I decided to put here a few words about making of a 1:26 scale diesel engine modell with a 3D printer and things can be found at (almost) every home.
This won't be the one and only way to build a locomotive modell, I try to be short and accurate. I suppose the reader can use a 3D printer and a basic CAD software: the aim here is not to teach how to use those, I will try to focus on the things you need to know how to build a working, usable modell to your modell layout.
Before starting, a few words as a tip for your health:
1. Train yards are dangerous, no-go zones. Do not enter to any of the yards just to make a closer photo of a detail unless you are not allowed. Watch your step and be extremely carefull, follow the instructions of the workers, etc. Do not attempt to climb to any train here. Be aware of the catenary wires, allways keep the safety distance, do not try to go near to any working locomotives.
2. Wear safety mask when sanding your modell. Even PLA is named a safe material, fine dust is extremely dangerous. If possible, use an eye protector also.
3. The same safety wear must be used when using pants, aerosols or other material painting your modell. Spray paints are extremely flammable, do not use them in a small, non-ventiled room, near fire or hot surfaces.
4. Electricity is not your friend. Even toy trains using small voltage, be careful connecting the wires. Use insulating tape, solder the connections, avoid short circuits. When connecting your DCC panel follow the instructions provided.
Step 1: Design of the Locomotive
For this instructable I choose to show how the modell of the Hungarian State Railways class M41 (later class 418) engine was made. The original is a diesel-hydraulic driven unit, used widely in Hungary. It was designed in the 70's, has a simple shape and an easy to do paint scheme.
I choose to buy the driving units for the locomotive because I don't want to spend all the time making cogwheels and pickup shoes. Driving units are available in factory new ready made, reliable and cheap. Here I used the PIKO driving units with two axles one motor and pickup shoes, originally made for their G scale BR218 modell. The measurements of the axles, wheels, wheelbase are almost identical to the M41 engine, fits just perfect.
Before drawing, I made a lot of photos of the original engine. Make as many photos as you can from every detail, they will be your guide through drawing. Acqiure original blueprints of the loco, if possible, and make notes while you taking the photos of the measurements of the main parts. The more data you have the more detail you can draw.
The center, the base of my drawing was the two driving units. I took the measurements of them and made a sketch of them, then I drawn the chassis around them. I divided the chassis onto 4 major parts: roof, main frame, body, fuel tank (later the body and the roof glued together).
The main frame is a 4mm thick printed sheet with the necessary holes for the motor blocks. The 4mm is thick enough to hold the driving units. The cut-out for the driving units are drawn to a special shape - the driving units has to be capable of tight corners on the track.
The next step was the body, I started with the driver's cab. Those are identical on both ends, so I just had to mirror it to the other end of the locomotive. I printed each driver's cab at once without the roof.
In the middle of the drawing, a small side note. There are size limits of each printer. You must know your printer's limits, as I know mine, so I made the design keeping in mind I have to print the loco divided into smaller parts and those parts must be connected - glued - screwed to each other, resulting in a strong body at the end.
Between the driver cabs there is a machine room. I cut it into two parts because this way it fits into the printer. I made the side vent panel also in two smaller parts. I designed walls into the machine room making it stronger. Those walls has their cut-outs in the main frame also.
The roof was printed separately, divided into three parts. I used glue to fix it onto the body. I tried to add as many small details to it, like the vent grilles or the hook-ups.
Under the frame there is a fuel tank and battery box, I used that part to carry the speaker of the engine (it is a Visaton FRS-7 speaker): The main frame and the chassis is put together with 6 screws.
The bogie frames were designed around the motor block. Here you must keep in your mind that bogies has to be able to take the curves. Do some testing while driving, rotate your objects into the maximum radius curves which you'll be using, to see all parts will fit there.
Step 2: Printing, Assembly and Paint
Printing the parts is time consuming process. My way is to print one part at a time, while printing the next part you can do the after works of each part. You can print the parts in groups: bogies, main frame, chassis, accessories.
Use as small portion of glue as possible: draw holes and other kind of parts which can go together and fix your part, this will make the whole chassis more durable.
After printing you must sand down the surface to get rid of the thin lines. Usually I sand down the raw part, then apply a very thin layer of filler or base paint then sand it down with water again. The next layer can be a thin layer of paint, then sand down the problematic places again. I use 1k acrilic paint in RAL colours to keep it as close to original look as I can.
After painting I apply the stickers (water transform thin ones to the glossy surface) and fix them with one layer of matte lacquer.
Step 3: Finishing Your Modell
When all the parts ready and painted, the best part is the final assembly. In this phase I build in the electronics (DCC controller), speaker, LED's and connect the wiring to the driving units.
When all ready I make a trial run in a stand, and then I put the loco to the tracks. You might have to fine-tune the DCC settings: volume, LED brightness, min and max speed, acceleration, etc.
I made three units of the M41 diesel locomotives, with a few small differences. Of course, when I'm building the last chassis I know all the problematic points of the design and tried to make it better with small modifications - so for me the last one is the best one.
The last four pictures in this section is taken from that last engine with one of the original engines in a maintenace shop near to me.
No locomotives were harmed in the making of this "tutorial" :)