Wednesday 27 December 2017

Murrumbidgee River Bridge - Part 3. 2nd span, and painting

Modelling can be very tedious at times.  Having finished the overhead beams on the first span, the second span was just a long slog on the workbench.  As I was now confident on my technique, the remaining girders were mass produced, and then fitted to the second truss span over a period of 3 weeks 

The Murrumbidgee River flooded last year - this is a picture taken of one of the billabongs during the flood, Note the gum trees, as this is some of the effect I hope to have around the bridge model.

Girders ready to be installed.  The above represents around 12 hours of workbench time

The instructions of the kit recommended dunking the completed bridge span into paint, and allowing it to drip dry,  Whilst  this had the advantage of speed, I have doubts over this method, as well as having sufficient quantity of paint.  In any case, the brass had to be cleaned of flux, fingerprints, cobwebs, and anything else that shouldn’t be there.

Rinsing under the tap was all I had done so far, but evidence of green verdigris in certain areas, showed that the tap  does not get rid of all the flux.    So the search started for a suitable container where I could scrub with a toothbrush, and use chemicals if needed.    Each of the spans is around 50cm in length.  To cut a long story short, (and no, the house bathtub is not suitable),  I made my own out of timber, and plastic film.  My inspiration is thanks to a display of R/C boats that I saw at the Newcastle model exhibition many years ago.  They had made a large shallow pond  on the floor of the exhibition hall using black plastic and bricks

The timber is scrap I happened to have lying around  

The plastic is double skinned to prevent leaks in case the upper plastic film is damaged.

Scrubbing was with cream cleanser, and toothbrush.  Followed by a rinse.  Then repeat.  Check for any verdigris, and if present, break out the ammonia and salt solution.  Wear gloves.  Rinse again.

After cleaning, the next stage was to attach the rivet shim to the upper, and lower chords of each span.  The shim needs to be carefully cut to size.  Checked for fit, I then glue the shim to the span.  Why glue?  Applying heat in the form of a soldering iron will cause the shim brass to expand, and then contract whilst it  cools.  I am not skilled enough to be able to control this, and any distortions in the rivet overlay will be difficult to correct.

Underside of the span, showing the rivet overlay.  Each of the main uprights are secured to the span with a pair of triangular gusset pieces made from scrap brass.

The glue I am using is 6 minute, 2 part epoxy from Great Planes in the USA.  Mix 2 parts equally, smear it over the bridge girder, and attach the rivet overlay.  Weight it down.  After about 10 minutes, remove the weights, and repeat on the next rivet section

Prior to painting brass, I like to provide a key for the paint with a light vinegar etch   The term used is “pickling”.  The vinegar is a mild acid, and the process gives a microscopic roughness that allows better adhesion of the paint.  My homebuilt container was half filled with fresh vinegar – about 1200ml.    As the bridge span is not covered by the vinegar, I am turning the model every 15 minutes to a new side.  You may notice a slight “pink” tinge on the brass after the pickling process, and this is a good sign.  After each side has been treated,   rinse the span and allow to dry

I am hoping that the vinegar treatment would allow me to skip the “self etch black” primer coat, and go straight to the final paint coats.  The self-etch black is not the easiest of paints to spray, and the bridge is rather a big model  The paint I am using is Floquil “Grimy Black”, thinned with turpentine, , and sprayed with a Badger 200 airbrush.   Pretty nasty stuff, so make sure that things are well ventilated.   Note. I know I should be using Xylene as the floquil thinner, but my experience with that product, besides expense, is that it dries far too flat. The turpentine , if anything, takes a bit longer to dry, and dries with a slight satin sheen.  And the other question is why Floquil?  Well, I have accumulated a supply of this excellent but now discontinued product, and to not use it would be rather sad, particularly as it sprays well onto brass, and gives a nice finish.

The main problem (other than the size of the span) with painting the bridge is the space between the 2 lattice etches on each side.  One had to try and avoid getting a shadow mask effect, so the airbrush angle of attack needs to be varied with each pass over the bridge.  My suggestion is that you do not attempt to complete the full span in one spray sitting – after you think you have painted the lattice, then allow the span to dry, and inspect for imperfections in a bright light.  Once you are happy with the lattice, then paint the remaining beams. 

One span painted.  I am happy with the colour, as it is close to the colour I remember on the real bridge.  The second span is still to get the pickling treatment, and then the paint. Note too, how the painted span only has 3 overhead support trusses. .  This is because the real bridge "shared" a support at the end of each span. .

The second  span should be a bit easier, as I had previously painted the insides of the lattice black (see part 1).  I will have to give the first painted span an additional coat, because, as expected in good light, there some areas where the brass colour  is showing through.  However, the effect of even this initial paint coat  is magic.  It is good to get a few wins on the way

The next part will be the wooden track support, and track.    

Until then, happy modelling

Saturday 9 December 2017

Murrumbidgee River bridge – part 2 – the overhead support beams

I am not sure when the Whitton truss bridge across the Murrumbidgee River at Wagga had its overhead arch girders replaced with steel beams, but it was before 1970.  And I do not know the reason, as the double track Whitton arch bridge at Albury over the Murray River still has its arches

Shortly after being built in 1879, this NSW Archives picture shows the Wagga Wagga bridge, and the timber trestle

In any case, I needed to model the Murrumbidgee bridge as it appeared in 1970, and that meant scratch building the overhead “U” support beams.   The first part of the Murrumbidgee bridge blog post is way back on the 26th June, 2017.  I finished that blog post saying that I was waiting for ABS styrene from China, or a source of brass “C” channel or the correct size.

The bridge as it appeared in my June 2017 blog. 

To cut a long story short, the styrene “C” channels from China were substandard, and I was unable to locate the correctly sized brass “C” channel.  However, I don’t like to leave projects sitting around for too long, and I wanted a distraction from the pain of shingles (yeah, that is another story), so a start was made. 
The styrene “C” channels, whilst poor, could be modified.  I cut out 7.5 cm sections that were more or less straight from the 50cm lengths supplied; and reduced the height down to approx 1.5mm.  These I put into a jig to hold them at the correct distance apart, whist I glued the styrene spacer plates on.  Unfortunately, removal of the jig stressed the joints out, and the whole lot disintegrated.  Better to find that out now, than after I had spent any more time on them.

The failed girder attempt with styrene

As no suitable brass “C’ channels were available. I had little choice than to fabricate my own from strip brass.  In this case I was very lucky.  I had in my supply of North Yard brass strip, enough 1.5 x 0.25 x 150mm, and 3.0 x 0.25 x 150mm strip pieces  to fabricate the 3.15 metres of the “C”  channels of the correct size for the bridge.  Yes, that is a lot of brass strip

Parts needed to make one 150mm strip.  This completed strip will be cut into two 7.5cm sections. This is a fortunate measurement, as it eliminates waste of the brass.

Once the “C Channels” were made, I fitted them into the distinctive “U” shape of the prototype, and matched a pair back to back, and soldered spacers to hold them a scale foot apart. 

Three 7.5cm sections make one half of the "U" beam

One completed "U" beam.  The spacer plates are 3mm strip brass.

  All these measurements have been guesstimated from the prototype photos I took during the bridge replacement over a decade ago

New concrete support piers were formed whilst the Whitton Truss bridge was still operational. The actual replacement of the bridge took around 3 days. The spans of the truss were cut into sections, and removed by crane. The old piers were then removed, and new concrete beams were laid onto the new concrete supports, and the track was then re-instated.

After being gas axed, the bridge truss sections were craned to one side awaiting removal. This view shows the interior construction of the girder quite well.

To match the prototype, I will have to fabricate 7 “U” beam supports, 4 on the first girder, and 3 on the second. 
 “U” beams were secured to the bridge using triangle gusset shapes.  These were made from sheet brass, but soldered to the bridge with 144 degree C solder, rather than the 60/40 solder I had used earlier.  I did this for 2 reasons – a)  I did not want to desolder the assembled “U” girder upright, and b) 144 degrree solder can flow with my small 25 watt iron on the large heatsink of brass that is the Whitton truss bridge side

The "U" beam fitted to the end of the bridge. The bottom had been secured to the bridge via a brass plate, although the triangle gusset plates on top are still to be fitted.  I was also concerned that I may have reduced the clearance too low, but I tested it with a USA outline diesel, and things should fit, provided I use lower profile sleepers.  But, I won't be able to run double-stacks through the bridge, 

One span, with two "U" beams fitted, and braced, and after removal of the temporary brace

The result was very strong, and I felt confident to remove the temporary brace, designed to stop the 2 bridge sides from collapsing inwards with handling.
I made a minor change to the assembly order of the subsequent “U” girders.  Basically, the 2 uprights were installed, soldered, and braced, before the overhead beam installed.  This was a lot simpler than my earlier method.

Detail soldering of one of the venter uprights

Angle braces were fabricated, and soldered to all 4 “U” beam assemblies.  These are purely cosmetic on my model.

I have been using fluxes to aid the soldering, and these need to be removed.  I have scrubbed the girder with kitchen cream cleanser, and washed the lot in water.  Further treatment will be needed prior to painting, but at least the model is no longer a clothing stain hazard. 

I have not yet finished the first girder.  The distinctive rivet overlay supplied in the kit will need to be cut to size, and glued [soldering heat would cause distortion of the thin rivet overlay]   to the flat brass on either side of the center uprights.  In some ways these uprights actually correct a problem in the kit, as the rivet overlay is around 6 mm too short to cover the area on the upper, and lower bridge chords where it was expected to go. I do not know if this was corrected with the newly re-released kit

As you can see, I still have to fit "U" girder supports to the second span.  Followed by painting, and then the sleepers and track can be added.  

This process is very time consuming.  I have not added up the hours I have spent – but my last “U” beam assembly took just over 5 hours to fabricate, and add.   I am sure that talented modellers might have used 3D printing to make the “U” beams - I am not in that category

Happy to report that after 3 weeks, the shingles pain has much reduced.  Whoever said Model Railways is therapeutic must have had me in mind.  I will continue with fitting the “U” beams, and with luck, will have painted the bridge for my next bridge blog post.

Happy modelling