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  1. #1
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    Question for the "fillet pros."

    It's been stated more than once that the fillet (filler thickness) should be 3 times the thickness of the tubes being joined. My question is where the "thickness" is measured. We are talking bronze brazed here.

    Assuming you are joining two tubes of .9mm wall, the thickness of the filler would be less tan 1/8". Or do you combine the two tubes (1.8mm) and times the thickness times three (approx. .211")?

    In either case, if the "filler" thickness is measured at it's "fattest" point it appears to be a much smaller fillet than what I see the "fillet pros" doing, assuming they provide a consistent radius at the joint.

    Does this mean (assuming the 3x number is correct) we can get by with smaller fillets than we commonly see?

  2. #2
    Andrew R Stewart Andrew R Stewart's Avatar
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    I have read of 3x and 4x as being the minimum before. I also have read the this is measured along the tube surface from the miter to the edge of the fillet OR measured through the fillet from the miter to the center of the fillet's curved outer surface (which would mean the first way to measure would be larger). But i agree, too, that most builders end up with fillets larger then either of theses measuring methods. Also I think most (I know I do) try to end up with an internal fillet in addition of the big outside one. This internal fillet insures that the miter has been fully flowed with filler, so tube to tube contact is complete.

    It has been my understanding that the x3 or x4 size is driven by the strength of the filler VS the tube's strength. So if a tube is rated at 100 units of tensile strength and the filler at 33 units then the x3 size is correctly the minimal size. I believe this is why weld fillets can be so much smaller then brass ones, as the weld filler has essentially/nearly the same strength as the tube.

    I'm curious what those with real knowledge have to say. Andy.

  3. #3
    Decrepit Member Scooper's Avatar
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    I grabbed this illustration off the web several years ago and failed to note the source, so I can't provide a credit. The 4x tubing wall thickness radius is based on the UTS of the tubing material compared to the UTS of the filler material to give the joint 100% strength parity with the tubing.

    Last edited by Scooper; 12-31-13 at 08:36 AM.
    - Stan

  4. #4
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    It is, I believe, a rule of thumb based on the comparative tensile strengths of steel and brass. Obviously they both vary a lot, so not that useful a rule.

    I've never seen a frame fail through a fillet, so it's not something I particularly worry about, I'm more concerned with getting smooth transitions for aesthetic reasons - and to reduce stress raisers.
    http://www.kinetics-online.co.uk

  5. #5
    Randomhead
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    I think anything less than 3x probably looks funny more than anything. I'm probably wrong, but I think brass is about 1/2 the strength of most steels, and if you get good penetration into the mitered area, that increases the strength more than the fillet does

  6. #6
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    Quote Originally Posted by unterhausen View Post
    if you get good penetration into the mitered area, that increases the strength more than the fillet does
    I've just started to practice fillet joints in anticipation of building my first practice frame in anticipation of building my first frame...so, just getting started. I've found a local builder who has been in the business for a long time and he said essentially the same thing.


    In addition, he said:
    I'm using too much heat
    I'm using wrong filler
    I'm using the wrong flux
    My gas pressure is too high
    I'm clumsy

    Other than that he's been very encouraging!

  7. #7
    Decrepit Member Scooper's Avatar
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    Here's an excellent Framebuilders' Collective five-part series on fillet brazing by one of the masters, Dave Kirk. Lots of illustrations.

    Kirk - Fillet Brazing
    - Stan

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