How on earth do they do that? Machining a chimney

[JonL]

I confess, I've wracked my brain and I still can't work it out. How on earth do you machine the saddle shape for the base of the chimney on a live steam locomotive? I've tried the search function and couldn't find much either here or Google. It's really bending my brain. The only thing I can think is that you machine what you can and then hand finish, but I've seldom seen a badly finished one.

Can someone please put my out of my misery?

[andyhigham]

You essentially have two choices, do it by hand or CNC

[JonL]

Hand it is...

[dscott]

The gunmetal castings are almost there and with a NEW file or two can be worked on.
Cast iron is messy but cheaper!
I find a spray can of gloss black shows any problems against a white sheet behind the vice as you work.
Other ways is to bash out of copper for the base and build it up from sections placed over a central
core which covers the petticoat pipe. angled inside to 17 degrees would work well.

David and Lily recovering from a day in Swindon, the town is looking rough.

[Jim]

I mounted the Britannia's chimney on a mandrel turned to fit neatly into the chimney and secured with some Loktite for extra security.

The saddle was then fly cut to sit neatly on the smoke box. The more tricky bit was in forming the flare at the base of the chimney, that's where the jeweler's files were used.

Sadly I don't think i took a photo. 

I should add I'm a Luddite where CNC machining is concerned

Jim

[silverfox]

I have a similar problem. the smokebox radius is ok

The 'edge' at the front and rear is nice and thin

The 'bits hanging down at the side' are very lumpy. So it is hours with a file then!!

[simplyloco]

This is what I did, but you have done that already!
John

modeleng.proboards.com/thread/4388/brit-progress-smokebox

[barlowworks]

I have only done this in O gauge but when feathering in the flare I initially used a carborundum disc in the mini drill, jewellers needle files to get a smooth finish, a rubberised carborundum grinding disc again in the mini drill to get out the final file marks then finer and finer grades of wet and dry until smooth. Worked fine in O gauge, just more of it in 5 inch gauge.

Mike

[stevep]

This cropped up a while ago, and we talked about the method described in ME by Sir Ron West (?)

[JonL]

Ok, these are all good tips, it sounds like for me its going to be hard graft. I'm just a little worried about getting it wrong and ruining a casting. I've got Gunmetal.

Thanks to everyone yet again for your help; I am in your debt.

[JonL]

Apr 23, 2019 8:55:59 GMT simplyloco said:

This is what I did, but you have done that already!
John

modeleng.proboards.com/thread/4388/brit-progress-smokebox

I'll be reading and re-reading that!

[mbrown]

Apr 23, 2019 16:24:51 GMT stevep said:

This cropped up a while ago, and we talked about the method described in ME by Sir Ron West (?)

Sir Ron East (wrong point of the compass!) - on his Boxhill. ME c.1968.

Malcolm

[steamer5]

Hi Nobby,
Heres another way of doing it.

Chimney Base by Kerrin Galvin, on Flickr

I THINK a side & face cutter would work for this once you had the shape correct, then maybe you could rotate it SLOWLY & the face cutter would clean off any ridges providing they are small.....

Cheers Kerrin

[JonL]

Thanks Kerrin, I think that might be beyond my means and brain power. However it's certainly interesting to read about.

[andyhigham]

You could use that method but use a sliding connection to a rotary table. Add a spring to keep it in contact with the peg