MODEL ENGINEERING IN THE STYX 13

By Journeyman

January

My club had a New Year’s Day steam up. Nobody brought a loco to run, defeated by the constant rain and cold, so the youngsters rolled out the diesel outline loco and tested the newly-laid track through the tunnel which was declared perfect. On the way home we spotted daffodils in bloom on one of the roundabouts. Sadly they were not accompanied by any spring feeling in the air.

New year’s resolution!  I must make 2013 the year in which I try and complete Boxhill after some 42 years of construction, albeit with a gap of at least 30 years.

In the last paragraph of Styx 12  I showed the finished side tanks on Boxhill. These were one of Martin Evans’ conundrum drawings. I found it impossible to visualise the tanks from them as he obviously tried to fit all the detail into too few drawings, resulting in a jumble of dotted lines to represent the differences between the two tanks. Rather than cut metal only to find that I had misread things I decided to make a cardboard mock up, which together with modifications supplied by my mentor allowed me to see how they were to be made. I have done the same for the cab and bunker tank.

As I have mentioned now and then, my background is as an ornamental blacksmith, and perhaps the greatest difference between that kind of work and engineering is precision. For example, in making a gate of a given width it is important, for reasons of appearance, to have all the bars equally spaced. This is best achieved using dividers and ‘walking’ them backwards and forwards along the rail of the gate which has been carefully cut to size, making small adjustments until the requisite number of ‘paces’ (i.e. bars) is arrived at, and marking accordingly. You don’t need to know a measurement to get it right. Furthermore, hardly anything needs to be made to a tolerance of less than 1/16”. My experience with model engineering has taught me the importance of accuracy, attention to detail and cleanliness : even the smallest bit of grit can have an adverse effect on the fit of critical parts, (see Styx 10 ref. collet chuck) and early mistakes are hard to rectify. I really discovered this recently when, as an exercise, I made a master square in the form of a block about 3” square and ½” thick mild steel (courtesy Colchester SMEE magazine The Link ). The smallest deviation on the dial indicator showed up inaccuracies that needed correction to the angles, and only after scrupulously careful setting up of the measuring jig was I able to get consistent results, a lesson well learned.

As the job progressed the workpiece was always de-burred and thoroughly cleaned before being offered up to the dial gauge and all machine surfaces were similarly cleaned. I also made some low profile clamps to a Harold Hall design for machining the large faces to ensure that they were parallel, by clamping directly to the table.

Another comparison with the blacksmithing is complexity. You may perhaps have found yourself looking at a piece of historical ironwork and wondered how it all went together. Like, say a locomotive, there is a sequence to the construction, and ways of doing the job to minimise the number of times assembly and dismantling is needed before finally, and in the case of the gate irrevocably, fixing everything together.

Where things differ is accuracy. Care taken over the making of the gate frame (it must after all fit the space for which it is being made !) is followed by the addition of decorative parts such as scrolls, and to a degree these two aspects are independent of each other.

The loco by comparison needs all the parts to fit as you proceed and bits that aren’t quite right cannot easily be tweaked to fit without affecting the fit of the parts that follow. A scroll can be bent a small amount to fit between two bars without it affecting the final appearance of a gate or upsetting symmetry, but try doing the equivalent to the width of a frame stretcher or connecting rod and ‘Thomas’ could give trouble for months, or perhaps never fit together to yield a well functioning model. So learning this difference has been one of the important though much less obvious changes of approach to which I have had to adapt; every part must be spot-on when it is made according to the drawings, (assuming they are correct !!!!)


Injectors

These cunning little devices provide as much discussion at the track as almost any other part of our locos. Some work like a dream and never give trouble, some give intermittent trouble and some never work whatever you do.

There are those who swear by them and pour scorn and derision on mechanical pumps, and those who wouldn’t touch them with a barge pole. But just how do they work, and why are they so troublesome ?

I’ve been trying to understand for ages, and it has to be said that no books that I have read get anywhere near a clear, unambiguous explanation unless you have degrees in maths and physics. For me, these devices don’t pump equations and formulae into the boiler, but water, and I can’t believe that there is no way to explain to a maths and physics duffer like me just how they do work. The nearest I can get is that is a lot to do with pressures, mass, velocity , kinetic energy, and momentum, and for an injector to work properly, the whole system from the water supply to the final delivery end must be free of air and have the shortest pipe runs possible with no abrupt changes of direction such as elbows.

Perhaps a MEWS reader can provide a lucid explanation accompanied by simple diagrams.


Drilling

Having become rather bogged down with brass sheet and angle, and rivets and soft solder while working on Boxhill’s cab, I decide to make Artisan’s lever operated tailstock chuck as published in MEWS. It proved to be a good exercise in making tapers and the most tricky part was the clamp bracket. The chuck is a Rhom capable of holding drills down to 0.4mm dia, small enough to make my own injectors should I feel like it!

I made and fitted Boxhill’s hand pump according to my mentor’s modifications. He designed a pump that fits into the rear tool box, which makes it readily accessible should it need attention.

The editor’s recent musings on the Festival of Britain, reminded me that my old Dad thought it was all a sinister Marxist plot to distract attention from the establishment of the NHS which was furiously opposed by a large part of the medical establishment.  He refused to go, so my Mum took us instead!  I can remember being fascinated by the toffee wrapping machine, the Skylon and the Dome of Discovery, which had far better exhibits than its farcical successor,  but sadly don’t recall Britannia.




 

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