One entry on the finishing off list - "speaker mounting and means of removal". I've kicked this one down the road for a long time, now it needs to be tackled.
The original plan described many WT posts ago was to insert the DCC circuit board and speaker on a subframe through the underside of the firebox and push them forward inside the boiler with the speaker ending up in the smokebox. It became clear that the DCC module wasn't going to fit that way so it's been relocated to the tender (i.e. kicked a bit further down the road) , where there's enough room. On trying to accomplish the same with the speaker it became clear that that wasn't a practical approach either. Since I definitely want the speaker at the noisy end of the locomotive (human hearing is astonishingly accurate at determining the direction of a sound) the only other way to fit and remove the speaker is through the smokebox door. Fortunately I had not fitted the smoke box door at that point. You've no idea how much work this is going to create!
There's a casting supplied in the kit that represents both the smoke box front ring and the door. Many posts ago I did some work on it to add the smokebox liner rivets. I didn't think too hard about it at the time. Since the smoke box door now needs to be removable the way this casting fits to the front of the smokebox becomes critical. And here's the first problem. On the prototype the smokebox front ring is a steel pressing that sits inside the smokebox wrapper plate and it riveted to it around the outer flange. What you see is the rounded corner of the ring pressing and the edge of the wrapper plate outside it. The plate is 3/8" thick on the prototype, equals 0.22mm on the model. The way the smokebox is made up on the kit presents double thickness at that edge: the base etch plus a half etched overlay. The resulting edge is way too fat and it inevitably shows the imperfect lamination of etch and half etch. Also the smokebox front casting doesn't quite replicate the prototype in the way it fits the smokebox. It doesn't quite slip inside the smokebox wrapper plate and it doesn't quite cover the front edge of it either, it's sort of undecided. There's one more thing that doesn't look quite right, the door seems a bit flat. More GNR Gresley than LMS Stanier. Hmmm...
For me the smokebox front if the 'face' of the loco (although not in a Rev W. Awdry sort of way, and in passing it has to be noted that although Henry the Green Engine was clearly a black five the Fat Controller never saw fit to acquire an 8F) and it's easy to lose the character of it if something's not quite right. So it'll have to be rebuilt from scratch. Hadn't planned on this and it looks a bit daunting.
The first job is the thin the smokebox wrapper by carefully (very carefully - one slip and it's a big mess!) undercutting the inside etch with a cutting disc and peeling out that thickness of material. Nerve wracking stuff.
After cleaning up the inside diameter can be measured and a smokebox front ring drawn up. I just copied the prototype using drawings in the Wild Swan book. The ring is easily, if rather wastefully, machined from a lump of brass, and the rivet holes drilled diameter 0.5mm on the milling machine.The inside of the ring is chamfered as per the prototype (the pressing was machined here to provide a seat for the door to seal on) so that there will be a small recess visible around the door when it's fitted. It's there on the prototype and almost impossible to replicate on the model unless you make the ring and door as separate parts like this.
Back in the lathe to part off and you get one of these...
The original casting is on the left. It's a decent casting, shame I can't use it. The MOK smoke box is formed around etched circular formers and as a result it comes out almost perfectly round. The machined ring is a gentle push fit into the wrapper with hardly any gap around the edge. Looks much better and worth the effort.
Took a bit of thinking to work out how to make a new smokebox door. I measured the casting and compared it with the profile taken from the Wild Swan book. The cast door is definitely too flat. The dome of the door is a spherical surface apart from the flange at the outside edge. I imagine the traditional way of doing this (unless you have a CNC lathe!) is to mount a length of bar in the chuck and go at it watchmaker style with a toolrest and gravers and a profile template. Never tried that approach to turning and I don't have the tools for it. I came up with another approach that guarantees the geometrical accuracy of the profile and relies much less on hand work.
First step is to draw the door profile in CAD. Then from the top centre of the curve step off parallel lines at 0.1mm intervals. You can use bigger intervals if you want to spend less time doing numbers on the lathe, but you'll have more hand finishing work to do and there's less control over the profile. Divide the door profile into what is effectively a stack of discs each 0.1mm thick and of a known diameter.
That lot is tabulated in a spreadsheet so I don't lose track of things when I'm on the lathe. The lump of brass bar is turned down to the outside diameter of the door and faced off. A diameter 2.5mm hole is drilled through the centre for the dart boss. Then it's the best part of an hour twirling handles with industrial efficiency and trying not to be hypnotised by the DRO numbers - one cut too far and you have to start again.
You're sort of making a contour map of the door.
The surface is then taken down with file (yes - file on lathe = heresy) carefully until all the rings have just disappeared. A scrub with wet & dry (more heresy) and a Garryflex block finishes the door profile. It took a while but it was easier than I thought it would be.
The diameter is reduced behind the depth of the door to be a close fit in the previously made ring, and the whole thing is parted off.
The handrail knob holes are drilled on the milling machine and a small part is turned to represent the boss for the dart locking handles.
There's at least one more episode of this...