Saturday, 18 November 2017

November 2017 meeting

Alistair, Alisdair, Andy, Stephen, Jim, Graham and new boy Chris assembled in Edinburgh this month. The temperature left no doubt that it was November, but the sun was shining.

This month's talk was by Jim, who swapped his Swiss needle files for the computer variety, in order to explain the gentle art of designing etched kits to us. Jim has commercialised some of his designs through his Buchanan Kits label, but he also produces one-off models for his Kirkallanmuir layout using etches, which he prepares himself using AutoCAD.

First of all Jim took us through a brief description of the principles of etching. Rather than repeat this blow for blow, the interested reader is referred to Bob Jones' excellent etching treatise on the 2mmSA virtual area group website. You can't access the VAG? Join the 2mmSA and see what you're missing!

Jim has used both of the main UK photo-etch suppliers, PPD and PEC, each of which has its own advantages and disadvantages. PPD dip their etches, PEC spray; PPD use rolls of metal, PEC use flat sheets; PPD offer smaller minimum sizes and setup costs, PEC are cheaper pro rata in bulk. The differences were explained to us. Search the VAG for "Etchers Advice" to read a series of very useful reports from experienced etchers in February 2016.

 Jim had brought along his most recent etch design and the outome in 10-thou nickel-silver: £109 from PPD, who are very responsive: two weeks from artwork submission to give a quote for approval, with five working days quoted to make the etch (in fact the order turned up two days after payment). PEC are typically slower.

The artwork for the etch is displayed on the monitor, and the finished result is on the table. On the left is a signalbox for Kirkallanmuir; on the right, a Caledonian Railway brakevan. Odd spaces are filled in with useful parts like hurdles for loading cattle, telegraph pole crossbars, and so on.

"Etch that, Jimmy" ... a traditional Glasgow greeting.

Etching artwork consists of four congruent layers, controlling how the nickel-silver is cut by the chemical etchant: leave the metal as full thickness; half-etch from the front; half-etch from the back; or etched right through. Jim uses AutoCAD 2007 to prepare his artwork, with two added preliminary layers: 0 for an outline drawing of parts using a zero-thickness line; 1 for a construction view (just the outline of areas of etch which belong together, used for placing the designs compactly in the final sheet layout); then finally the four layers copying layer 0 and filling in its shapes by hatching, to tell the etcher what to do: 2 to show where metal should be etched right through; 3 for "etch from front"; 4 for "etch from back"; and 5 for "etch right through".  PPD and PEC use different conventions for whether black mean "etch through" or "full thickness". Personal preference for one or the other when drawing may therefore influence the choice of etcher. Another factor is the drawing format they will accept: PEC for example standardise on AutoCAD 2000.

Jim took us through the design of a wagon, starting by drawing the floor outline on layer 0 with zero-thickness lines spaced 0.15mm apart for planking, step-copied as an array to be over length then trimmed back. Next, the inner layer of the three-layer etch for a side is drawn, and mirrored over to the other side; then the outer layer similarly; then finally a strapping layer. Each side Z-folds together like a concertina, with fishtail hinges which are filed off after the layers are sweated together. Jim explained the difference between a fold (90 degrees) and a bend (180 degrees), and how to get the dimensions of these correct. Then the ends are added in the same way, and solebars, headstocks, axleguards, axleboxes, and end stanchions. Veritable origami in ten-thou metal sheet.

Other tricks of the trade included tab-and-slot design - the slot 0.1mm oversize,  the tab tapered to make fitting easier. Multilayer parts don't always fold perfectly so a little tolerance is a good idea: multilayer axlebox bearing holes slightly oversize, and more generous hole sizes on the inner layers of a multi-layer part. This also give somewhere for the solder to go. Bolthead relief on outer layers is best added by half-etching from the back, then pushing a needle through from the back to create a pimple. Small multilayer parts are much more easily manipulated if an etched handle is included, removed once the part is soldered in position. Fitting of parts like coupling hooks and brake levers is eased if a tiny step is designed in at the appropriate point so that they locate correctly on assembly, avoiding the need for a shaky hand to hold them in place while soldering and the consequent risk of a dry joint.

There were more words of wisdom and experience, but at that point I went off to prepare lunch. Jim had brought along his latest build of his own etches, in the elusive Dixon livery which was a very common, but little-photographed private owner livery around Glasgow. Usually a shot like this has to be excused with a  caption on the lines of a "cruel enlargement". Not necessary here: the closer you get, the better it looks.
After lunch everybody got their heads down for a bit of modelling, while Alistair invigilated, as befits a retired dominie. It was dark by the time we stopped for more discussion, tea, cake, then off home.


Quite a productive day for us all. Next month, we'll meet in Glasgow.

No comments:

Post a comment