Introduction

 

The last few months have been busy ones for my household, with little time to think about model-making. The spells of hot weather have not been conducive to spending time in my workroom either and, on top of all those things, I have had another failure of the feeder mechanism in my 'Geeetech' printer.

 

Broken Filament Feeder

 

I think the root cause of the problem was a faulty temperature sensor in the print head, which caused the feed tube to jam and over-stressed the feeder mechanism. It has happened before and I obtained a replacement from China but that is a process which takes a few weeks.

 

In the meantime, I have made a temporary repair with the help of superglue and some reinforcing strips across the crack. I suspect, however, that my printer is beginning to feel its age, so I have started to thing about a replacement. I shall stay with an FDP machine, since I appreciate its simplicity and the absence of toxic and somewhat messy materials. I realise that resin machines provide a superior finish but, for my purposes, the extruded filament method is adequate in realising my modelling ambitions.

 

Re-Visiting 6-Wheel Carriages

 

I read quite recently that Didcot Railway Centre has taken delivery of  GWR Coach No. 820 on 7th May 2022.. This coach had previously been held at the NRM's depot in Shildon. More information and photos at: https://didcotrailwaycentre.org.uk/article.php/509/news

 

It’s of particular interest to me, since one of my first scratch-built models was a coach of this type, built to Diagram U29. My model goes back to the end of the first year of this blog (Dec.2013) when I acquired a Silhouette Cutter and started looking around for something to model.

 

In January 2014, I wrote “The first step was to select a suitable prototype….. I made a shortlist of potential subjects - all 6-wheel composite coaches - and chose diagram U29 as a particularly attractive subject.” I set about creating drawings of the coach side for my Silhouette Cutter and soon came to the conclusion that “Diagram U29 is somewhat 'the design from hell', since every panel seems to be unique! It certainly gave me plenty of practice in using the drawing tools.”

 

Eventually, I put together a passable model with laminated, printed sides on a folded brass underframe. The clerestory roof posed several construction problems and I was never wholly satisfied with my attempts at folding brass sheet into an appropriate profile. For some reason, I do not seem to have shown a photo of the finished model at the time but pressed on with other coaches to form a complete 19th century train. I have found, however, a photo of my model in service at North Leigh:

 

my model of Diagram U29 with ink-jet printed sides

 

A few years later, in August 2019, while I was exploring the possibilities of 3D-printing, I revisited some of my earlier models and created new 3D-printed bodies for them. My main aim was to create a better clerestory roof, since my folded brass construction could never be persuaded to fit very well.  At that time, I had gained little experience with the 3D modelling software and made rather heavy weather of the design. I also used some rather laborious techniques in printing the components of the model. In fact, my attention was being distracted by the call of Broad Gauge modelling and I never completed my re-design of the 6-wheel standard gauge carriages, although I did create a model of the Family Saloon, diagram G13, with sides printed on ink-jet printable vinyl, applied to the 3D printed body.
 

Another Re-Build

 

I decided to return again to Diagram U29 after reading about he arrival of one of the prototypes at Didcot, using some of the additional ‘lessons learned’ in my exploration of 3D printing, mainly with Broad Gauge models.

 

The first improvement was to add the oil lamp tops to the roof of my model, since I now know how to do this by creating cylindrical bodies with the ‘Revolve’ tool in 'Fusion 360'. I then added the ventilation holes around the sides of the lamp housing by using a combination of the ‘Hole’ tool and the ‘Pattern on Path’ tool, to create an array of openings around the sides of the housings. I then ‘sank’ the housings into the roof and used the ‘Combine’ tool to create a single body comprising both the roof and array of lamps.

 

The second modification was to remove the raised mouldings from the sides of the carriage by using the ‘Split’ tool. I kept the raised mouldings as separate bodies and left the carriage sides flat, so that I could apply 3D-printed vinyl ‘decoration’ to represent the livery and lining. The revised model in 'Fusion 360' is shown below:

 

Re-design of U29 Tri-composite carriage

When it came to printing, I decided not to use the option within the ‘Cura’ slicing software to create support structures inside the clerestory. After my first print in 2019, I found that the infill was tedious to remove. Now that I have a much better feel for the overhangs that my printer can accommodate, I simply incorporated compartment partitions within the clerestory and then added some extra partitions between the windows, where there was a long gap. The arrangement I used is shown below. The compartment partitions can remain but the others can easily be removed.

 

Underside of Clerestory Roof as printed.

 

Having become used to very short print times with my early Broad Gauge models, it seemed unusual to experience a printing time of over 2 hours for this roof. This model had to placed diagonally on the printer bed, to fit within the 130mm x 130mm size limit. I was relieved to find that my temporary repair to the feed mechanism held together throughout this time!

 

The Oil Lamp Tops also printed well, although the ultra-close view unsurprisingly shows the ‘banding’. This is not noticeable in practice and the plinths hold the lamps vertically, above the sloping roof.

 

3D Printed Lamp Top integral with Roof

 

Ink-Jet Printed Sides

 

I used my 'Silhouette Portrait' Cutter to prepare the printed vinyl sides, using the methods I described back in 2014. The only difference is that I now print onto self-adhesive vinyl rather than photo paper.

 

I hadn’t used my cutter for a considerable time but it started up immediately. The original cutting mat was badly worn and I found that I could buy a pack of three replacement mats for less than the price of a single 'Silhouette'-branded mat. Described as ‘standard hold’, he new mat had quite a strong hold but the cut parts peeled off cleanly.

 

The detail achieved by my 'HP6980' ink-jet printer is remarkably clear with clear text at the small (4mm) scale of my models, as shown below:

 

Ink-jet Printed Carriage Side - detail

 

The new 3D printed body with its clerestory roof is shown below. This body will fit onto the same 6-wheel ‘Cleminson’ chassis that I used for my original ‘fabricated’ model. I painted the roof with a fairly thick mix of zinc white acrylic, which has filled the bands between the print layers successfully. In earlier models I have used a white vinyl covering but this was not necessary here.

 

My 3D-printed Carriage with ink-jet printed Vinyl Sides

 

The overall structure is far more robust than my original fabricated model and the roof, in particular, is a far better fit to the rest of the model. The interior detail includes partitions and seats, which all contribute to the overall rigidity of the structure. I feel that this model could be handled by my young grand-children with a good chance of surviving intact. That would not be true of its predecessor!

 

Mike