Electroforming a dome

Hi, I want to build a model of an old French 2-4-0, one of it's features is a very large dome nearly the diameter of the boiler. I have been wondering how to make this and remember reading some articles in ME a few years back on making an early aero engine and the writer electroformed the cylinder water jackets. The problem is I can't find it. Can somenone please tell me where the articles are? I've looked in the index but no results. Thanks!

Stephen Wessel's ENV aero engine. ME 4577, 4579, 4581 refer (2018)

Thanks Nigel, that's the one. I thought it was an Antoinette which explains why I couldn't find it.

An interesting process ! but casting might be easier, though for this item. The thickness would leave you with a very thin and easily damaged component in soft copper. One might also consider beating it out of sheet. Keep us posted. Noel.

Hi, I've thought of spinning it from a piece of 3 inch copper tube which i happen to have but I don't know how to spin something inwards. I've managed to spin a dome from a sheet but not this size. LBSC says bash it out from sheet but I've always suspected that he over simplified how to do things!

Copper should be easy enough to bash out from sheet using a sandbag and domed hammer etc then moving to a former of some sort for finishing maybe. You would have to keep annealing it as you went I should imagine. Herbert Dyer, a prolific contributor to ME for about 50 years, wrote a good book on the subject that is still available secondhand at reasonable prices if you shop around a bit (and avoid Amazon!). LINK

Nigel, I haven't got the first of those articles. Would it be possible please to send me a copy of the article or if not could you see if there are specific instructions in it as to how to mix the electrolyte?

John, I have watched several youtube videos where a dome is formed in metal using hydraulic pressure. Two thick steel plates with the copper sheet sandwiched between. The top plate has a hole the diameter of the dome, the bottom plate has a hole for a grease nipple. When the whole lot is bolted together and a grease gun is used to pump grease into the former, the copper, properly annealed, should start forming a dome. I have seen a video where a headlight for a motorbike was formed this way.

Lee

That;s interesting Lee, I've someone use a sandwich of wood and bashed the dome shape into thin steel with a mallet.

John,

Some of the domes I have seen formed were over a foot wide, in steel. They weren't a half round though, it was the making of a musical instrument called a hand pan. Nice sound.

Lee

A few years ago I bought a rothenberger boiler test pump from a fellow, he was neither a plumber or boiler inspector so I what he had used it for - he showed me some very interesting 2 stroke expansion boxes. You drew a development of the shape cut it out of may be 18 or 20 gauge steel sheet, welded the edges together with a hydraulic fitting in one end and then just blew it up ! equal pressure at all points so no wrincles - perfect ! Once you got the sheet thickness right it would be easy. Nornal hydraulics will run at 2500psi and hand pumps to 10,000psi so there is HUGE force available ! Noel.

One full size loco I worked on had the dome made in 2 pieces - a hemisphere top silver soldered or brazed to a cylinder. The joints were caxtellated and easily seen close up. The bottom of the cylinder was flared to sit on the boilrt.

Martin

All you need is a former and some sheet gilders metal. Very easy to use if you have even rudimentary panel beating skills.

Andrew.

I think that Jewellers use things called Doming Blocks and Punches but I do not know if they come in large enough sizes or would suit your purpose.

Might be worth looking into the idea with jewellery tool suppliers (or make your own?).

Edited By V8Eng on 03/08/2022 16:44:08

I hadn't thought of gilders metal. Is that an alloy of copper?

I know it as gilding metal - it might help to find a source. It is a copper alloy which works rather like silver, less inclined to split as brass can. Heat to redness to anneal, and if possible, leave in the work-hardened condition if the part is subject to attack by housewives or small children.

There are Jewellers Supplies firms which are likely to carry stock of sheet.

Cheers, Tim

Try Edwards Metals in Birmingham - They list it.

Edwards Metals