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Current Featured Engineering Project:

Steam Whistle Valve:

Todays job; we have to machine a new steam whistle valve from solid billet manganese bronze complete with stainless steel valve and bronze spring.

The billet starts life as a 6" x 8" round stock - due to time restrictions and cost we found it was quicker and cheaper to use round rather than cast some material into block. - It is placed into the vertical milling machine as we start to block the piece into a rectangle. We take care to ensure that the rough piece remains 'square' on all sides throughout machining to aid setting.

We continue machining the billet to gradually produce the basic shape of the part - at various stages we finish machine the material to the finished height and width of the areas of the component. To produce the round section between the fore and aft sections the material is placed into our centre lathe and turned, we have centre drilled both ends at the blocking stage which enables me to clock the part to ensure that the turned section is perfect alignment with the drawing. The sides of the part are rough machined around the area where the webbing will be sited, we leave a minimal amount of material on the faces for later finishing as we form the webs.

I produce our own springs at the same time using the centre lathe. I have pre-turned a piece of stock to a diameter that will be smaller than the core size of the finished spring (as the spring expands a lot as it is removed from the mandrel). A hole is drilled to allow the end of the wire to lock into the middle of the mandrel. I set the lathe feed to the required spring coil pitch (in this job we are running in reverse) and insert a piece of square bar with a small hole into the tool post to act as a guide. The wire is fed into the mandrel through the guide and the lathe run slowly with screw cutting gears engaged - this produced a nice tightly wound spring which is then cut from the mandrel to be finished.

Meanwhile, the whistle valve is nearly finished milled to size with only some detailing to do. The part is re-placed back into the centre lathe using centre drillings to clock the various areas of the component to be machined. We screw cut all of the threads into both ends for the end caps and at the same time produce the valve seats and spindle drillings, we then produce the steam in and steam out ports along the topside of the material. We use jaw extensions with thick pieces of aluminium packing to stop the chuck jaws from marking the relatively soft manganese bronze -this also helps the part to move easier in the chuck as we clock the part into the correct position.

Once the last of the turning is complete, the part returns to the milling machine for detail finishing and to have the webbing shaped - this is mostly done using a good sharp three fluted endmill and a long series radius cutter. All the finishing chamfers are applied and radius ed off to give a hexagon type appearance to the part. We finish the piece by applying a hard polish with butterscotch to give a satin effect. The end-caps and internals are machined and fitted along with the external stainless steel steam inlet nipple and lever pivot. The part is then cleaned and assembled ready for the two tone whistle to be fitted on the locomotive.

 

 
Component is featured on the website home page (December 2012).

 

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