Making Brass Fittings Fit

[Serjt-Dave]

Hi All.

I'm working on trying to fit cast brass fittings namely the safety valve bonnet and dome to the boiler but I'm having trouble getting a good all round fit. Is there a way of getting them to bed down on to the boiler so that the edges touches all the way around without any wobble etc. Or is it a case of filing, hammering, swearing at them till they do?

All Best

Dave

[Jol Wilkinson]

Dave,

I cut away the casting sprue on the underside of the boiler fittings, wrap a piece of emery paper around the boiler/smokebo (abrasive side out) and rub the casting along/around this to get a close fit.

Jol

[Will L]

Hi All.

I'm working on trying to fit cast brass fittings namely the safety valve bonnet and dome to the boiler but I'm having trouble getting a good all round fit. Is there a way of getting them to bed down on to the boiler so that the edges touches all the way around without any wobble etc. Or is it a case of filing, hammering, swearing at them till they do?

Start by fettling the cast brass item as best you can, or as Joel suggests. Even then you often don't get a perfect fit, but you can certainly improve things a lot.

Once that's done, I like to bed them down on the boiler in 100 degree low melt solder.

This gives you two advantages.

The first is the fact that you can actually get a very heavy/solid bit of brass firmly soldered in a bed of solder into the boiler without unsoldering everything else you've done. And do it with a soldering iron. Unlike other low melts, 100 degree stuff wets brass so you don't need or want to tin everything first.

The second is that it is a good filler so you can ensure there are no gaps round and under the edges. Once its cooled down, even with 100 degree stuff, this can take a while, you then clean the solder on the boiler back to the profile of the casting. Finally you feather the edge of the brass with a round file. You can reduce it to practically nothing, so much reducing the lip round the edge. With care I suspect you can file right through the brass so you actually only have the solder left, but this will be tricky to do well as the solder is much softer than the brass.

[davebradwell]

The problem is very common because as the casting cools it shrinks slightly and this pulls the sides together and reduces the radius. I would advise taking great care as it's possible to push the curve off-centre and you get a very funny looking part.

My chief weapon is a fine half round file with a curve that matches the most common diameter of boiler (5-9 over cladding) on my choice of prototypes. It has a slight taper so there's room for adjustment but it's also handy for banjo domes sitting on a tapered section. Will also admit to turning a mandrel to same size as boiler to avoid damage to model during any fettling - I prefer to dress the outside before fitting. Smooth files are essential here to avoid unsightly scratches.

DaveB

[martin goodall]

It is worth bearing in mind that boiler fittings were not always a particularly accurate fit on the prototype.

[Serjt-Dave]

Thanks guys for your advise.

I have done what was suggested and this has worked really well. The safety valve bonnet has a couple of deep nicks fore and aft which makes it look like the rim had lifted but it's not. Once soldered into place I will feather out these edges. I wasn't sure about cutting off the sprue at first but pulling my big boy pants on my whizzy disc soon removed them. The boiler was made from a brass tube and the piece that I cut off to allow the motor and gear box to fit I used this to do the fettling on as also I had already fitted the boiler bands on the boiler.

I'm now in a good place to solder the boiler in place using high temperature solder and then use low melt solder to attach the dome etc. I'll have to remember to use low melt on the other smaller fitting when they get fitted.

Thanks Again

Dave

[Edward45]

Dave B is quite right about shrinkage, a problem made worse with larger casting. Domes in particular can come out looking very odd. Although more difficult to cast, hollow castings do mitigate the problem, but then you loose the locating spigot. I have looked at a two part casting. This would have the item itself cast hollow with a separate locating fitting, one end fitting into the casting the other end with the spigot. Would put the price up a bit!

[davebradwell]

Slaters do my brass castings and I made cores so that chimneys have a hole through and domes are just a thin shell - probably just over 1/2mm thick. Locating spigots aren't any benefit as they prevent the bottom curve being adjusted to match the boiler, and chimneys look pretty dumb if they are cast solid. It's easy to end up with too large a hole through the chimney in order to get a good casting and the last couple of locos I've made have had correct size liners fitted. A locating spigot requires the hole to be in the correct place on the boiler and this is easier said than done. Any reasonably sized hole interferes with the rolling of the boiler anyway. If you must have a spigot this could be turned as a separate part and fitted later.

Use of cores does have a side effect in that the diameter of the fitting shrinks slightly less than the height but a twizzle in a bit of emery sorts this out while improving the surface.

DaveB

[Crepello]

Slaters do my brass castings and I made cores so that chimneys have a hole through

Could you expand on this please? I.e. is the core a tooling aid you've supplied to Slaters?

[Terry Bendall]

davebradwell wrote:
Slaters do my brass castings and I made cores so that chimneys have a hole through

Could you expand on this please? I.e. is the core a tooling aid you've supplied to Slaters?

I expect Dave will be along later to explain exactly what he does but the term "core" in this contect referes to casting of metal. A core can be any shape but in this context will be cylindrical. In traditional engineering casting, a core is usually made from silica sand bonded with oil and other substances to give cohesion. The core is shaped in a core box and has to be baked in an oven to make it solid. The core has to withstand the molten metal being poured in, but brittle enough to be removed once the molten metal has cooled solid. There needs to be supports made in the mould to support the core which are known as core prints. A fairly easy way of making a core is to use Plaster of Paris.

Terry Bendall

[davebradwell]

Thought I might be able to just put up an old drg but they're in faded pencil and covered in calculations so try this sketch:

Img251.jpg

The upper drg shows the completed chimney master with a protrusion each end to form the "core prints" in the rubber mould. This is removed from mould and the lower "core" dropped in. When wax is squirted into the mould a hollow component is produced after the core is extracted for re-use. For a dome you get a blind hole by just having the LH core print - there will be a limit on how far the core will cantilever out but tall NER domes are fine.

A degree of precision is required as if the core is oversize or a poor fit then the mould won't close properly. Undersize and you'll get flash around it. A good surface finish is needed so the core can be extracted from the mould and you can't have an undercut where the taper meets the smaller diameter. My lathe has imperial dials so replace 0.312" by 8mm with 3mm groove for metric. Groove enables core to be gripped and pulled out. I put bigger chamfers on core than master.

Can confirm that top of a B1 dome is 20-25 thou' thick - 0.5/0.6mm so that can be taken as a typical figure.

DaveB

[Crepello]

Thanks, Dave, for putting in the effort to illustrate that. It's pretty much what I'd
visualised, but wasn't sure it was feasible for a contractor to adopt in production.