Hi Paul,

From the little experience of soldering and brazing I have had the main issue was getting the piece up to temperature fast enough to avoid distortion, oxidation and melting. I did a bit of silver smithing and what I found worked well was finding some way to build a heat reflective shelter around the work.

I havent done much brazing / hard soldering work recently however I might suggest a rig such as this:

https://www.ebay.co.uk/itm/271433881978

You could make your own from superlux an asbestos replacement commonly found in woodburners and available from such shops as spares.

Paul,

I've owned a Bullfinch 404 brazing torch for some years now and routinely SIF bronze small (ferrous) parts in preference to using silver solder (which I do use for non-ferrous parts). The 404 does all my normal brazing work, although I do have a Sievert-like torch for boiler work.

The 404 uses just propane but manages to achieve a higher heat (over 950C) than a conventional propane torch. I have a few photos in my albums (under Brazing) that show a few parts brazed with this torch. Below is a part that I fabricated from laser cut mild steel and SIF bronzed together. It was an early attempt and clearly not perfect but that was down to me and not the torch. It's a very strong joint (stronger than silver soldering) and much cheaper than using silver solder.

Bullfinch 404 Brazing Torch

The price has gone up a bit since I purchased mine but if you braze small parts regularly then the cost is quickly made up by using normal propane and SIF rod (£31.20 inc VAT for approx 60 x 1 metre rods) instead of silver solder.

Regards,

IanT

Hello Paul, There are 2 things in heating metals, I will call 1 the spot temp the other is the amount of heat ! For brazing it you need both. I would suggest a propane burner depending on the size of work of about 1" dia and a 4Kg bottle as a good starting point. You will need an appropriate regulator, adjustable . The key is speed ! If the heat is too slow then the flux will die before your up to temp, it will also cause unnessessary oxidation. A hearth of a suitable size can be made using a refractory material, even theralite block will work in a 1" angle frame. The technique very much depends on the size and type of material. A small intense flame is ideal for thin steel or jewellery, but quite useless for a large chunky job. One point worth mentioning is that of bottle freezing, the gas starts as a liquid and just like boiling a kettle it needs heat to release the gas. Gradually if the gas is drawn off fast the bottle will get so cold it will stop boiling off the gas = no pressure ! Using a propane furnace, even with a 47Kg bottle, once the bottle was about 1/4 left, pressure would drop below a working value as not enough heat could be drawn from the surrounding air. Sitting the bottle in WARM water can solve this problem. Good Luck. Noel.

Sad to see Ian is perpetuating the myth, in all innocence no doubt, that I have tried my best to demolish.

Ian, please could you explain what you mean when you say that a 404 “manages to achieve a higher heat (over 950 degrees C) than a conventional propane torch”. Conventional propane torches typically have a considerably higher flame temperature than 950C, so according to your information that would make the Autotorch unusually cool.

I am perhaps fortunate to still be able to afford to use oxy-acetylene equipment as a hang on from the days when we owned an elderly Landrover which eventually needed the chassis replating with steel sheet salvaged from a a leaking domestic heating oil storage tank; a job that took many hours grovelling about underneath with a welding torch.

Today I am no longer able to afford the full size bottles of the past and now make do with the smaller versions instead but the ability to weld or braze as required it too useful to be without and SIF brazing of cast iron is a regular task. The high heat output of this system makes this a treat to use and gets things up to temperature quickly.

As a useful aid I have found that " coal paint " to revitalise coal effect gas fires works well in reducing the oxidation on surfaces that need protection.. The product I have used is called Hotspot Coal Paint, an aerosol can that claims to protect to better than 1250 degrees F. In practice I have had acceptable results at a good red heat which is well above that claim

Brian

Personally I prefer oxy-propane with a Type 5 Welding Shank and a Propane mixer, Adams gas do rent free oxygen at very reasonable prices through a number of different gas suppliers who all sell Propane as well.

Much more easily available and somewhat less hazardous than acetylene to handle (and no insurance issues) if you're nervous of it, but also cheaper and better suited to area heating (as required in brazing) due to the softer flame.

The only manufacturer I'm aware of still doing manual brazing for production work runs on oxy-propane (piped to each bay, with economisers), and I'm sure they'd have switched to the Venturi-type torches for cost reasons if it was comparable or superior in terms of performance.

However in my opinion, it's extremely rare that brazing is actually a better option than welding, given the choice of process.

So in your shoes I'd think an inexpensive inverter welder was a far better use of space and money than a brazing hearth, especially as both MIG and TIG can be used for brazing with suitable wire if you really need it for joining dissimilar metals…

Controversial with some forum members I'm sure, and no good at all if you're already committed to a project which requires a "silver soldered" boiler, but it's an additional perspective at least.

Edited By Jelly on 05/06/2023 13:39:08

One of the tools used in the jewellery trade is an electrolytic brazing torch. Mains electricity (via transformer & rectifier) splits water uinto hydrogen and oxygen, and these are fed to the torch, mixed and whoosh. Concentrated heat (no nitrogen getting in the way) and no impurities.

But the main way to get the best value from any source of heat is to keep the heat in. This relies on good heat-proof insulation, as well as a torch which is the right size for the job. Charcoal is a good insulator, produces its own supply of extra heat, and uses up any spare oxygen so the flux doesn't have to work so hard.

Cheers, Tim

Edited By Tim Stevens on 05/06/2023 17:52:54

Hi, one has to understand the difference between temperature and heat, a steel spark from welding will have a temperature close to or even above 3000 C, but will have very little heat, and both will dissipate very quickly, as it will have very little energy. Your central heating boiler on the other hand, will have a good deal of heat, but the temperature will be relatively low in comparison, and the temperature will fall quicker than the heat dissipates, but both will be extremely slower than the spark. The temperature of any flame will be the same whether it is a small or big flame, but a bigger flame will give out more energy and thus raise the temperature of your work, quicker than a small flame, but of course, if the energy is conducted away faster than a small flame is giving out, your work may never reach the temperature that you require. Therefore, large section of metal will require much more gas than a smaller section. Oxy-Acetylene has the hottest flame, but a small Oxy-Acetylene flame will not compete in raising the temperature, with a large Propane flame on larger sections, and may never get it to the temperature you want. Heat will always travel to colder things, not the other way round. The temperatures that the Bullfinch has quoted, is what is possible to raise your work too, and of course the flame temperature will always be hotter than what you are heating.

Regards Nick.

Sorry for precising your request Paul but your requirements seemed very similar to mine (at the time I was looking for a suitable torch) which is why I suggested the Bullfinch 404. As Bullfinch state (in terms of the temperature of the "flame" ) – the 404 sits between an 'ordinary' propane torch and a MAPP torch.

They advise that a propane torch "under general conditions" operates at 800C but can get upto about 1000C. The equivelent temperatures for the 404 are given as being between 950C and about 1200C. I use SIFbronze No 1 which has a melting point between 875-895 Degrees C. Clearly (as Jason has demonstrated) a propane torch can melt SIFbronze but (in my experience) the 404 makes SIFbronzing that bit easier. I also feel (perhaps subjectively) that it is quicker when silver soldering too.

As Nick suggests, the heat output (Kw power) point seems to me to be a little bit misleading. The Sievert head that Jason used has a power rating of 7.7Kw but it also uses about twice the gas (600g/hour) as the 404. A higher power head can obviously replace any heat loss (from the part) more easily than a lower powered one.

The mid-power head I normally use (4104) is a 4.76Kw head (and uses 340g/hour). There was a larger Bullfinch head (4105) available that produced 6.16Kw (and used 440g/hour). Clearly the 4105 produced more "power" than the smaller 4104 but both heads operate at the same temperature. In fact you can get a lower powered head for the 404 (4103) that is a 1.84Kw (133gm/hour) alternative. So different (heating) powers but the same temperature range. Clearly, larger parts need more power but this is also very much subject to how well you can conserve heat during the braze.

If you need to do larger parts then perhaps (as Jelly suggests) welding is a better approach – less so for small (more detailed?) parts though.

That's it. You asked for our thoughts Paul and these are mine. Others may differ in theirs of course.

Regards,

IanT

Below – one of two endplates for an engine-building frame I built from a laser-cut kit. All the sub-assemblies were SIFbronzed but the final assembly was admittedly done using silver solder (I didn't have a brazing hearth to keep the heat in at the time). It's 6" x 4" x 7" (in 2mm mild steel) but all done with the Bullfinch.

The advantage of the Sievert type torches is there is a far larger range of burners available so if you dis want to braze something larger or of thicker section then it is just a case of buying the burner not a complete new handle/neck/hose and burner or a welder which you would need to do if the 404 were found lacking. I very rarely change burners so ther is really no need for a small one, the standard can do all but the largest work.

I'm not sure if gas consumption is all about what the torch burns in an hour, the higher Kw rating will mean that the torch gets the job upto temp in less time so therefor you are not running it for so long which will negate any savings from a lower hourly usage.

It would help if you could give an idea of the type of things you want to make, one persons small is anothers large, even a boiler could be anything from a 2" dia pot boiler to a 7-8" one for a 3" traction engine.

If you are mainly doing thin sheet work like Ian shows or small section stuff then the 404 may do you. I wonder if the 404 would be able to silver solder let alone braze something like this which is 10mm thick plate at the ends and 5mm wall tube. Silver soldered with the 7.7Kw Sievert?

Or a flywheel like this with 14mm dia rim and 8mm spokes

At the other end of the scale something like this small 20mm tall brass fabrication can be done with the same 7.7kw burner. The 630deg temp of silver solder means you are a long way from risking damage to the brass part which is far more likely at brazing temperatures. It starts to get to a point where the brass and bronzes become soft enough to distort even if thin parts don't melt particularly if you ar eapplying any form of clamping to hold the items together.

I don't know how well someone could mig braze that but I would not want to try. The other thing to think about if you did go for the welding or mig brazing option depending on what you want to make is how well does the filler material flow. Weld will just produce a fillet and what I have seen of mig brazing it again seems to form a fillet and not flow like the filler rod has in my test piece posted earlier. So if you want to fabricate say a cylinder for a model steam engine and solder on a block to form the port faces will you be able to seal that to the curved cylinder wall so you don't get air/steam passing between ports, certainly not with weld and I think unlikely with tig brazing.

Typical cylinder fab, 24mm bore brass and 660 leaded bronze again stuck with silver solder and 7.7Kw burner

Edited By JasonB on 06/06/2023 07:25:46

This is a fair point about MIG and TIG brazing, whilst they preserve the option to join dissimilar metals, they do use a different approach to joint design, which is more closely aligned to welding; although for TIG brazing it certainly is possible to do conventional capillary-action brazing of small parts, (much in the same way you can with carbon-arc if you're a masochist).

In your example shown above, making the part using one of those processes would ideally have a joint with a wider fillet (toe-length 80%-100% the thickness of the base material), and owing to the size would almost certainly need to be done with a small TIG torch (definitely a No. 9 with a small cup, possibly an MR70 micro torch would be a better option for someone working on items of that size all the time).

I would however fully expect the TIG Brazed example to take a fraction of the time required, and require no pickling or further clean-up.

.

The point about the choice of joining process dictating joint design then carries over into the cylinder example, where a welded approach would be to use a weldment with individual joints at each port welded from the inside (or both sides depending on thickness and shape) then machine as a second op if the surface finish of the ID was critical…

Which is far more economical (and again faster) on a larger item, but challenging to downright impossible as you get down to very small assemblies like your 24mm bore example.

.

Very small items are generally the one place that brazing (and in your shoes I would go for pre-placed powdered braze in a furnace rather than use a torch because I'm much too ham-fisted for such small parts) can achieve things that conventional welding processes can't.

If we ever get to the point of laser welding machines being affordable at a consumer price point, I would expect the scope of the advantage brazing has there to diminish somewhat further, but there's a way to go yet.