There are a lot of things to consider when making a scale model.
When people see the computer build of the Holliday I did on this site, most people would think that they should be able to just send it to a 3D printer or a CNC machine and have their own scaled copy of the famous engine to sit on their desk.
But there is a lot more work than that–a lot of issues to consider. The most governing issue is the size. Larger model will accommodate more details, but is expensive and inconvenient for most people. A model too small won’t have many of the details that are unique to the engine. So there’s a balance somewhere.
What about the materials? People think metal like aluminum, steel, and cast iron are authentic and durable, but they have terrible details and tolerance, and are hard to modify.
I also have to redesign the parts to make them suitable for small scale. This is because very small details like bolt heads or cotter pins can not be reproduced by machines. So, I have to take the full scale parts and delete or simplify the details (of course, I’m working with a different copy of the part!).
And the material? Plastic gets my vote. It’s the easiest to work with and modify and add details. They’re also lighter in weight which saves on shipping.
And which parts are important to the modeler? With a copy of the engine plan book, one could build a model with every single detail that is found on the real engine. But, when scaled down, some of those parts are impractical. And scratch building it would be tough. How would you go about making the drivers, boiler, saddle, etc.?
But, there can be a CK Holliday semi-kit, and modelers wouldn’t have to worry about any of the above. And maybe it will look like this:
Note that the parts like footplate, side sheets, running boards, and deck are very flat pieces that I think the modeler can fabricate himself (just cut them from a styrene sheet, or even thin metal sheet). The parts will be made of polyamide (PA2200) plastic–strong but flexible enough to work with (they’ll take some abuse).
The intention also is to allow the modeler to build his kit like the real engine, which means the saddle you see above is just that–the saddle. The modeler will have to furnish his own finishing, such as steam chest cover, on top of that, which I think is great so anyone can customize the finishing to match his favorite 4-4-0. Also, the modeler will understand how the engine is really put together–he’ll have to assemble and key the drivers to the axles, and place the axle boxes in their jaws, etc. Not much is “pre-done”, except the boiler is “pre-jacketed” which one can paint–or even better, wrap it with thin metal sheet–just follow the contours!
(Speaking of the jacket, you can see how it has cutouts for the domes and the running board brackets already!)
There are still a few more parts I want to add to the above (crossheads and rods comes to mind). Then, I will experiment with some laser cutting wood to provide for the cab.
The above picture is just a computer preview. I’ve sent a few parts to be fabricated so we’ll see how they turn out. Standby for updates and more details on the semi-kit!
UPDATE: New rendering of the prototype below. The 1:20 semi-kit will contain just about what you see below, except for the cranks and the crosshead guides, which anyone can make (they’re included in these pictures just for purpose of completing the assembly).
The cylinders and domes covers can be painted, or better yet, wrapped in gold foil to look like real brass.
Note the backhead has cutouts for the throttle plate, washout plug, and some of the plumbing.