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Multiple Pass Welds - When are they necessary?

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  • Multiple Pass Welds - When are they necessary?

    This might belong in the motorsports section but it doesn't seem to get a whole lot of traffic.

    These are full-floater housing ends that are welded to a 3.5" x 1/2" wall tube. The housing ends are not a press fit, but they slide onto the tube about 1" with very little wiggle room.

    This is a single pass TIG weld. It was welded at 190 amps on my Dynasty with the pedal pegged almost the whole time.

    This is roughly 11" of weld around the entire tube. I know it's hard to say without a failure, but what is YOUR take on extra passes to eliminate the undercutting and help build up a larger fillet?

    Adding an unnecessary second/third/fourth pass adds to additional surface warping, and extra time and $$$ spent on supplies.

    Not adding a second/third/fourth pass when I should could lead to a failure.

    Any suggestions are appreciated!!!

    A few shots from various sections of the tube.



    Last edited by tmorgan4; 03-29-2010, 09:28 PM.

  • #2
    When you miss the crack, where your welding. But your not having that trouble. I'm teaching my wife to weld and that came up.

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    • #3
      From my understanding a multi pass weld is used in a few situations. One is when the filler specified will not produce the correct bead size and root depth. The second situation I can think of is over heating. If a material needs a large weld bead but heat input must be kept to a minimum then a multi pass weld would be used with adequate time for the temp to drop off and complete a new bead.

      Calculations can be made to determine the size of the root required.
      i.e. a 1/2" root fillet of 7018 has the ability to hold 7400 lbs per linear inch of weld.
      Last edited by kcstott; 03-29-2010, 10:07 PM.
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      • #4
        Originally posted by tmorgan4 View Post
        This might belong in the motorsports section but it doesn't seem to get a whole lot of traffic.

        These are full-floater housing ends that are welded to a 3.5" x 1/2" wall tube. The housing ends are not a press fit, but they slide onto the tube about 1" with very little wiggle room.

        This is a single pass TIG weld. It was welded at 190 amps on my Dynasty with the pedal pegged almost the whole time.

        This is roughly 11" of weld around the entire tube. I know it's hard to say without a failure, but what is YOUR take on extra passes to eliminate the undercutting and help build up a larger fillet?

        Adding an unnecessary second/third/fourth pass adds to additional surface warping, and extra time and $$$ spent on supplies.

        Not adding a second/third/fourth pass when I should could lead to a failure.



        Any suggestions are appreciated!!!

        A few shots from various sections of the tube.




        It's hard to tell from the pics, but are you adding filler on this first pass or are you fusion welding?
        If you are using filler, what type?

        Griff

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        • #5
          Fillet Welds

          Even though, this is a circumferential weld, it would be classified as a fillet.

          A fillet weld is one, that forms a "triangle" between the work pieces, and is most commonly illustrated as a "Tee."

          Fillet welds size, is always based on the thinner of the two members.

          The joint cannot be made any stronger by using the thicker member for the weld size.

          A rule-of-thumb, for full strength design is (w = 3/4 t). Weld size is .750 of the material thickness.

          If your flange face is 3/8", and you are welding to 1/2" tube, your fillet weld size would be 5/16"

          To put things in perspective, this weld could be successfully made with a 1/8" electrode @ 130 amps, in a single pass.
          "Bonne journe'e mes amis"

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          • #6
            Right, it could be welded for full strength but does it need it? I am obviously just guessing but I would say its adequate especially with the type of fit.

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            • #7
              Originally posted by Sberry View Post
              Right, it could be welded for full strength but does it need it? I am obviously just guessing but I would say its adequate especially with the type of fit.
              There are strength designs and rigidity designs.

              Rigidity designs can use the 50% or 33% formulas.

              50% of full strength weld (w = 3/8 t)

              33% of full strength weld (w = 1/4 t)

              If he's building a competition car, Sanctioning Body Rules would prevail.

              Overwelding is one of the major factors of welding costs. with fillet welds, it's possible to have a weld too small, or large. That's why it's necessary to determine proper size.

              The following is a quote from Omer Blodgett's Book "Design of Welded Structures" Section 7.4-2 "Determining Weld Size" (fillets).

              "The AWS has defined the effective throat area of a fillet weld to be equal to the effective length of the weld times the effective throat. The effective throat is defined as the shortest distance from the root of the diagrammatic weld to the face......the leg size of a fillet weld is measured by the largest right triangle which can be inscribed within the weld........this definition would allow unequal-legged fillet welds (this would be to gain throat area, hence strength) when the vertical leg cannot be increased.....Another definition stipulates the largest isosceles inscribed right triangle and would limit this to an equal-legged fillet weld."

              Mr Blodgett is the most respected PhD on the planet in Structural Engineering circles. I refer to his data, often. I encourage his reading for those who would like to take the "guess work" out of their welding.
              Last edited by davedarragh; 03-30-2010, 09:33 AM.
              "Bonne journe'e mes amis"

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              • #8
                Very good info! Thanks for taking the time to help me out.

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