As part of my continued adventures in digital fabrication/scratchbuilding, I’m working on a Seaboard whale belly Pneumatic Hopper. Not that I don’t have my hands full with the G-39/38 ore Jennie project, the Erie Dunmore Cabeeses, and a TTX 85’ piggyback build, plus a bunch of other, simpler, painting/decaling/weathering projects. But this car has always been a favorite of mine. It just looks cool and unique, sort of a cross between a covered hopper and a tank car.
There is a brass O-Scale version of this car, that occasionally shows up on eBay for gobs of $$$, and there was a most excellent article in an older O-Scale magazine, showing how to heavily kitbash a Weaver 50’ modern tank car into one of these. If you’ve been reading my other posts, my use of digital fabrication techniques, involves a lot of front end design and testing work, but once done, it’s easy to built multiple cars.
This build will be a little different than my other projects. It will still involve the use of a laser cutter for many of the parts, but also rely more, on the CNC router, and 3D printer.
The car body is more complicated than it looks. There are five segments to the lower section and a single continuous radius top section. All of the five bottom segments have the same radius and if it was a wood half-round, you would cut each of the pieces on an angle where they joined to get a smooth transition, however, if you look at the prototype, the joints are vertical. This type of metal work, is less pipe fitting, and more duct making. Years ago, in high school mechanical drawing class, I learned how to calculate and draw the patterns for this type of thing. Fortunately for me, since I’ve forgotten how to do that, and since I’m not building the car out of sheet metal, 3D CAD figures it all out for me.
I use a program called Fusion360, by Autodesk, for 3D drawings. It was fairly easy to generate the body of this hopper. Scale drawings were published in Mainline Modeler years ago, and are also in on of their Freight Car Plan complications. As with everything, I scanned the drawing and then printed it out full size O-Scale. I do this as I draw everything in metric (my laser only understands metric as do the 3D printers, my CNC understands metric or standard). Having the full size plans, I just measure what I see with a metric scale. The body of the covered hopper was drawn fully, without the conical ends. I then split the body in two (virtually) and created an .stl file that my CNC software could generate cut instruction for.
The idea was to cut out a form for the body and the ends, that could be used as a pattern to vacuum form body halves out of .040 styrene. These four parts would fit around an acrylic laser cut honeycomb type structural assembly. Initially the patterns were going to be maple, however, I had some HDU foam scraps laying around from a large sign I built, so I cut from that. HDU is high density urethane. It might look a little like our favorite pink foam, but its much denser. The 1.5” foam I used, weighs 20 lbs per square foot. It’s also expensive - over $300 for a 4x8 sheet. It sands and drills great, and cuts nicely on the CNC. The foam is “carved” with a 1/4” ball nose (rounded) bit, that makes passes back and forth at a speed of 450 inches per minute. These parts took about 10 minutes.
As I was sanding the foam, I started thinking about all the hassle of vacuum forming, the internal bracing assembly, and then the inevitable bondo work filling all the seams between the plastic parts. Then there is the worry about the styrene shell deforming over time, especially after stretching the plastic during forming. Why not just glue the four pieces together and call it a day. Once primed and painted, I THINK, the foam will be durable enough for handling and running these cars. The foam is not affected by solvent based paints, so multiple coats of a grey auto-body primer, with sanding between, created a nice hard shell. It’s solid feeling, but not overly heavy, that it would have a high center of gravity around curves.
The photo shows the laser cut roof walkway pieces, which is as far as I am right now. Updates will follow, interspersed with G-39/38 and Dunmore Caboose updates.
