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© February 4, 2004 

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Designing the Rio Grande Southern #20 (part 1)


Full scale Rio Grande Southern #20 narrow gauge 4-6-0. Tom Artzberger photo.

by Tom Artzberger, Pike Lake and Eastern Hartford Shops

 

After building several 1 ½ scale steam locomotives I decided to take the plunge and try a narrow gauge locomotive in 2 ½ scale. The purpose of this article is to explain how I went about designing the locomotive and to talk about some of the tools that I used to accomplish it. Later Part 2 will cover the actual construction of the locomotive and the manufacturing methods utilized.

First a little history. The RGS20 project started out as my desired to build one locomotive for myself, however things quickly got out of hand as others found out about the project and one quickly grew into a production run of 18 locomotives. Why the 20? When I decided to do a narrow gauge locomotive I looked around at what was available and what others have done in the past. I wanted something a little unique but that had a lot of information available so I could do a faithful reproduction. The 20 filled the bill perfectly. It is a very popular engine, was still in existence at the Colorado RR Museum and a lot of historical data was available. The 20 was it.

The first thing I do when starting a project is to gather all of the information that I can find on the subject. In the case of the 20 a trip to Colorado with the camera and tape measure started to fill in a number of question marks. I did a search of various magazines and books and found some additional details. It was then networking time. A couple contacts on the Internet lead to a couple of others and presto I found a copy of the original erection card and frame drawings.

Next I consider good design practice. What has worked in the past, what did the prototype use. If cast iron was used for a part for a hundred years on locomotives is there a good reason to change now. A good resource on locomotive design and good design practice is So you want to build a Live Steam Locomotive by Joseph F. Nelson (available from the Discover Live Steam book store).

It was then time to start putting lines on paper or in this case creating the geometry of the parts in the computer. About 25 years ago computers became powerful enough that simple drafting could be done thus ushering in the age of computer aided design or CAD. The first systems were simple drawings systems that the designer to draw lines and arcs etc. to create drawings. The first systems were 2D duplicating the drawings practices of the past. A little later a third dimension was added (depth) allowing one to draw in three dimensions. This still did not totally define the model and the next stage in CAD development was the surface modeler. A surface modeler ties together the boundary lines and arcs to form surfaces adding additional information to the model. Surfaces were necessary to create CNC tool paths. The next step in CAD Development was the solid modeler that adds additional information to tie the surfaces together to obtain a complete description of the model.

The 20 were designed completely in a solid modeling program called Pro/Engineer. In Pro/E one starts by creating the geometry of the individual parts. These parts can then be assembled into subassemblies and subassemblies assembled into assemblies until the complete locomotive has been constructed. A by-product of this process is conventual’s 2D drawings and tool paths for driving CNC machine tools.
Tom Artzberger and the Pike Lake and Eastern Hartford Shops are producing this locomotive for sale. Rough castings, fully machined parts, drawings, and more are available for the RGS20.  Contact Tom Artzberger and the Pike Lake and Eastern Hartford Shops for more information.
One can also do interference checking to check the operation of the various parts and to verify hole alignment. On the RGS20 project I started by modeling the frame, creating the frame side plates, frame extensions, and binders. Next came the frame cross members and castings. They were then assembled into a frame subassembly, which eventually became part of a chassis subassembly, which eventually became part of the locomotive assembly.


Figure 1-  frame binders
(click image to view full size)
The same procedure was used on all the other parts that make up a locomotive.  Figure 1 shows one of the frame binders. This is a 2D drawing with 2 views shown. Each view was drawn independently and if a change is made both views must be updated and anywhere that part is used has to be updated to show the change. A lot of manual work and easy to forget to change the part on some other drawing.


Figure 2- 3D solid model of the frame plate 
(click image to view full size)
Figure 2 is a 3D solid model of the frame plate. Figure 3 is a drawing of the frame plate showing 3 views. All three views were generated from the solid model by specifying the direction of the view that you wanted. No geometry was redrawn and if a change is made to the model all the views and dimensions are automatically updated. When assemblies are made, as in figure 4, the changes are picked up without any intervention.

Figure 3- drawing of the frame plate showing 3 views
(click image to view full size)

The changes are propagated throughout all of the assemblies right to top level. In CAD slang, change once change everywhere

Once all of the major components were modeled and assembled, finial checking was performed, do holes line up, do surfaces mate as they should, does parts line up? It was then time to start making chips. As additional detail parts are designed they will be added to the model.  Continued

Figure 4- changes are picked up without any intervention
(click image to view full size)

 

by Tom Artzberger, Pike Lake and Eastern Hartford Shops

Images by Tom Artzberger

Part 2 describes the methods used to make parts. 
Read Part 2

 

Rio Grande Southern #20
Narrow gauge 10 wheeler project

The Construction Project Continues

Last call! The second production run of the RGS20 in 2-1/2 scale 7-1/2" gauge will be starting soon.  See the Hartford Shops web page at www.pleshops.com for additional information or to reserve one now.

 

the end

 

 

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