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 Post subject: welding vs brazing
PostPosted: Sun Oct 09, 2011 4:00 pm 
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So it would seem that brazing would be more akin to soldering than welding, although soldering is done at a much lower temperature. I've read that most bicycle frames are built with tubes fitted inside joints then brazed, so filler metal filling the gaps at high temperatures. It's also said that there is less distortion with brazing than there is with welding, less heat localization. Lotus, among others, used brazing for chassis building instead of welding. I was looking at pictures of the Maserati T61 birdcage chassis and it's hard to believe the thing was brazed, and tiny 10-15mm tubes at that. Built so that all joints are at the ends of the tubes so to be in compression or tension to minimize bending forces. I've further read that the chassis is fairly delicate and needed to be checked regularly.

Can a brazed chassis stand up to use on the road? How frequently must they be repaired?

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 Post subject: Re: welding vs brazing
PostPosted: Sun Oct 09, 2011 6:40 pm 
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Brazing heats the area less than gas welding, but mig or tig heat a smaller area than brazing to a greater temp.

IMHO, mig is the way to go for building a chassis or exhaust system. It is much faster and cheaper than brazing. For detailed precision work, tig is best.

Also, mig and tig are appropriate for butt welding, but brazing should be lapped or used for sealing and / or support of a welded area, such as a tank or header tube fillet. Brazing can also be used where welding cannot, such as dissimilar metals.

Once brazed, forget about ever welding in that area again.

Paint will have a tough time adhering to some brazing rods.

In general, strength-wise, a butt welded joint is not as strong as a lap brazed joint.

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 Post subject: Re: welding vs brazing
PostPosted: Sun Oct 09, 2011 7:00 pm 
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It's a big subject. I think everything that MV9 said is correct. I would add though that cars are usually ( at least the ones I've seen ) fillet brazed. That would include the Lotus 7 / Caterhams and the Formula Fords I've worked on. You can certainly do good work that way and it is something I've enjoyed because the brazing material looks like gold when your melting it on. :)

You can disassemble brazed parts more easily. That shouldn't really come up all that much though.

MIG will do a good job on these chassis and it is economical and useful for other things.

You can also get brazing wire for your MIG machine and I've been curious about that but not so sure there is any real advanage for our chassis...

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 Post subject: Re: welding vs brazing
PostPosted: Sun Oct 09, 2011 7:15 pm 
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Historically the English seem to use brazing and silver soldering more, even on weapons from WW2.
I'm building a 4130 chassis now and silver soldering tabs beacuse I don't like the idea of pounds of slabs welded to tubing as is normal for sheetmetal tabs.

Horizon', what kind of material do you braze with?

BTW, silver solder looks nice when flowing too. When it gets to the shiny melting point it's a satisfying experience!


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 Post subject: Re: welding vs brazing
PostPosted: Sun Oct 09, 2011 10:05 pm 
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Location: Sunny-Okanagan, Canada, eh?!
I understand that traditionally, these were "Bronze-Welded," not "brazed." The filler rod is different, and can actually be fillet welded, unlike typical brazing.

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 Post subject: Re: welding vs brazing
PostPosted: Mon Oct 10, 2011 1:14 am 
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A Locost is typically built from 1" square tube, some larger depending on how much power the builder plans to put through it. The birdcage used small tubes, 15mm being approximately .625" tubing (.5906). I read that it used chrome-moly tubing so they could get away with a thinner wall and less weight. 10mm tubes are tiny (.3937).

I suppose the important question is this. If a Locost were built in the style of the Maserati T61 'birdcage', smaller tubes, maybe .625 and .500, maybe some larger in key areas, and welded, would it, could it be durable enough to be street driven? It would have 2-3x as many tubes as a typical Locost chassis. I get that racing chassis' are usually built for cars that drive on relatively smooth circuits.

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 Post subject: Re: welding vs brazing
PostPosted: Mon Oct 10, 2011 1:34 am 
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The brazing done by Caterham and many other race chassis builders is called Bronze welding. It's done with a nickel/bronze rod, not silver bronze which is used for bicycle brazing, and much easier ti find, or any other alloy with bronze. This rod will build a fillet and adhere to the steel at a pounds per sq inch a good bit above the strength of the steel, can't remember the numbers right now. It is strong enough to rebuild broken gears too. that's one of it's stated uses. I have some of the rod and will post the name and number later when I can get to the box. It cost's about $185 for a 5 lb package. That is enough to bronze weld a chassis though. I did mine that way. I find that I can do a much neater job brazing over using my mig. I don't have a tig. I tacked the corners with my mig and filled in with the brazing. So far it seems to be working fine. I have 600 miles on the car so far. :lol:
Wayne


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 Post subject: Re: welding vs brazing
PostPosted: Mon Oct 10, 2011 2:27 am 
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Quote:
The brazing done by Caterham and many other race chassis builders is called Bronze welding.


I think terminology varies. My recollection is that brazing refers to work that does not melt the base metal.

Elewayne did you use a gas type flux? I've heard that they use something that connects to the torch's hoses and fluxes thru the flame. It's not common gear in the US though, so far as I know.

Since the fillet is larger then the gage of the metal I'm not sure very exotic material is required for 1018 square tube. I'm interested in this, but researching is tiring. There are many types of brazing materials, in a variety of shapes and sizes. When you go to the suppliers sites, you can be confronted with a list of 40 types for similar purposes.

I've only used fairly generally available rods and also some silver soldering and copper/phosphorous rods.

Elewayne how much gas does it take to weld a frame? Please do get us the type you used. I know my local Lincoln dealer has a few types and said they would order rods too. They didn't have nickel/bronze MIG wire, but it seems silicon bronze is becoming available. Maybe that had a problem though, perhaps brittle?

Some are better for fillets and others for wicking into joints. This is generally in the description. One clue is the range given for the melting point. Materials with a large range say 100-200 degrees from starting to melt to completely melted will make better fillets. The "solidus" temperature is when it hardens and the "liquidous" temperature is fully melted.

As for Charles original question about frame tubing size, if you build a Locost out of birdcage size tubing, it needs to look like a birdcage to work - if that makes sense. If you used larger then normal tubing you have less tubes. So a Cobra with 2 4" tubes or a Locost with one or two dozen 1" tubes or a birdcage with more then a hundred ( ? ) tubes. If you carry this to it's logical extent, that's how you get monocoque cars out of sheet metal. A Lotus Elan or Europa sort of has 1 tube as it's backbone.

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 Post subject: Re: welding vs brazing
PostPosted: Mon Oct 10, 2011 3:21 am 
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Miatav8,MstrASE,A&P,F wrote:
Brazing heats the area less than gas welding, but mig or tig heat a smaller area than brazing to a greater temp.

IMHO, mig is the way to go for building a chassis or exhaust system. It is much faster and cheaper than brazing. For detailed precision work, tig is best.

Also, mig and tig are appropriate for butt welding, but brazing should be lapped or used for sealing and / or support of a welded area, such as a tank or header tube fillet. Brazing can also be used where welding cannot, such as dissimilar metals.

Once brazed, forget about ever welding in that area again.

Paint will have a tough time adhering to some brazing rods.

In general, strength-wise, a butt welded joint is not as strong as a lap brazed joint.


Sure everything here.

Brazing/Bronze is an old way to do it because it was the only way for most back then and now firms are saying this and that about it purely for the purpose of charging you more money so you can say it was done the same way it has been "traditionally" and some even use "art" in their spiels - it is not, it is a slower, more fussy, not as strong way to do it as well a everything above.

Modern MIG, TIG and ARC are superior in everyway and ARC welding is still the best but can be difficult for thin wall applications and of course messy to clean/chip after. Many engineering jobs specify ARC in certain critical areas, well some of our jobs do anyway but 95% is MIG.

MIG and a 4" grinder is your answer, TIG if you want it pretty.


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 Post subject: Re: welding vs brazing
PostPosted: Mon Oct 10, 2011 10:17 am 
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Last week, I began looking up pictures of the birdcage to see how much detail I could find, then Friday night, sat down to see if I could model one. When I finish it, I plan to put it up on grabcad. Mine is not a great match as no good measurements are available, and I don't have the time to model period specific drivetrain. I built it keeping in mind what is available today and what would likely be used if the car were actually built. My model has an 86.6" wheelbase, a correct track width front and rear at 49.2"/47.2". From there, I estimated dimensions for the frame, based on 16" wheels and guestimating 24" diameter tires. The outermost frame tubes are 50" on center, an intended track width of 86.6" and an overall length of 114". The floor behind the rear bulkhead is angled up at 6 degrees, while the section in front of the front suspension is angled up 5 degrees. A V8 engine for this chassis is probably a bit too big, but I think it would probably take a Duratec up to a 30 or maybe even a 35. Width and length should be fine, although I'm not sure about height. A Duratec 35 could be had for $450 or an EcoBoost Duratec 35 for $3000. If the 35 fit, that would be a fun little car.

The body panels in the last picture are not in the right position. They're positioned in the center point of the tubes instead of on the surface of them. The arc for the sides is also not correct. It would need to be separated into at least two arcs instead of one, to enclose the pieces inside the pods and make a smooth transition to the rest of the body. The dash panel needs to be angled forward about 5 degrees or so. The tubes for the suspension are incomplete. Both ends need quite a bit of work, but the rear end needs to be figured out. The original used a DeDion axle, but I would opt for a Ford 7.5" due to availability, but given a 47.2" rear end, it would likely need customized axle shafts.

The second image for me, represents about 15 hours of researching online by looking for images and measurements, about 15 hours of work in Solidworks and 5 1/2 hours of rendering time.

Image
Image

1920x1080 versions:
http://battletechlive.files.wordpress.c ... eflect.png
http://battletechlive.files.wordpress.c ... dcage4.png

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 Post subject: Re: welding vs brazing
PostPosted: Mon Oct 10, 2011 10:43 am 
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Having done production fillet brazing on bicycle frames (mountain, road AND tandem) I can definitely say that I would not want to do anything more than perhaps a bracket on a car frame that way. One of the problems is that you have to work with gravity. While you can "push" the weld uphill to a certain degree, in order to have a good looking weld you need to be constantly rotating the workpiece. We used a gimble mounted clamp that allowed us to rotate the frames in any direction...even so, it tended to be an exhauting proposition. TIG* welding you can essentially do standing on your head. A lot less movement of the workpiece is required to get good welds. (Even so, a wise old timer once taught me to find a comfortable position, and weld TO that...i.e., don't start in a good position and weld OUT of it)

Then there is weld prep and cleanup. And fumes. All are easier to deal with when you are TIG welding. We used in-line gas flux and it pretty much required welding in a gas mask.

I still love fillet brazing; you can make beautiful and very fatigue-resistant joints with it. But I sure wouldn't want to do a car frame that way.

*TIG is what I'm familiar with. I have yet to try MIG.

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Last edited by Acerguy on Mon Oct 10, 2011 12:01 pm, edited 1 time in total.

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 Post subject: Re: welding vs brazing
PostPosted: Mon Oct 10, 2011 10:46 am 
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Nice modeling, BTW! :D

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 Post subject: Re: welding vs brazing
PostPosted: Mon Oct 10, 2011 10:58 am 
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Are you going to be able to do FEA in Solidworks? I suggest starting now so that you can get a start on the learning curve. That motor is very bulky, it will be difficult to fit in the frame your drawing. A I4 would be a better choice, IMHO. This is a huge task your undertaking so you need to make other choices to help you out, not make things harder for you.

You need to calculate how much metal you have carrying loads some places. A 1/2" round tube probably has between 1/4 - 1/3 the amount of steel in it as a 1" square tube. It has much less material to resist bending because without the flat side it doesn't have something like a flange. A square tube is like an I-beam, but with two shear webs.

So the whole vertical load of the car appears on the 2 little tubes at the top of the frame, in compression. It's not clear how that gets to the rear of the car. It looks like an I-beam that's been sawed 3/4 of the way thru where the cockpit is.

I would spend a lot of time looking at birdcage pictures and also reflect on the fact it was not all that successful and nobody else has ever tried to go down that road.

I think a much better locost frame can be designed, but would suggest larger tubing and simpler is the better approach. More safety, more strength, easier to build and suitable for more applications. All that should be possible.

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 Post subject: Re: welding vs brazing
PostPosted: Mon Oct 10, 2011 11:34 am 
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The "Bircage" Maserati is a design that has always fascinated me. When you look at it, it seems to have more in common with a unibody than a traditional spaceframe. Kind of like an origami made with toothpicks instead of paper.

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 Post subject: Re: welding vs brazing
PostPosted: Mon Oct 10, 2011 11:49 am 
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The frames are difficult to understand when you see one by itself, but when a floor is added and some of the other panels, the birdcage design is almost pretty.

Sort of my thought was to replace the longest floor tubes with heavier and/or square tubes. Width at radiator is 26" on center and width at the rear of the fuel tank is 32" on center. Then, build a roll hoop to take the loads as the floor itself really provides for very little strength. The common shape I see is a hoop to define the position of a windshield, then a hoop behind the driver with sections of tube joining the two, and sometimes tubes angled down to the trunk area. My idea is more along the lines of an Audi TT. Two curved 1.5" tubes that run lengthwise, the front end connecting to the floor tubes near the pedal box, arcing up, over, back and down to the trunk area to connect beside the fuel tank. The two hoops then connected by four straight tubes, one inside the dash, one above where a windshield will be, one behind the passengers, also used for seatbelt mounts and one behind the fuel tank, also to double as impact protection.

For the engine, horizenjob is probably right. A supercharged Duratec 23 makes essentially the same power as a NA Duratec 35, a claimed 265hp. Although my general belief is that a larger naturally aspirated engine producing the same power as a smaller forced induction engine, the larger engine is typically going to be more durable.

I can do FEA in Solidworks, but I haven't taken the time yet to figure it out. Last time I tried, it told me I had a configuration problem with one of my beams.

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