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© October 25, 2006 

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Designing the Rio Grande Southern #20 (part 2)

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

by Tom Artzberger, Pike Lake and Eastern Hartford Shops


Last time (read part 1 of this series) I described the processes used to design the Rio Grande Southern #20.  It is now time to start building. I had a few goals in mind when I started the project.  I wanted the model to be true to the prototype as much as is reasonable and yet be relatively easy to build and affordable.  These goals was kept in mind throughout the design phase and carried thru in building the locomotive.

Most of the parts were machined on my CNC vertical machining center (KITAMURA Mycenter 1) and CNC Lathe (Graziano SAG12) with simple jobs handled by the Bridgeport and drill press.  The machining center has a rotary forth axles that allow parts to be machined on multiple surfaces with one setup.  For example all the smoke box holes were drilled in one setup (200 holes) and the cylinder block will be totally machined in two.

Fixtures were made when required to hold or drill parts so that accuracy could be maintained.  Some fixtures were relatively simple while others complex. Fabricated metal parts were sawed and bent on the press brake while sheet metal parts were punched on the CNC turret punch and then bent and welded as required.  All parts were made from the geometry files created in the Cad system (Pro/Engineer) along with the NC tool paths, which were down loaded to the machine controls.  All programs and fixtures were saved for future use.

The author and the CNC vertical machining center KITAMURA Mycenter 1

CNC lathe Graziano SAG12
Patterns were made of aluminum or wood.  Most were mounted on match plates although some of the larger ones were loose. Most of the castings were made in gray or ductile iron although a few were done in aluminum. I started out by making the parts for the frame.  I considered several methods of making the frame, one method is to build it up out of bar stock or a second cut it out of steel plate or bar. 

I rejected the first because it would be difficult to make it like the prototype and decided to cut it out of ¾ steel plate.  There is several ways to do this and I have used them all.  The popular processes include flame cutting (including plasma and laser) then machining or to machine it out of the solid.

 I decided to use a process called water jet cutting. In water jet cutting an abrasive is mixed into a very high pressure water jet (50,000 psi) and the jet is slowly moved along the path to be cut.  The abrasive cuts its way through the material leaving a very fine accurate cut. Tolerances of .002 or .003 can be maintained.  In fact the cuts are so accurate that no milling was required on the frame rails. The cost is higher than flame cutting but the time saved in milling all of the surfaces more than makes up for the additional cost.

Because of the design being available on the computer all I had to do was email the frame geometry to my vendor and he cut the frame right from that geometry.  The only machining required was drilling holes. They were done on my CNC vertical machining center. Total machine time for a ready to assemble frame rail was about 15 minutes.  Figure 1 shows the frame rails ready to assemble.


Holes being drilled in the frame using the KITAMURA Mycenter 1

The drivers were cast in ductile iron and turned on the CNC lathe.  When I make wheels I start by facing the backside and boring the center hole to size.  I then turn the wheel around and face the front side. Finally the wheel is placed on a mandrel and the outside profiled.  Once the drivers were turned they were placed on a pin on the machining center table and the crankpin hole drilled and reamed in the correct position. A broaching fixture was made that located the keyway in the main bore relative to the crankpin. 

The axles were next.  They were turned on the CNC lathe and then a fixture was used to mill the axle and eccentric keyways.  How I usually do this is take a piece of square stock and bore a hole thru to accept the axle diameter. A couple setscrews hold the axle in position.  The keyways are milled on one end then the fixture is rotated 90 degrees and the keyways milled on the other end.  I decided to key the eccentrics to the axle as the prototype does.  This eliminates the chances of an eccentric slipping and aids in setting up the gear correctly. Ball bearings were used on all axles including the front truck
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.
Next up was the front truck.  Again the frame was water jet cut as was the swing links and equalizing bars. Casting were used for the center pivot journal boxes and wheels. Machining was straightforward.

Main springs and equalization was the next components tackled.  Rockers were water jet cut, hangers were bent up out of 3/16 x ¾ steel.  The spring leaves were cut and punched from .050 x ¾  1095 hardened spring steel.   The locomotive was sprung like the prototype and is fully equalized.

Front pilot truck, frame, spring rigging, drivers are being assembled.

Boilers are being built by Ridge Locomotive Works (Marty Knox) and are a true wagon top design with a steel shell and copper tubes.  Calculations show them to be a well-proportioned design and should be good steamers with a very large steam space.

As the locomotive was proceeding work began on the tender.  The tender frame was fabricated out of channel and bar stock and bolted together just like the prototype.  A wood deck was added to complete the frame.  The tender trucks were constructed in line with the prototype.  The only exception is that I use ball bearings in place of  the half brasses and wedges that were prototype practice.  The tender tank was supplied by Locoparts and included all of the parts to construct the tank.  The tank is riveted together. Locoparts also supplied the hand pump, valves and other tank components.

Work is now under way on machining the cylinders and valve gear parts with the goal of being able to test fire the locomotive by summer 2004. A future article will report on the first tests of the locomotive and how things worked out.

Frame, drivers, pilot, cab, boiler and smokebox come together.

Cylinder castings.


by Tom Artzberger, Pike Lake and Eastern Hartford Shops

Images by Tom Artzberger



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