Several years ago when I first got into 3D printing my own trains, I designed my very first O Scale freight car-- this 31-foot McKeen mail trailer (the sort that the Southern Pacific had in service in the 1910s). It was a bit crude but an excellent learning experience. I've been contemplating redesigning this kit since last year, and with the positive response from some of this Forum's McKeen enthusiasts, I have finally gotten around to working on it.
Rather than trying to untangle the Gordian knot of the original CAD drawings, I spent some time making them up fresh and new. To optimize for 3D printing with UV-cured resin, I designed the body in two sections that can be glued together.
I designed a new FDM printed frame to improve the way that the sliding doors fit the model. The new model has the same sort of ABS filament doors as the previous model, but now they ride in nice little tracks instead of being pinched in place with wires.
I have a large stock of these diecast tender trucks that I acquired for another project. They're just the right style for McKeen trailers. I designed a new coupler to fit them after measuring out the necessary shank length and came up with an assortment of suitable coupler and truck screws from my parts bins.
Here's another look at what the completed trucks look like. There's screw holes and mounting features for third rail pickups on these trucks, although I don't have any pickups left to use on them.
Here's what the two body sections look like in the raw resin.
Self-threading body screws are great for soft ABS plastic, but UV-cured resin is too hard for them to work right. I tap resin parts for machine screws instead, in this case #6-32.
After assembling the trucks, body, doors and frame, the prototype was ready for track testing. I was very pleased with the height match against my 72ft McKeen motor car.
Here's a comparison of the old mail trailer to the new one. The old design had printed-in roof details that were never very satisfactory. The new design has mounting holes printed into the roof which allow for separately applied details and also improve print quality (small holes in large parts provide air escapes during the resin print process).
Here's the new mail trailer on the track. My McKeen looks pretty neat with two trailers in tow, I'll have to see if I have any of that Colonial Red paint left so I can build up the prototype for my personal collection.
Incorporating a few minor improvements that I discerned during the prototype build, I just finished printing the first complete "Mark 2" McKeen mail trailer kit this morning. This photo shows what the kits come with. This particular kit and the prototype are both spoken for but I'm going to make another one or two over the course of this week. As always, send me an email if interested.
Thanks for reading along. If you're in the Northeast, don't melt in this week's heat wave!
