I just wanted to share what I've been working on: a series of HO scale drop bottom gondolas that were used by the Denver & Rio Grande Western Railroad from the 1920's to the 1960's. The bodies are one piece printed, with the truckframes printed separately, and the rest of the other parts sourced from traditional HO scale sources. There are three different variations so far, the standard 46ft car, a coke rack added to the 46ft cars in 1942, and the shorter, smaller cousin at 42ft. It's been a great challenge so far, and so many new ideas learned. Hope you enjoy!
Some of the hurdles I crashed into along the way that might help others:
Coplanar Surfaces: I used Sketchup for the drafting, and I used the component feature for the grab-irons on the cars at first, but had these weird seam lines that would extend randomly from the grab irons to the edges of the cars. After reading a forum post about coplanar surfaces, and how that would confuse Preform, I exploded those parts, and made sure the manifold was closed, which solved that problem.
Drilling & Tapping Holes: I am using #2-56 thread screws to attach the truckframes and the couplers to the cars, but its taken a fair bit of experimenting to get it right. The best luck I have had is by drilling out the holes with the proper bit size, and tapping the car while its still "green", before curing in the UV oven, using a bit of diluted IPA as a cutting fluid for the tap. Once the car is cured, the tap has a tendency to crack the resin around the holes. If I do have a crack, or the threads get messed up, I drill out the area with a 1/8th inch bit, and CA glue (IC-2000, or rubber-toughened) a length of 1/8th inch styrene tube into the hole, and thread it.