Narrow Gauge Road Engine – 6 axle – part 1

I was talking to my friend Andrew Gillette one day about the possibility of using my Prusa i3 MK3 3D printer to create modern style On30 Road Engines. This isn’t anything new – On30 itself developed from using HO mechanisms with O scale shells to create narrow gauge ‘on the cheap’.

We had found examples of EMD, GE and ALCO built locomotives for the South American narrow gauge market and thought we could come up with something similar. To this end Andrew supplied me with an Athearn (HO) SD40-2.

Narrow Gauge Road Engine – 6 axle – part 2

The first step was to replicate the inside to the SD40-2 shell to create a ‘glove’ that would fit over the mechanism just as the factory shell does. The narrow gauge shell then would be built over this.

Narrow Gauge Road Engine – 6 axle – part 3

I was happily working on the CAD for the shell that would fit over the mechanism – but at some point realized I needed to create the mechanism itself in CAD – to get at least an idea of where to ‘slice and dice’.

I downloaded a SD40-2 model from 3D Warehouse just for the trucks and gas tank. Everything else I created from measurements of the Athearn mechanism. I didn’t try to get the drawing exact .. just wanted the shapes ‘good nuff’.


OK. The stack of parts are – according to Andrew Gillette – Red Caboose GP-9 detail parts. Me .. I readily admit that I hadn’t a clue to that. I am often clueless .. I tend to wander around until I see something shiny at which point ..

The flat louvered parts are the battey/storage boxes under the cab. The idea is to glue these to the side of the print. In the photo we can also see some of the fans which will go on top of the hood.

Minimum Shell – Early in design

I added extensions to the shell for the coupler pockets. This matches the pocket location on the original Athearn shell – unless I were to modify the couplers this would be a ‘fixed’ location. The couplers slide into the slots at either end of the shell and a screw goes through the couplers up into the HO mechanism. This means the face around the coupler slot is ‘fixed’ in place. When I say “modify the couplers” I am talking about a coupler say with an extended shank which would allow that face to move outwards along with the extended coupler.

The surface in gold(ish) is the inside of the HO SD40-2 shell which I replicated as best as I could. This includes the coupler pocket location and all of the interior recesses.

I then made walls 1/8″ thick. This then was the ‘starting point’ from which I plan to build off of. This .. “starting point” simply meaning that any visible On30 shell would sit on top of this minimum shell.

Some Sketchuping

This is my working model(s) around the SD40-2 mechanism. The cab shown is an Atlas O scale U23B.

The idea is for a modern narrow gauge road engine. You can do a bit of research online and find various examples – many in South America of narrow gauge lines with modern (relatively) looking engines. In this specific case the Atlas O scale U23B cab sets the width of the locomotive – so you are working out from the minimum shell described above out to the U23B cab. The fun part being designing all the ‘bits’ in between.

Step Test Print

This is a test print for the steps that will go at each corner of the locomotive. I printed the test step on it’s back. This puts the supports on the backside .. hopefully meaning that we end up with a good print (good meaning ‘good nuff’) for the finished model. I printed this on my Prusa MK3 I3 in PLA using a .4mm nozzle and .15mm layers.

This photo shows the print still sitting on the PEI sheet. Yeah .. it is way .. way too tall but I was more interested in seeing just how well I could get it to print – the size (number of steps) can be modified.

In reality bits like this should be printed on a resin printer for the best resolution but the FDM printer works great for creating parts for checking fit – in which these ‘draft’ prints are just fine.

Cleaned up (some)

The supports pealed off mostly – cleaned up just a bit with blade and file. Some sandpaper would be good .. primer and sanding even better.

Shell Test #1

At this point I worked the model up to a point I could do a test print. Before I go any further I need to insure that what I have will fit over the mechanism. I will print this and then go from there after seeing what needs to be changed. The body itself is nothing more than a shell.

Test Print #1

I printed this last night and .. it works. The print fits nicely over the mechanism. I haven’t checked the fit of the couplers yet (need more coffee) but that is a minor issue at best.

The small ‘cab clips’ would snap into slots on the Atlas O scale U23B chassis. I have doubts that I can make such slots on this print – probably will just slice them off.

The ‘mechanism frame’ is a bit of the mechanism that will be covered by extending the bottom of the print out to create the frame of the locomotive.


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