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  1. #31
    Join Date
    Feb 2010
    Location
    Ballina, NSW
    Posts
    901

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    Interesting test mugget - better than I though it would be.

    My concern is that the joints may be highly variable in strength - how will you know if they're strong enough? If this is a table, will kids be jumping up and down on it? etc. etc.

    If you are just going to be doing perpendicular butt joints, you could also consider riveting small bits of aluminium angle inside the joint, which in combination with your repair rods will provide some additional strength and provide a little bit more insurance. If you put the rivet on the underside of the joints, they would barely be noticable.

    Cheers

    - Mick
    Capture.PNG

  2. #32
    Join Date
    Jun 2010
    Location
    Canberra
    Posts
    1,322

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    Nice to hear the J's on a vid.

    From the look of it, you're not getting enough heat into the piece you're butting on to, such that when the join breaks, it's actually the brazed fillet disconnecting cleanly from the surface of the main piece.

  3. #33
    Join Date
    Mar 2009
    Location
    BrisVegas
    Posts
    48

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    Cheers Mick - neat idea! And yep I was planning for all perpendicular butt joints so that may be a good way to go.

    The joints could vary in strength, but I think that just comes down to me needing to get the technique right. It seems that you really need to get it right on the first shot to get it looking neat (and probably have a good bond), otherwise if you try and "touch it up" a bit it just adds more and more filler. Already used nearly one rod on that test shown in the video, glad I bought the "trade pack"! Lots of excess & mess there though, not nearly as much needed if you get it right.

    Also this bench will be installed in the back of a van - lightweight duty only. No heavy hammering like in the vid, I figure that would transfer straight to the van floor, so doing that would be like hammering directly on the floor - none of that in my van! It only needs to hold probably no more than 40kg at once. 2,250 x 600mm benchtop, supported by 9 legs (2 legs on one corner where a vise will be. The vise will just be for holding blades for sharpening etc.). If you can imagine the legs free standing, then the long 2,250mm lengths of tube placed on top - so the forces should be pretty well supported, I figure it would need a fairly major sideways force to move anything?

    Anyway, the good thing about this method is that I can get this all done reasonably quickly. If I need something more heavy duty in the future I can just re-heat the joints and take them apart, wipe away the filler and get it welded up.

    RustyArc - You wouldn't believe that YouTube picked up on that song just from the small section in the 2nd part of the vid! (They let you keep the audio, but you've just got to let the band advertise on your video etc.)

    About the heat - How should it actually look when a good join breaks? Just not a clean split? The info for the Dura Bond says that it chemically bonds - should it leave a pitted surface? Or should the filler break rather than separating? I will try and rework that test piece to see what I can see...

  4. #34
    Join Date
    Dec 2005
    Location
    Canberra
    Posts
    490

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    A good *welded* join should break along the heat affected zone, typically at or very near the toe of the weld puddle...at least, that's for steel.

    Aluminium, with its range of alloys, tempers and hardness states is another matter, and can be much more susceptible to operator technique - if you quench your weld rather than leaving it to cool in still air, you can quite easily get a crack right through the middle of the filler puddle. Welding on aluminium can also reduce the strength of the alloy by anywhere up to 50%.

    However, a braze failure will typically look like the braze has 'pulled off' the joined metal; but good brazing technique also includes the design of the join so that this failure mode isn't possible...think of joint design for brazing as more like 'holding something in place so the base metal can take the load' rather than the braze actually taking the load. (look at how carbide tips are brazed to saw blades, or how bike frames use braze to hold tubes in place in gussets or lugs.)

    A better brazing strength test would be to braze up an aluminium 'T' shape, then bash on the top of the 'T' directly over the braze. The job of the braze is to hold the top of the 'T' in place while you hammer, not to withstand the force of the hammering.

  5. #35
    Join Date
    Mar 2009
    Location
    BrisVegas
    Posts
    48

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    Quote Originally Posted by Master Splinter View Post
    A better brazing strength test would be to braze up an aluminium 'T' shape, then bash on the top of the 'T' directly over the braze. The job of the braze is to hold the top of the 'T' in place while you hammer, not to withstand the force of the hammering.
    Ahhh... gotcha!

    I will test that out next. I was interested to see how the other type of test worked out anyway, but it makes sense to test this way because the "T" idea is exactly how I planned to put the bench together. This testing has gone to the back burner for now, but it still needs to be done, so I'll get to it eventually!

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