COUPLING RODS MADE IN THE OLD AND NEW WAYS

By Richard Lunn

Bristol SMEE member, John Harris, has demonstrated that the old ways of working are still alive and well. John is making a 7 ¼” gauge BR Standard tank engine and the following photos are a self explanatory story of the manufacture of the coupling rods.

The first operation was to mark out the shape of the coupling rods and bore the end holes using the DRO for accuracy.

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As seen in the previous photo the next stage was to chain drill the profile and make the bronze bushes, then (as shown below) the profile was cut out using a jigsaw.
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Back in the mill the profile edges were smoothed off.

Above, machining the joint in the end of the front rod and, below, using the lathe with the rod held in the toolpost to machine the corresponding slot.
The end of one of the rods with the bush fitted.

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The assembled front and rear coupling rods.

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Bernard North is also making a 7 ¼” gauge BR Standard but chose the easier route of having his coupling rods water jet cut by Luffman Engineering in Tiverton. This saved a lot of time and he just needed to mill them in the plan view, drill a few holes and draw file them to get a good finish. He used water jet rather than laser cutting to avoid the hard external layer that you can get with steel laser cut parts.

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Above are the rods as received from Luffmans.

The picture below shows the finish obtained by water jet cutting.

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They are cut from 20 mm steel plate.

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The above photo shows the rods being milled to width and below the result. A clever way of holding them for machining.

Here Bernard is milling the flutes in one of the coupling rods.

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This is a trial fit of Bernard’s coupling rods on the engine.

A six coupled (or more) loco has to have a hinged joint in the coupling rods otherwise if any of the wheels hit an undulation in the track the coupling rods would bend with the suspension movement.

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This is back to John’s engine again showing the coupling rods installed and below a close up. The engine is upside down.

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So there you have it. Two different methods of manufacturing the same parts.

Bernard’s way is probably easier but John has the satisfaction of knowing that it was all his own work. In terms of time Bernard’s method is quicker although he had to produce a CAD drawing for the cutter to use. But then again seeing as we are spending much of our time in lockdown perhaps the time element is not so critical at the moment.

Cost may not be as different as you may think because the cost of the material to buy is probably close to the cutting cost including material which would come from a large sheet.

To finish here is  a picture of John’s steam brake cylinder and mechanism.

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Article first published in The Bristol Model Engineer, January 2021.

 
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