HARRY’S GAME – 6-INCH REGULATOR REFURB
Maintenance of a 6-inch scale go-valve occupies our resident engineer in this month’s look behind the shed door at the Fairbourne Railway.
The last few weeks in the lockdown-induced quiet surroundings of the Fairbourne Railway workshop have seen me mainly focusing on the continuing 10-year overhaul of our Darjeeling style 0-4-0ST ‘Sherpa’, and after disassembling the boiler and its internals, it was time to take a good look at the regulator.
Checking the maintenance records there had been no reports of a leaky regulator from the drivers over the past season so I wasn’t expecting anything too bad, and this was confirmed after I stripped the regulator down fully – neither of the mating faces were particularly scored or pitted which was a good sign. There were just the usual wear marks from use, however with the regulator out of the boiler I took the opportunity to reface it and lap it in to hopefully give another 10 years of trouble-free use.
The design is a nice simple rotating disc regulator, with three teardrop ports spaced equally around the mating face, giving nice proportional control. There is a replaceable bronze insert in the moving half and the stationary section is a piece of cast iron, with a replaceable mating face too.
I started the job by setting up the stationary section in the four-jaw chuck of the lathe and faced across the sealing face of the regulator – this removed the little bit of pitting and grooving that had occurred.
Figure of eight
I then went over to the surface plate and with the usual figure-of-eight motion on some Wet and Dry paper, polished the machining marks out of the cast iron. Using a figure-eight motion prevents any bias occurring in the polishing, wearing down one side faster than another.
Unfortunately the countersunk screws that are used to hold the replaceable face on the valve are at the surface of the cast iron. Since these are harder than the iron they will wear the bronze more, but this shouldn’t be an issue as it should simply wear a circular section of the bronze and not between any of the port faces.
To get an idea of how much material I would need to take off the bronze face, I quickly rubbed this over on the surface table as well and you
can see the results in the pictures.
This necessitated making a couple of passes on the lathe before it was back to the surface table for final finishing.
The regulator was then reassembled, remembering to put PTFE tape on the central pivot to prevent leaks as it goes between the steam space of the boiler and the regulator J-pipe. I tensioned the spring to ensure it held the faces together but still allowed for nice easy operation.
I then re-assembled this onto the components of the J-pipe that sits in the dome and set it aside until the boiler has been re-tubed.
Regulator gland
When I took the regulator gland off Sherpa, I discovered that it was past its best, with a corroded bore that had started to mark the stainless regulator rod, and a corroded end which squeezed the regulator gland packing out around the gland rather then compacting it to seal on the regulator rod.
All of this encouraged me to replace the gland, and to replace it with an SAE660 bronze one at that. Unfortunately we did not have any castings lying around for this gland so it would have to be machined from solid and again I ran into a problem – the only stock material we had that was big enough to machine the bolting flange from was not thick enough to machine the full length of the gland. As a result I was forced to fabricate the gland from two pieces.
The first task was to machine the centre section that the regulator rod runs through. I turned the outer diameter to fit into the gland housing before boring out the bore to be a good sliding fit on the regulator rod.
I machined the face that compresses the gland with a taper on, to help compress the packing material onto the regulator rod. Once this was complete, I turned it around and machined the main diameter down to 10mm bigger than the cored section of the large diameter bronze stock.
Safety step
I then machined a step into the front face of this so that if the braze ever did fail, the centre section that has the pressure acting on it would be held in place by the matching step on the outer bolt flange.
After the inner section was machined to size, including putting weld preps on the outer facing surfaces, I put the larger piece of bronze into the chuck, faced it off and then bored the middle to fit around the centre section, including the emergency step.
I couldn’t do any more machining to this piece as it was too thin to turn
the outer diameter so I took the piece out of the lathe and then Tig-welded the back side of the gland up. This is a very similar technique to Tig-welding steel, it just happens to be in bronze in this case – you can use a variety of filler rod, but I used some SAE660 to be a perfect match with the base metal.
After completing the welding I put the gland back into the lathe, holding it on the outer bore of the smaller diameter, allowing me to machine the front face and outer diameter of the bolting flange.
I machined it down until the thickness of the flange matched the end of the centre section, with the small weld prep visible. I then Tig-welded the front face before cleaning this up on the lather again.
If it looks right...
Next job was to mount the flange in the rotary table on the mill and drill the two bolt holes into it, before milling it to the final shape that I liked onto the outside. I did this by drawing the shape I wanted with a Sharpie pen onto the bronze, then rotating the table until the drawing was perpendicular to the table.
Having machined this edge, I rotated the table by 180 degrees to get the parallel edge. I then added the angle the table was sat at to the original angle the table had been at to then produce the lozenge shape.
This was then further finished by roughing the radius at the top and bottom by determining by eye where the cutter touched the gland and using the table to machine the radius, then lowering the table, turning the table through 180 degrees and taking the same cut. Once this was done, I finished the radii off with a file, completing the new gland.
To go alongside the gland, I mounted the regulator rod in the lathe and skimmed the length that sits in the gland to remove the pitting and roughness left by the previous gland, finishing this off with a piece of emery cloth to leave a good polished surface for the new gland material to run on. Another job of the many in a 10-year overhaul done!