Gauge 1 Diesel Multiple Unit

This page last updated: 11 December 2018.

Having begun the construction of my front garden railway on a postage stamp (intended to reflect the Rhymney Valley line in south Wales from around 1964) at the start of 2018 I needed to resolve how it carries passengers.  The Rhymney Valley line was converted to diesel multiple units in around 1958 and, with the help of Noel on the WRRC forums, the immensely detailed www.railcar.co.uk, and my memories, I determined that I needed a class 116 DMU, gangwayed (i.e. converted to have a corridor running all the way through) in all blue livery except for yellow-painted ends.  However, no-one makes such a beast so I had to construct one myself.  I began work from these drawings:
Stuart Mackay of www.railcar.co.uk helped me with additional pictures and Chris Moxon of preserved.railcar.co.uk helped me get in contact with Alan Pitt of the Great Central Railway near Nottingham where Dave Watts, the co-owner of a class 116 DMU (one of only two that remain) very kindly allowed my to take detailed pictures and measurements.

For the modelling part, I joined the Gauge 1 forums to ask for advice.  David Halfpenny pointed me towards David Leech, a Canadian modeller who has a retirement business scratch-building coaches and has attempted to build a class 121 DMU using 3D printing.  David Leech has given me lots of great advice on the use of 3D printing, the more traditional bent aluminium sheet method and many, many other aspects of Gauge 1 coach building.  I also purchased a copy of "Carriage Modelling Made Easy" by David Jenkinson which describes another method for making coaches.  I decided to 3D print the ends and initially I thought I would make the body of folded aluminium sheet but in the end it turned out that 3D printing would work for the entire body.  Drive will be provided by a couple of Fosmotors on one bogie, the power and control for which David Leech believes can be hidden underneath the DMU.  Here's what I think I have to do:
Having never done this before lord knows how it is going go turn out.  But I have to start somewhere.

Drawing

The dimensions of the vehicle are based on the drawings from www.railcar.co.uk enhanced by a visit to the Great Central Railway for a detailed photographing/measuring session.

Motor
                Open Second Body
Cabin dimensions
Motor Open Second Brake body
Motor chassis
Trailer
                Composite body

Trailer chassis


3D Printing: The Cabin

I spent a month or so designing the 3D printed cabin. It took some considerable time to get my workflow correct in Blender, the de-facto free 3D design package (primarily intended for animation); it is arcane and complex but there is a lot of community help out there which makes it ultimately useable. Since Blender is intended for animation it is not so good at maintaining a "manifold" object, i.e. one which 3D printing software can understand and print; a real closed shape, without one or two-dimensional protuberances, which can very easily appear when creating complex meshes.  Anyway, here is the finished article and the final print from my Prusa 3D printer, along with many of the test prints, all printed in PLA, the default material for 3D printing work (which is long-term biodegradable).

Cabin in Blender
Cabin printed

The "optimal" resolution (balancing print time and quality) of my printer is only 0.15 mm and you can see this very clearly in the rivets, probably the part of the model which took longest to get right.

Rivets in Blender
Rivets printed

I tried increasing the resolution: here is the riveted area, photographed under a microscope, at 0.15 mm, 0.10 mm and 0.05 mm resolution, print times increasing from 3 hours to 12 hours:

Rivets at 0.15 mm
                resolution
Rivets at 0.10 mm
                resolution Rivets at 0.05 mm resolution

0.10 mm layer height seems a sensible compromise.

While showing this off at work one lunchtime, I was asked why I don't 3D print the rest of the body. Given what I've learned about 3D printing, and what I don't know about folding aluminium sheet, it is worth considering. The entire body of one coach works out to be about 600 mm long while my printer maximum vertical dimension (the body would need to be printed vertically to avoid a large support structure) is about 200 mm, and I'd want to stay well below that as shapes can become unstable and print a bit "raggedy" as they get higher. So it would need to be split up into 4 or 5 sections. This is quite possible since the coach side is split vertically at each door section (see below).  I could use a guttering strip around the top and tape of some form on the roof to hide the rest of the joint.  And the bodies have repeating sections which would reduce the amount of work required.

Door view Body
                  permutations

I had a quick go at a test print of the "A" section above and that worked out pretty well. Before I do this for real, though, I need to work on the chassis so that I can mould-in the connection between the two.

Body test 3D print

Of course, I also need to start 3D printing in my target material, ASA (a UV-safe version of ABS).  I had originally expected to get the final parts printed on a more expensive, higher resolution, 3D printer, so I sent the front section to be printed on a few other 3D printers (all in ASA):
Professionally
                  printed
Dremmel print
Printed on
                  my printer (in ASA) for comparison
Professionally
                  printed, detail Dremmel print,
                  detail
My printer
                  for comparison, detail

The Fortus 450MC prints are definitely much higher quality, the individual steps being more finely registered.  The Dremel prints have a higher definition around the window area but are otherwise pretty similar to those from my Prusa printer.  But how much all of this matters will depend a lot on how I finish the prints.  The suggested way to remove the "ribbing" of the 3D print is to use sandpaper, which would be really laborious if I'm going to make loads of 3D printed parts, so I thought I'd try my hand at acetone vapour smoothing.  Placing an ASA (or ABS) 3D print in a sealed box containing kitchen-towel soaked in acetone for a few hours effectively melts the surface of the print.  However, you lose definition along the way.  In a test, I found that if I left the print in the acetone long enough to smooth the ribbing (between 3 and 4 hours) then I also lost the detail around the windows and, even if I might prevent that with some form of acetone-resistant coating, the result still wasn't really good enough, too indiscriminate for such a small and detailed print:

Before
                  acetone
After acetone Before
                  acetone, rivet detail After acetone,
                  rivet detail

So, looks like it's loads of sanding for me.  Here's a test piece after 15 minutes of sanding with sanding sticks, one coat of primer, another 15 minutes of sanding and another coat of primer.  Good enough; the secret is to be bold on the coarse grade (400 grit).  It has somewhat lost the rivets, and in the paint rather than the sanding:

Sanded finish

Having satisfied myself that this was the right approach, I started adding detail.  First, I added a slot to the Blender model of the cabin into which I could insert windows (cut from the 0.5 mm plastic sheet used for dolls house windows); these windows aren't going to be pushed out by mistake when handling the model.

Window slot
Window slot

Then I added a dashboard...

Dashboard Dashboard

...and some ridges to the cabin and cabin roof so that they would mate together somewhat.

Ridges in cabin
Ridges in cabin
                      roof
Ridges in action
Assembled cabin


Bogies

I purchased from Tenmille an AG140W bogie kit and from Fosworks a pair of MOT110 nose-hung Fosmotors (6 V, so that I can treat it like a standard radio control model set up, similar to that of my autogyro) on 34 mm diameter wheels, plus a pair of plain-old 34 mm diameter wheels.  Assembling the bogie with the undriven wheels was fairly straightforward.  I filed off the "SR" emblem from the axle covers of the casting and left out the optional step across the middle (which is only required for brake vans).

Undriven bogie
                      assembled
The original

To fit the Fosmotors in the other bogie I drilled a 4 mm diameter hole in the side of the centre piece of the bogie, roughly as shown below, in order to hold the end of the Fosmotor suspension bracket.  I also had to file a slot in the edge to get the Fosmotors to fit; it's all a bit snug.  I powered the pair of Fosmotors from a bench power supply and made sure that they were both running freely, especially at low speeds, with all of the assembly screws as tight as I was going to make them.

Hole and slot in side of
                bogie centre piece
Fosmotors fitted
Fosmotors fitted, top

I made the steps up to the cab at the front of the powered bogie by taking a nice sharp tin snips to 1 mm aluminium plate.  I cut a piece 20 mm by 50 mm and then cut/formed it as below, glueing it into on the bogie with CA like all the other decorative parts.

First fold
Cut and fold more, front
                  view Cut and fold more, rear
                  view
Glued into position


Chassis

I decided to CNC cut the base of the chassis on my High-Z/S-400T CNC milling machine from cnc-step.de.  I began by making a few drawings, including castellations that represent the wooden steps up to the doors.

Motor
                  open second base
Motor open second brake base Trailer composite base

Note: the bogie attachment holes are 6 mm as that allows me to use the same holes to bolt the plate to the milling machine while it is being cut (and in fact I had to make an additional 6 mm hole in the centre of the plate for that purpose as well).  Here's a preview of the motor open second base, drawn in VCarve, the software which happens to come with my milling machine, and the resulting work.

Base
                    plate preview Milling in
                  progress
The milled
                  result

I cut the first version in SWG 10 brass plate (3.2 mm thick, which just about scale-matches the 110 mm depth of the upper part of the visible chassis frame), thinking that I could then braze a length of brass bar to each end to which I could attach buffers etc.  However, this was way too heavy (over 1 kg) so I re-cut it in 3 mm thick aluminium plate (weighing less than 400 gm) and will attach a bar to the end by some other means.  The VCarve file for the motor open second base can be found here; I had to cut it in two "tiles" as my machine could only cut 400 mm lengths and, in the final "cut-out" cut with the 2 mm milling bit, I manually reduced the feed rate to 25% as, no matter what I did, I couldn't persuade VCarve to set my desired glacial feed rate of 0.5 mm/second in order to avoid damage to the tiny bit.  The spindle was run at 1400 RPM and I used 2 mm and 4 mm Only One PM60 end-mills from Cutwel.  Don't forget to put a sacrificial piece of thin MDF or hardboard underneath the plate as it needs to cut through.

I drilled 6 mm holes in the centre of the bogies and mounted them on the chassis using 40 mm long M6 cheese-head bolts.  I used two nuts (each 5 mm high) at the top to achieve the correct spacing, then a washer, then the bogie itself, followed by a nice large washer and two more nuts to act as lock-nuts.

NutsAssembled

I don't have a speed controller yet so I just wired the motors directly to a 7.2 V battery pack I had lying around and made a circle of Peco G45 track to see how it would go.  It certainly went; click on the link below to see how and refresh this page if no YouTube video image appears below this text, sometimes it doesn't load on the first attempt.

 

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