And I am pleased to say that the construction went quite smoothly with no major hiccups. Figure 7 shows the boiler and splasher in the metal. Decide for yourself whether I got it right or not entirely wrong.

Figure 6. Another view of the drive train to the rear axle.

There are many solid modelling packages available, many of them aimed at the industrial user and costing huge sums of money. I use Fusion 360 (www.autodesk .co.uk/products/fusion-360/overview) which is free to cheapskates, I mean to hobbyists like me. With the free version you don't get much support, but you do get a fully featured, industrial strength package. One of my friends uses OpenSCAD (www.openscad.org), which is open source (and free) and uses a very different approach, which might appeal if you prefer writing scripts. There are other contenders, which your chosen search engine will show you, but I have no experience of them.

Yes, the learning curve is steep to begin with. There are many documents and videos available on the web to explain the features of Fusion 360, but like all such packages, learning can only be done by doing. It took me three months of winter evenings, when the workshop was not so attractive, to become reasonably confident with it and use it to generate the sort of models illustrated here. Your learning time could, of course, be more or less than mine.

Figure 7. The actual model. The way the splasher merges into the boiler is not true to prototype, but I decided that it was the lesser of the two evils.