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Wagon Making

DocRef: JC13a

Rolling Stock Construction.

Casting techniques for easier batch construction.

See also DIY Casting and Scratch Building

As we all know, repetitive processes become tedious and that applies to reproducing identical parts in wagon construction. If building several examples of a common wagon type, the measuring, placing of strapping and numerous cosmetic bolts and rivets becomes a real chore and then mistakes can creep in. The best way to batch build is to cast the identical parts. This may be the parts for a complete wagon or just parts which are identical on different wagons (e.g. the end of a van which may be to the same basic design for several different diagrams).

The starting point is a dimensioned drawing, which should be as large as possible to show details, from which to determine the way in which the various parts will be best cast to fit together as neatly as possible. Also take into account that any detail which involves deep recesses or 'undercuts' will need to be produced as a separate component and later added to the completed major castings. An important point to take into account is that the moulding and casting process usually results in 'shrinkage' (i.e. the final casting turns out smaller than the original pattern). I generally allow for 1% shrinkage on all components and make the patterns 1% larger than scale in all dimensions, this includes the thickness where parts abutt in final assembly.

Making the Patterns for Sides, Ends and Underframe.

Each separate component requires a pattern from which a mould is made. The number of components for any particular project will depend upon the complexity of the wagon and the profile, in particular any deep recesses, such as doors and similar detail. On a normal open or covered wagon, the sides can usually be cast as a single piece, as can the ends. Details such as ventilator hoods can be separate components, which in some vans will allow for a single end pattern to be used for more than one diagram. At this stage, decide whether the headstocks are to be cast in the same material as the ends. The strength of the final casting will play more of a part in arriving at this decision than the design of the prototype.

  • A 'chunky' early design with heavy outside timber framing and timber underframe will usually lend itself to having the headstock cast in resin as part of the end casting.
  • A more modern wagon, with a thin steel underframe, may be better with cast metal solebars and separate metal headstocks, for greater strength.

These are judgements left to the builder, having weighed up the various factors.The major points of potential weakness in a wagon comprising a set of individual castings will be the joints between sides and ends together the 'bending' effects upon the headstock of the draw bar and the compression of the buffers. In a model with a commercially produced single piece resin body casting, (e.g.Williams Models kits), it is acceptable for the stresses imposed by draw bar and buffers to be transmitted through the unitary body casting, but in a multi component body with glued resin components, I consider that to be unwise.

My usual method is to have the draw bars acting through brass angle brackets bolted through the thick (3mm) styrene sheet floor, thus removing stresses from the ends and sides of the body. Buffer compression forces are also then absorbed by the floor where it abutts the headstocks, as well as by the proximity of the solebars. The components for the body and underframe can all be cast from flat, open, moulds. Start with a base of thick styrene card larger by 2cm all round than the component being produced. Build up the pattern in layers of appropriate thickness, adding details such as planking, strapping and bolt heads, etc.

Remember to allow for the 1% shrinkage and avoiding heavy 'undercuts' which may damage the mould when removing the pattern. Planking is scored using a 'board scraper', these are available from art supplies shops with a variety of cutter profiles and a depth gauge to ensure evenly spaced and parallel planks. For adding bolt head and rivet details, combinations of plastic rod, brass rivets and moulded or cast bolt head detail, (available from Cambrian Models in plastic and Brandbright in brass), can be employed. Before drilling thin strapping of styrene card which has been attached with solvent, remember to allow plenty of time for the styrene to harden. Some details, such as door latches, label clips, etc., can either be incorporated into the pattern for casting or added as separate components when assembling the wagon.

When the pattern is completed, inspect closely for imperfections, (watch out for any small scratches, bits of swarf or solvent marks), and repair with filler if necessary. Check to ensure that all edges are sealed, since any major 'creep' of the silicone rubber under the edges of strapping, etc. could result in tearing of the mould.

Be warned, even the smallest mark will be reproduced on a resin casting!

Next build up the sides of the pattern to contain the mould rubber mixture. Allow a minimum of 5mm depth above the highest point of the pattern or the completed mould will lack strength.

Pouring the Moulds.

The type of rubber to be used for the mould will depend upon the material to be used for casting.

  • Silicone rubber, soft and very flexible, will be suitable for casting in resin.
  • Room temperature vulcanizing rubber (RTV) will be necessary for hot castings of white metal alloy.

See the article on 'DIY Casting' for further details of materials and suppliers.

Having mixed the rubber compounds in accordance with the supplied instructions and using the advice contained in the Appendix to the above article to avoid problems with trapped air, and observing relevant safety notices, the moulds will need to be left to cure. The curing time varies greatly. The RTV-101 compound will usually be ready to be removed from the pattern within 6 hours, this depends upon ambient temperature. The soft silicon rubber T28 compound takes around 24 hours to cure sufficiently for the mould to be removed again this is temperature dependent.

The moulds should not be used immediately, since they will take time to fully cure and attain full strength. Three days is the time I usually allow for the rubber types quoted above.

Casting Resin.

The moulds will now need to be inspected for imperfections, in particular for areas where 'creep' under the edges of strapping or other detail, may have occurred and thin strands of excess rubber may need to be trimmed away with a very sharp knife. With silicone rubber T28 it may also be necessary to trim away, on the reverse of the mould, any ridge around the perimeter which has been formed by the meniscus where the rubber met the mould walls. Set the moulds on a level surface, check this with a spirit level, and mix the resin according to the instructions, always following the advice in the article above to avoid air bubbles.

Dust the mould lightly with French chalk (talc) and pour the resin. Working quickly, as the resin will begin to cure within a few minutes, run a smooth-ended rod over the detail of the mould to release any air trapped (e.g. in rivets, etc.), then vibrate the work surface for a few seconds, again to release any air.

According to the type of resin employed, and the ambient temperature, the casting will be ready to be released from the mould between 30 to 60 minutes.

Casting White metal alloys.

In this part we are dealing with castings from 'flat' moulds e.g. solebars, headstocks and similar, where the reverse of the casting carries no detail. The mould will require to have a large pouring plug cut at the side or end where the metal is to be poured. It is better that the metal is introduced at a point where it will flow down through the mould, rather at a point where it is required to be forced, by the weight of molten metal in the plug, either upwards or sideways through branches from the main core of the mould. There will need to be small channels cut from the lowest point of the mould, and from the top of any side branches, leading upwards to exit at the top of the mould and away from the pouring plug. These channels permit displaced air to be vented.

To cast you will need a plain (flat surfaced piece of RTV-101 to match the outline of the mould and provide the backing piece to it. Dust the mould with French chalk and clamp firmly in the vice, but do not squeeze out of shape. Select an alloy appropriate to the type of casting (see Appendix to the above article for suppliers) and melt according to instructions and observing safety precautions.

  • Pre-heat the mould, either by heating gently in an oven or by pouring two or three castings, which if not perfect can be scrapped and remelted.
  • Pour the molten metal into the mould so as to fill the plug, it is the weight of the plug forces the metal into the recesses of the mould.
  • Remove and inspect the casting.

Do not expect a perfect casting at the first and every attempt, there will be a failure rate which will increase according to the complexity of the mould, thin and intricate patterns may have a failure rate as high as 90%. You may need to enlarge the pouring plug aperture or increase the venting channels before achieving an acceptable casting. Solebars and headstocks are generally not a problem, the failure rate being almost zero, once the mould has become very hot.

Multi-faceted castings.

This where the fun starts! If you require a buffer guide housing, then you obviously cannot employ a flat mould if you want a single-piece casting, you could, of course, produce in two halves and solder or glue. Taking the buffer as an example, prepare your pattern, adding the ribbing and bolt detail as required. Cut an appropriately sized cube of modelling clay. Paying particular attention as to how the ribbing will sit in the mould, so as not to produce deep undercuts, impress the pattern, lengthwise, into the clay to half its depth. Build up a wall of styrene card around the clay block, to retain the RTV. Insert four vertical spigots to act as registration points to ensure accurate alignment of the two parts of the mould. Pour the first half of the mould using RTV-101. When cured, remove the clay and spigots, leaving the pattern embedded in the half mould.

Build up a new wall to retain the next pouring of rubber and liberally coat the mould with a release agent, a silicon spray type or 'Vaseline'. Pour the second part of the mould. When it has cured, CAREFULLY, separate to two halves of the mould, you may need to use a knife to start the parting line.

Producing white metal castings from this two-part mould is no more difficult than for the solebars as described in the preceding section.

An enclosed two-part mould will not easily produce acceptable resin castings as bubbles produced during the curing process cannot easily be expelled without a vacuum chamber. ''' Completing the Models.'''

The castings may need some fettling to remove 'flash' and repair other imperfections. Resin is best joined using thick cyano-acrylate ('superglue') with white metal or resin joints being made with either thick CA or epoxy-resin ('Araldite'). If employing a white metal underframe, this is best soldered for maximum strength. Floors can be cut from styrene sheet, wood, or even metal, according to personal preference. Van roofs, I usually produce by rolling brass sheet, use aluminium if you prefer, and fixing in place with epoxy-resin. Other components such as axle boxes, brake parts, etc., can be produced using these techniques. This is only a brief guide, written with the intention to encourage experimentation with this method of efficiently producing batches of model wagons.

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Page last modified on February 27, 2018, at 11:06 PM