Equipment for brazing?

The last time I tried brazing was at school, we had natural gas torches using air compressors. I never seemed to get enough heat in to my work piece quickly enough to make a good brazed joint, silver soldering no problem, but using brass filler rod no chance!

Now in present day, what do people use to braze in their model Engineering work shops please?

A plumbing friend has suggested ‘Mapp gas’ saying that it’s a bit naughty for soft soldering in the plumbing world, but might do the trick for the higher temps required for brazing small items. (You will be pleased to hear that I am not into major constructions such as boiler making).

Other alternatives seem to involve small rigs of oxygen plus some other fuel gas which looks a bit more serious, I have used oxy-acetylene for welding as an apprentice and as a result a healthy respect for handing such gasses, their bottles and fittings was drilled into me, I’d rather not get into this sort of gear. Not least because it wouldn’t get the use to justify the cost/risks.

I am also looking for ideas for a small portable hearth.

As I say, really just for small jobs.

I would appreciate your thoughts.

If it is small items then would silver solder do the job, I tend to use that for all my fabrications. And technically silver soldering is brazing.

It is not so much the temperature of the gas that matters but the total amount of heat that a given burner can put out so a reasonable size propane torch can get parts hotter than a MAPP one

Small hearth can be made with a few Vermiculite blocks and plate, have a look at CuP Alloys 

Edited By JasonB on 05/06/2023 09:40:40

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.

+1

In my small torch equipped workshop, the problem is getting enough heat to melt Brass into the job fast enough. Too slow, and the gradually building heat destroys the flux before the job is up to temperature. The flux cleans the metal and then keeps it clean by keeping air out until Brass flows. Cooking Brass slowly also causes Zinc to evaporate, leaving high-melting point Copper behind. Speed is of the essence.

As most metals soak up a lot of heat it can take a small torch long time to get a joint up to temperature, especially as metals are also good at conducting heat away. Best way to heat quickly is with a whopping big torch, but a heat reflective shelter helps enormously.

As far as I know true MAPP gas (methyl-acetylene-propadiene propane) is no longer available. It was produced as a by-product of a chemical process that's been replaced by a more efficient equivalent. The new process doesn't make MAPP on the side, and MAPP made from scratch is too expensive to burn. What's sold today as MAPP today is probably a substitute that's a bit hotter than the usual mix. Worth trying - might be enough to help a small torch to do the job.

In terms of power output, small blowtorches are tiddlers:

• Small DIY camping-gaz cartridge type - about 600W
• Sievert cartridge PowerJet - 900W
• Sievert General Purpose Torch (fed with domestic sized Calor gas cylinder - 7700W
• Big Sievert fed from a large gas cylinder - over 100kW

A job that's impossible with a 600W blowlamp is easy with a quick blast from a 7.7kW torch.

Dave

Edited By SillyOldDuffer on 05/06/2023 10:48:41

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

PS

A useful source of SIFbronze No1 is here.

SIFbronze No 1

No connection except satisified customer etc. Interesting to note that the price of SIF doesn't seem to have gone up too much - whereas 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.

You can of course use an old carbon arc torch with SIFbronze if like me you still have one from the 70's.

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.

Well, I'm not claiming to be an expert Bill, although I have attended a welding course, which included oxy-acetylene brazing, which is where I first used SIFbronze. I have also owned the 404 for quite a while now and been very happy with it's performance. In practical terms, the Bullfinch can manage brazing work that I couldn't manage with one of my 'disposable' gas torchs or with the heads I have for my Sievert (like) torch.

All I can do is suggest that Bullfinch probably do know what they are talking about and they state this in relation to their Bullfinch Autotorch (e.g. the 404).

Regards,

IanT

Technical information: flames

Although the flame temperature of an LPG (propane or butane) air burner at the hottest point is approximately 1900^oC, the actual temperature a heated component can attain is much lower than this and depends on the burner, and the thermal properties of the component and its surroundings.

Acetylene-air and MAP//Pro gas-air will give flame temperatures 200-300^oC above this and component temperatures 200-300^oC higher too.

The Autotorch brazing burners can give temperatures up to 950^oC under general conditions and up to 1200^oC in ideal conditions only. They can therefore bronze braze under most conditions as well as being able to make faster soft and silver soldered joints. They are particularly good for hard/silver soldering of copper pipes.

The Autotorch 2300 series burners, the Autotorch System 2 burners and the Standard torch burners can give temperatures up to 800^oC under general conditions. They are, therefore, ideal for soft and silver soldering and lead welding, but cannot be used for bronze brazing.

The Autotorch acetylene burners can give temperatures up to 1100^oC undergeneral conditions and 1350^oC under ideal conditions and can be used to do bronze brazing as well as the other general uses of torches discussed above.

MAP//Pro gas used on the FirePower, AutoPerformer and Performer torches will generally achieve temperatures between the Autotorch brazing burners and the Autotorch acetylene burners.

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

As I said earlier and Bill has also said it it not so much the temperature as the heat output in watts or more likely Kw that matters. The 404 is only rated at 4.76Kw but the standard size 28mm Sievert burner is rated at 7.7kw. So although the 404 will melt the rod for smaller jobs on thinner material it will be found lacking if your fabrications are a bit larger so you may end up having to buy a second torch if they do which cancels out any cost savings.

Just stuck a couple of bits of scrap together, 2.5mm plate offcut and 16mm dia bar end, Sif flux and unknown brazing rod

I'll post a closer shot when it is out of the pickle - the higher temp does need a bit more clean up than silver soldered joints.

Where the smaller flame size of oxy is useful is when you want to form a large fillet as it gives you more control of the alloy as you build up the fillet and you are less likely to get a big blob of molten brass on your boots, also makes it easier to get in close with the rod as the heat bounceback from larger burners can be an issue but I suspect fillet brazing is not such a requierment for model work as it would be for bike frame building

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

Here it is cleaned up, quite a nice fillet which this type of rod tends to leave, flowed out more onto the flat material as that got hotter first despite trying to get the heat aimed at the round stock.

Looks like the higher temps affected the round stock which may well have been leaded steel which has now got quite a pitted surface, the flat is unaffected. So that is another thing to think about when choosing how you are going to join parts, some are better suited to a particular method than others.

Clean up is another consideration on small fabricated parts for models, I find the fillet left by silver soldering and even one like above do not really need any further work and if they do it's quite soft metal so easy to file or use a dremel on, welding (at least my efforts) need a lot more clean up and you can't weld leaded steel that I will often include in a fabrication.

Edited By JasonB on 05/06/2023 14:22:40

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

I managed to braze the frame of my wheelbarrow using a MAPP gas torch. Would have been better if Id removed the metal body from the frame as it did conduct away heat, but I needed it to align the frame. I did manage it and the braze is still holding strong. Better than the original weld!

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.

Thank you everyone, I can see that getting heat in quickly is the trick, like soft soldering, the mass of the iron tip makes all the difference in transferring heat energy in quickly without spoiling all of your prep.

so if I can recap:

I could just stick with silver soldering,

a propane torch like the 404 plus a good heat reflecting hearth should do the trick for light brazing, but a bigger burner like the sievert might be a better option.

lower melting point sif rods and flux might make brazing easier, (thanks for the demo @JasonB )

Mini ‘oxy-Acetylene’ opinion might be fine for small works, high temp modest kW output

TIG is an option, wow I had not thought of that one!

it’s quite a lot to consider, but thank you for the advice, it’s a good way forward from my current understanding

Paul McD

Edited By Paul McDonough on 05/06/2023 21:11:34

Edited By Paul McDonough on 05/06/2023 21:48:53

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.

Thank you @ianT your explanations are very helpful.