More progress

A special section just for steam engines and boilers, as without these you may as well fit a sail.
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DetroiTug
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Re: More progress

Post by DetroiTug » Sat Jan 18, 2014 3:02 pm

Ken,

Thanks for that. I'm putting a replica car chassis together which is slip fit tubing joints and it is all brazed together. Quite a bit of heat was used to draw the bronze back in to the joints. I tried simply cleaning it up with die-grinder driven 2" sanding pads and scotch brite wheels but it was too aggressive. I'll try the sand blaster route and finish up with the scotch-brite wheel. Need the surface to be really smooth, it will be a high gloss automotive finish over the top.

-Ron
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Re: More progress

Post by barts » Sat Jan 18, 2014 4:35 pm

There is a world of difference between brazing with bronze alloys and brazing with silver ones....

Bronze alloys (in general) permit a large build-up and don't 'wick' into joints like silver-bearing alloys do... These are used when the joint design permits or requires a fillet (see below). All that is needed for a good joint is careful fit-up; tubes can meet at any desired angle.

Silver-alloys have very low surface tension and will wick into thin (.003" or so) joints very aggressively. Properly made, these joints can have shear strength of 50ksi or better, but adequate surface area is required. Note that 'lugged' bicycle frames are often silver brazed, but the lugs must fit closely; loose joints have little strength in comparison to tight ones.

In the case of the link, since there's a close fit and sufficient area between the boss and the link, silver bearing alloys are the right choice. Note that these tend not to spread on their own w/o capillary action or deliberate 'pushing' with the torch, so they tend to stay confined to the joint area.

Both forms of brazing are very handy for steamboaters; they permit fabrication of parts and then subsequent assembly w/o the distortion and stresses engendered by fusion welding.

- Bart
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Re: More progress

Post by DetroiTug » Sun Jan 19, 2014 8:03 pm

Bart,

Thanks for the info. It is brazed up with bronze already, it seems very strong. The joints have only a thou or two clearance, it appeared the bronze went back/wicked in the joints pretty well. Had I known about the Silver-alloys prior I would have probably gone that route. I used some a long time ago on some refrigeration lines and it seemed to work pretty well. The original chassis was "furnace-brazed" much the same as these bicycle frames were in this video below, they said it was removed by chemical process? What chemical would remove bronze and not the steel?



-Ron
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Re: More progress

Post by barts » Sun Jan 19, 2014 8:40 pm

Wonderful video - thanks. OSHA would have a fit :o

A bit of googling reveals:

http://books.google.com/books?id=f4sfAQ ... ts&f=false


This is not something I've heard of though; furnace brazing usually relies on careful joint design and braze pre-placement to control the molten metal, rather than supplying extra and then removing it.

- Bart
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Re: More progress

Post by barts » Sun Jan 19, 2014 8:53 pm

I'm quite cautious with relying on capillary action with brazed joints ever since a rather dramatic failure (no injuries, luckily enough) in a small coaster car from my college days. I find it difficult to determine how much of the socket is actually brazed. Silver solder is also much more sensitive to proper joint design and cleaning; I acid-etch parts before silver soldering, and of course pre-apply a floride-bearing flux to the entire joint area. When the silver wicks its way around the joint, you know things are right. Of course, perhaps if I'd use the same technique with bronze, I'd get better results, but I've always used the 'heat the rod, dip in granular flux' method I learned in high school with bronze/brass. I also find it difficult to balance overheating any zinc in the brazing alloy with getting the fluidity needed to creep into the tight gaps.

- Bart
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Re: More progress

Post by Mike Rometer » Sun Jan 19, 2014 10:25 pm

It is always best to treat powdered brazing fluxes in the same way as Silver fluxes, i.e. mix with plain water. You can still dip the rod in either the dry or wet flux, to help if you wish. Another tip is to play the flame on the dipped rod to make the flux run back up the rod aways, but if the job is properly fluxed it isn't really necessary.

The flux can't work where it isn't, and not pre-fluxing means that you are expecting the flux to wick into the joint before the spelter, which also mean that it has time to oxidize with the heat before the flux gets to it, and probably will. Result, poor joint!
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Re: More progress

Post by marinesteam » Mon Jan 20, 2014 3:48 am

DetroiTug wrote:Ken,

Thanks for that. I'm putting a replica car chassis together which is slip fit tubing joints and it is all brazed together. Quite a bit of heat was used to draw the bronze back in to the joints. I tried simply cleaning it up with die-grinder driven 2" sanding pads and scotch brite wheels but it was too aggressive. I'll try the sand blaster route and finish up with the scotch-brite wheel. Need the surface to be really smooth, it will be a high gloss automotive finish over the top.

-Ron
Ron,

If using bronze with a close fit joint (in your description, slip fit tubing joints) make sure you are using a flowing bronze filler. There are many types of bronze fillers and some are intended for high build, fillet joints. I would use silver for close fitting joints as I believe that it flows better than similar bronze fillers.

Tubing cleans up really well with long strips of abrasive cloth (shop roll) but being sure not to get to much braze material around the joint is key to easy cleanup. I suspect your filler isn't quite flowing as well as you would like it to either due to the wrong flow, incorrect flux or improper cleaning and you're needing to overwork/overheat the joint.

Ken
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Re: More progress

Post by DetroiTug » Mon Jan 20, 2014 4:30 am

The problem is these are parts with a lot of mass (nodular casting to a 1.25" Cromoly tube, the tubes slide in to the castings about 2")and trying to heat one area and not the other is about impossible - acetylene torch is too slow for that. As far as brazing and controlling the appearance, the end of the outer tube could be immediately brazed to the inner tube. But I knew that would not be sufficient as the inner parallel mating surfaces were still too cold to draw any bronze. I had to pour heat to the outer tube back from it's end to draw the bronze back in. That is when it got everywhere. I went back over it and heated the exterior surface and quickly wire brushed it, that removed about 90% of it. I thought there may be some trick to completely cleaning the surface, from the references provided it appears this was an electro-chemical process - the inverse of plating. Not a practical solution. I'll just sandblast it and scotch wheel it.

Apologies, my question sort of took your thread off course.

On with the York! :)

-Ron
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Re: More progress

Post by Jack Innes » Mon Jan 20, 2014 4:53 am

Ron,
I believe ammonia will dissolve brass, particularly when it is a thin coating like you have remaining on your chassis. The ammonia will promote rust on the steel but will not attack it.
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Re: More progress

Post by marinesteam » Mon Jan 20, 2014 2:43 pm

Ron,

If I'm picturing this correctly, what you have is basically like a bicycle frame with investment cast lugs.

If this is the case, apply all of your heat to the casting until it is an even very dull red glow, then apply the filler to the edge of joint, in one spot until you see a small bead of molten filler form around the entire joint.

Keep the torch constantly moving on the part with the most mass to keep it heated evenly, the smaller mass parts (tube) should heat by absorbing heat from the large mass part. A large rosebud tip also may be helpful in getting more even heat into the large casting.

If the large mass part isn't thoroughly heated when you start brazing the joint quality is doomed. The large mass part must be able to keep the filler molten so it can flow around the entire joint. Trying to heat the rest of the joint once you start brazing in one spot trying to pull the filler around is problematic.

Preforms (filler placed in the joint before heating) or paste filler could also be helpful. I would try to place a ring of filler (if you have finer filler wire) into the joint before fluxing & heating. Flux then heat the joint evenly until the filler flows and then get the heat off immediately. If possible, fixture so that there is a bit of force pushing the tube into the joint as long as there is an internal stop. Otherwise paste filler could be applied at assembly and use the same method without pushing on the tube

Too high a temperature is much more of a problem than having a joint heated for too long. So work on raising the temp evenly without lingering on one spot. That said, the flux does have a limit on how long it will be active, this is where the difficulty using too small of a torch comes in that you cant get to brazing temp before you have used up the flux. Some fluxes are designed to hold up to longer heat (Harris Black).

If your casting as really heavy as compared to the mating part, you may just have to live with the amount of excess braze material flowing onto the tube. A lot of hand work will be needed to clean up the joint. You can use files on the heavy overflow areas then come back in with coarse cloth. The trick is to get the braze that your removing and even thickness so the you can work toward removing the layer of braze with out breaking through to the base metal in spots. Ideally, you want to reach the base metal just as you have removed all of the layer of braze with your fine abrasive. It's easier said than done, it's better to have excessive, but smoothed braze than to remove base metal.

Hope this helps,

Ken
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