Britannia 70047 "Derek Stokes"

[Jock McFarlane]

Jan 4, 2019 23:00:03 GMT simplyloco said:

Here they are: just as I remembered from my yoof!

www.cromwell.co.uk/shop/hand-tools/tap-extractors/3-flute-tap-extractors/f/1222

That looks like the simplest way to remove a broken tap with minimum collateral damage. I have added the Cromwell site to my favourites but hope never to go there.

[JonL]

I've had suggested to me today that that screw may be slightly redundant anyway as its not on a pressurised part of the steam chest, however I will have a go at getting it out.

Thanks for all of the suggestions, yet again the value of a good forum is proven.

[JonL]

I managed to get it out! A combination of three prongs of locking wire pushed down the flutes and heat applied to the casting got the remains of the tap to spin free. Turned out there were not many turns in there. I don't know why it snapped, my 6 BA tap ran in just fine. I'm glad I managed it, it would have bugged me.

Tonight I started making the blocks that will hold the cylinder at the correct height on my lathe saddle for the "boring bar between centres" technique advocated on this forum and at my model engineering society. I've machined two spacers at 15/16 to hold the block perfectly at the centre; I'll do those and then reduce the spacer height accordingly to allow me to do the valve chests.

[Jim]

That's great news Nobby. Well done and what a feeling of satisfaction too.

Jim

[mbrown]

It sounds as if the tap did not break by applying too much torsion but by an accident like knocking it sideways. I can just hear the cries of "Oh dear!" from the previous builder when that happened!

Well done for getting it out without damaging the block.

Malcolm

[JonL]

I agree Malcolm, the fact that once it started moving it span out freely suggests it wasn't exactly jammed.

[JonL]

I have the knack of turning a small job into an evenings work. Photo taken during a test fitting so covered in swarf and oil! Made the countersunk screws and washers for the front connecting rods this evening. First matching pair took about 45 minutes, second pair 40 minutes, last pair about 15 as I already had the mandrel and everything by then. Three pairs because after I had made the second one I realised the first one was not as good as it could be.

[JonL]

Tonight I used the vertical slide to machine out the ports between the exhaust section of the piston valve outer sleeve and the manifold in the back of the casting that collects the waste gases before sending them on to the chimney. It went relatively smoothly. I still pine for a mill, but this does me for now.

I also virtually finished my between-centres boring bar. I edge ever closer to the point where I will be machining the cylinders, but making the tooling is taking a while. Seems a shame to only use it once!

[JonL]

I also lapped in the backplate to the casting. The result was a seal that, to my great surprise, was capable of holding lung-applied suction for a good fifteen seconds. At least one part of my locomotive might not weep!

[mbrown]

Nicely done on both components. Lapping to that standard is very good - most of us settle for a smear of jointing compound as it only has to hold exhaust steam. You could qualify as one of the old fitters who could put a main steam pipe joint together with no intermediate gasketing!

But watch what you do with that sausage!

Malcolm

[barlowworks]

There are 3 excellent videos about boring out cylinders on MrCrispin's YouTube channel.

Mike

[JonL]

Thanks Mike, I'll check them out. I like to go in fore-armed before cutting!

[JonL]

Jan 16, 2019 21:55:10 GMT mbrown said:

...most of us settle for a smear of jointing compound as it only has to hold exhaust steam...

Out of interest what is your Jointing compound of choice?

[mbrown]

I use various compounds depending on the situation.

There's a rather thin nutlock grade of Locative which I use on fittings or small areas that may need to come apart, but for larger areas like the cylinder backplates (mine are similar to yours) I used Red Hermetite which is now marketed under a different but similar name. Most of it squeezed out when the joint was tightened, but it is steam tight. If yours are lapped to that standard I suspect you won't need any, or possibly a smear of nutlock to be on the safe side.

Malcolm

[JonL]

Bit of a bittersweet one this evening, not sure how I feel about it all.

Using the between-centres boring bar worked a treat. Good control over the geometry, everything seemed to go brilliantly for the first casting. On to the second casting...

I machined out the bore to the required diameter, before I got excited and removed it from the rig I inspected the bore only to find a pocket of what looked like sand. I had no idea you could even get that so deep into a casting, especially bearing in mind I'm guessing the former that went through the bore was probably wood? I can't afford to go buying new castings, and figuring that this one was effectively now scrap anyway, I kept machining away the bore until the pocket disappeared. At this point the bore has become 80 thou in diameter larger than designed. Thats around 2mm... the pocket of crud was 1mm deep.

Now looking at it I don't suppose its a great disaster, but as it happened on the second casting I've now got cylinders of two different bores. And as you know a small difference in radius adds up to a much larger difference in area and therefore volume.

I'm at a bit of a quandary now. I don't really want to remount my other casting; although it has enough meat to go out to the same bore I feel i'm tempting fate. That one is really good! I don't want to ruin it! When I put the bolts onto the cylinder covers there appears to be enough space to move the PCD out a little with no detriment. I don't want to get a new casting, aside from the money I haven't got (wedding looming) I would either have to get a pair or live with having two different types, as I have no idea who the original supplier was all those years ago. At the moment there is a great temptation to carry on with my odd bores, but I wonder how much that bore difference will affect the smoothness of the running.

I need to have a think. Thinking potion (Beer) will probably be involved.

EDIT: Could a sleeve be an economically and mechanically sensible repair?

[simplyloco]

Jan 18, 2019 20:09:32 GMT JonL said:

Bit of a bittersweet one this evening, not sure how I feel about it all.

Using the between-centres boring bar worked a treat. Good control over the geometry, everything seemed to go brilliantly for the first casting. On to the 

EDIT: Could a sleeve be an economically and mechanically sensible repair?

There's a sleeve in my Brit for exactly the same reason...
Do not worry...
J

[JonL]

Thats like a loads been lifted from my shoulders. If you are happy to use a sleeve then so am I!

[mbrown]

I have some old copies of ME from the 1930s and there is an article in there by LBSC about how to remedy exactly this situation.

First, I am sure he says that a difference in diameter between the two cylinders isn't really of much account - I suppose you might get two "puffs" louder than the other two, but I bet you'd hardly notice.

Where one bore is wildly out - say, so big that it is impossible to drill the steam passages or perhaps it's bell-mouthed or otherwise not good enough, he suggests boring it out sufficiently to fit a liner. LBSC was talking mainly about gunmetal cylinders, so he suggested machining a liner in bronze, tinning it all over, heating the liner and the block until the solder is molten and pressing the liner home. I see your cylinders are cast iron, so you'd be looking at drawing a liner in with a long bolt (like fitting a piston valve liner) or perhaps heating the block, freezing the liner and shrink fitting it. The question then would be, is a 1 mm thick liner (to take up the 2mm difference) going to work or will it be too flimsy to get the interference fit right? You don't really want to have to re-set either cylinder for re-boring as getting it exactly lined up could be tricky, so it might be worth trying a thin liner machined from continuous-cast iron bar.


Personally, I'd be inclined to live with the 2 mm difference provided there were no knock-on consequences in terms of steam passages, cover studs etc. and, from what you say, it sounds OK on that score.

Hope this helps.

Malcolm

[Deleted]

I don't think that cylinders of different sized bores are such an issue, it certainly wasn't on full size and they could differ by 'inches'? Like JB, you could fit a sleeve, bore the other out to the same size or leave as is? Other's more familiar with your loco may be able to suggest which is best...

Pete

[ettingtonliam]

Theres always a chance with castings that a bit of sand will have become detached from the mould and carried somewhere it isn't wanted by the flow of molten metal. The former for the bore will have been sand, bonded together to form a cylindrical core.
As simlyloco has said, you aren't the first its happened to, and I'm sure you won't be the last, but solutions exist!