Preface: I've been going through my fleet over the last year or so and converting everything to Kadees. Many thanks to folks here for lots of good info. I am mostly done with rolling stock now, and starting to look at engines...
I decided my first attempt would be with my trusty old Williams CN GP9. This is one of my oldest and most favorite engines. It is a simple model, and not very detailed, but it always runs and pulls great. It was also my initial test unit for my conversion to DCC a few years ago, so it seemed like a good place to start for converting my engines to Kadees. I do a lot of mechanical design work professionally, so I figured I'd have a go at doing this using some 3d printed parts. I thought others might be interested in the process, as well as just the finished parts, so i'll include some detail on that for starters.
The first step for something like this is to model up the engine chassis, in order to define the interface for the new parts, as well as things like where the coupler needs to end up, how high everything is off the track, keep out envelopes for trucks/wheels, etc. The chassis for this model is a simple stamped steel sheet that forms the base of the engine as well as the front, rear, and side handrails. The CAD model of the chassis looks like this:
The red and blue parallelograms are reference planes that define things like the center, front and rear of the chassis, as well as key features like the kadee mounting height and railhead height.
I also needed a detailed CAD model of the Kadee 740, which is what I'm using as my standard coupler. I contacted Kadee about that, but they understandably did not want to share theirs, so I developed my own. The coupler model articulates, so I can check clearances against my new pilots, from engine to cars, etc. Here is what the coupler assembly looks like:
I also spent a lot of time on railpictures.net, looking at photos like these:
Lots of great ideas on that site! So, after a lot of thought about how I wanted to approach things (and the fact that my CN definitely needed a plow on it!), I fired up my favorite CAD tool. A week or two later, here are the front and rear assemblies:
I wanted them to be fairly realistic, even for this simple model, as I have a number of other engines I am planning to do in the future, so I wanted to make this a real, eh... pilot program, if you will, for future parts. So I added details like the coupler cut lever, different hoses here and there, grating on the steps, rivets, and so on. I wanted to see what I could really get out of the 3d print.
Of course, the whole point of the exercise is to be able to mount Kadee couplers, so the mounting interface for those is a key part of the design. I designed it to nominally require one Micromark Kadee shim, to allow for tolerances, slop in the chassis and trucks, etc. I also added a few locating features to pick up key points of the chassis, so that there is no guesswork involved in locating the parts when bonding them on. Here is a view of the rear pilot from the underside showing the coupler mounting interface. You can see the shim in tan below the coupler:
So with the design basically done, the next step is to submit the files for 3d printing. I am using Shapeways these days, and have been pretty happy with the products so far. They have some good automated checks that get run when you submit your model, so you can get feedback on any issues quickly, and make any needed changes. You can also share the model with others for review.
I selected the Frosted Ultra Detail acrylic material, which has good resolution and physical properties, at a reasonable cost.
So, after waiting (not so) patiently for things to arrive in the mail, here is what the bare parts look like:
The only post-processing required on the parts is tapping the four holes for the kadee mounting screws. The holes themselves are part of the model, so it's just a matter of running a 2mm tap down each of them.
I should mention at this point that it is very important to make sure the chassis of the engine is true, so that everything fits as intended in the end. In the case of my GP9, there were a few issues that needed to be corrected. First, the chassis was not flat, and had a slight twist to it. It is a ductile steel, so that was easily corrected by laying it on a flat surface and tweaking it until it was perfectly flat. Second, one end of the chassis was actually slightly higher than the other with respect to the track. This was due to a small difference in the plates on top of the trucks that hold the side frames on - the features on the plate that set the height of the chassis were about 1/32" lower on one of them. A few shims under the plate on that truck corrected that issue. Finally, there is quite a bit of lateral slop between the chassis and the trucks, which means the couplers can shift a bit from the centerline of the track. It remains to be seen if this will be an issue, but it is also correctable with some shimming inside the engine.
After addressing these issues, here is the initial test fit of the rear pilot:
The initial airbrushing on the front:
Assembly to the trued and freshly repainted chassis:
And finally, the completed project!
Everything fit exactly as intended, so I'm pretty happy with the design, as well as the quality of the printed part. I did come away with a few lessons learned, for future parts. First, the coupler cut levers are very small and delicate - I designed them to be 1" scale diameter, so they are 0.021" on the model. One of the parts was actually damaged in shipment (thanks to Shapeways for replacing it at no cost), and I'm a little worried about how long they will survive normal handling, etc. So I am increasing the diameter of those to 1.5" scale, or 0.031" on the model. Second, having all the hoses designed as integral parts of the pilot made detailing them difficult, so I am going to make them separate parts in the future. I expect painting them individually will be much easier, and will also facilitate weathering and nicer details on the pilots themselves.
That's about it, I think! If anyone is interested in these parts for their own engines, I'd be happy to share. A quick look at the Williams line up of diesels suggests that they may fit a number of other models in addition to the GP9. If there's any interest, I'll figure out how to set things up on Shapeways so that they are available there.
Now on to my MTH SD90, SD45, and a few others...