A Highland Railway Locomotive in 2 mm Scale

by John H. Ahern

[Editor's note: This article was passed to me recently by Stewart Hine as being of historical interest to 2 mm scale modellers. It is reprinted from the March 1950 issue of Model Railway News — a full ten years before the Association was founded (see the Association Yearbook for a fuller historical perspective). The model described is of a very small prototype and would have to be approached carefully even using today's techniques. This model features in the Inversnecky and Drambuie railway.]

Those readers who are interested in the smaller I scales will no doubt remember the Highland Railway Stroudley tank-engine Dornoch, the property of R. W. G. Bryant, and built by C. W. A. Mitchell, which was mentioned in Mr. Bryant's article in the May, 1949 issue. This little locomotive presents a number of unusual points, some of which might prove useful to workers in larger scales, and I think that a detailed description should be of general interest. It will be realised that workers in 2 mm. scale are at present thrown very much on their own resources. They have none of the commercially made equipment which is available in larger scales and for this reason must either display great ingenuity and skill or quit. It is definitely the scale for the man who likes problems.

General Arrangement of Motor

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Figure 1: Click to see larger version

With the permission and kind co-operation of the builder and owner I have made the drawings which are reproduced with this article. I should like to say that there may be one or two quite unimportant errors in the dimensions of the internal parts. This is because I did not take the motor to pieces and all measurements were taken as carefully as possible from the outside. In the side elevation, Fig. 1, I have indicated the main details of the superstructure but the main purpose of the drawing is to show the arrangement of the chassis and motor. It wilt be seen that the armature shaft is disposed vertically,

rather in the manner of the Hornby Dublo mechanism. The magnet is a block of Ticonal, about ¼ in. square in section and 3/8 in. long, clamped between thin soft iron pole pieces. I shall return to this in a moment, but I want to speak about the frames first. These are of brass, about 1/64 in. thick, and are insulated from each other by blocks of fibre which form the cross-members of the chassis and to which they are secured with 10-B.A. screws. To help sort matters out I have put a stipple on the fibre blocks in both diagrams.


For insulation the axles are divided in the middle and the method by which this is accomplished is shown in the small sectional drawing at the foot of Fig. 2. The half-axles. as we may call them, are forced into a length of Tufnol tube, turned and drilled from the solid, which has first been enclosed within a specially turned piece of 4 in. o.d. brass tube. A tiny circle of paper, treated with shellac, is inserted in the tube between the ends of the two half-axles as a precaution against possible shorting. It will be seen that the wheels are insulated from each other but in electrical contact with their respective side frames through the axles. Very thin springy washers are placed between the wheels and the frames to improve electrical conductivity. This is a little point which workers in larger scales might remember with advantage. There are no collectors in the ordinary sense; current pick-up takes place entirely through the wheels and the frames, and as far as I have been able to observe the system works perfectly. I have seen the engine running on a number of occasions and have never yet known it to falter or fail to start. In this respect it seems superior to many larger models. It will be seen that with this system there is no need for the wheel treads to be insulated from the centres, and if this were a locomotive with outside cylinders and valve-gear no insulation problems could possibly arise therefrom. As a matter of fact, Mr. Mitchell has built an outside cylinder engine, a Southern Railway "Schools", on, I believe, very similar lines. The wormwheel on the driving axle is a force fit on the outer brass tube and is therefore insulated from both half-axles.

The Permanent Magnet

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Figure 2: Click to see larger version

Returning to the motor, I have tried to show its construction in Fig. 2, and I think that if the reader will compare this with Fig. 1, the arrangement should be fairly clear. The block permanent magnet, as I said earlier, is clamped between soft iron pole pieces, and these are bolted to a block of brass just forward of the magnet. It has not been possible to show this block in the diagrams but its position will be obvious. A top plate of brass is soldered to the pole pieces and provides the upper bearing for the armature shaft. Mr. Mitchell centred the brass plate by trial and error, with the armature in position and packing strips of paper against the pole faces, until he was satisfied that the armature was centrally placed. The iron pole pieces were then soldered to the brass plate, care being exercised to ensure that nothing slipped, and the job cleaned up on the outside. The two holes in the brass plate behind the shaft bearing were drilled in the hope that they might provide some ventilation for the armature, but Mr. Mitchell says that it is extremely doubtful if they fulfil their purpose when the superstructure is in place. This unit, consisting of magnet, brass block, pole pieces, and top plate, is bolted to the fibre block which forms the rear cross-member of the chassis by a surprisingly hefty screw for such a small locomotive (8-B.A., I believe). Since the fibre block rises slightly higher than the frames the motor assembly is insulated from them; and it is, in fact, at neutral polarity. The commutator brushes consist of thin, flexible, phosphor bronze strips with shaped blocks of copper carbon attached to their ends. The method by which this is accomplished is so interesting and unusual that I have made it the subject of a drawing, Fig. 3. The shanks of the brush arms are pressed into slots in the fibre block, but it has proved rather difficult to show this detail clearly in the drawings. They are, of course, connected electrically to their respective side frames. The reader will understand from the foregoing that the commutator is situated below the six-pole armature. The lower shaft is a strip of brass bolted to the underside of the fibre block which forms the bed for the motor. It can be seen in Figs. 1 and 2.

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Figure 3: Click to see larger version

Summing Up

That, in broad outline, is the layout, and it is certainly an efficient and practicable one. The construction is really quite straightforward and should not present any serious problems except, perhaps, as regards the accurate centring of the armature. It will be clear that the locating of the top shaft bearing hole must have been carried out with great care since no further adjustment was possible after the assembly had been completed. It proves beyond doubt that motors more compact than any at present available commercially are a practicable proposition.

The armature windings are of 42 gauge wire, 172 turns to each pole. The armature is 4 mm. long and 11.5 mm. diameter. The magnet tunnel is 11.9 mm. diameter.

This locomotive has now been in use for several years, and it has been worked quite hard. It seems to be not a scrap the worse for it and has not revealed any mechanical or electrical faults. It is interesting to note that it runs at quite a reasonable scale speed, even with the 12 volt power supply turned full on. Mr. Mitchell experimented with three-pole armatures in two locomotives before adopting six poles as standard and tells me that in no circumstances would he revert to three poles.

The method of insulating the frames from each other is, of course, not new, but it has been very little used and I am inclined to think that it deserves more attention than it has received. It is questionable whether there is any more difficulty in making a frame with fibre cross-members than with metal ones; in fact, many people might find it easier. It is a decidedly attractive thought that the motor and the body of the locomotive, can be insulated from the current supply. In fact the only parts which are not insulated are the side frames and wheels and, of course, the brushes and armature windings; no short circuits can possibly occur through other parts of the locomotive. Perhaps I shall be trying something on similar lines in 4 mm. scale before very long.

[Copyright Note (1997): The copyright for this article must still reside with the estate of the late John Ahern. I have not, however, been able to trace the current owner or owners. The article has, therefore, been reproduced without permission but with full acknowledgement of copyright.

(2004): Mr G Russell contacted the 2mm Scale Association through this website. He informed me that his mother and her sister are nieces of John Ahern's wife, and the beneficiaries of the estate. He has kindly passed on permission to reproduce the article above, for which we are grateful. ]