Purge vs. Heatsink for Welding 4130- Mtbr.com
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  1. #1
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    Purge vs. Heatsink for Welding 4130

    I have been pondering this question lately and a purgeable heatsink would be the easy answer but here is my dilemma. My jig (Anvil Journeyman 4) is set up to purge but I can't use the classic style 660 SAE purgeable heatsinks that Ive made. Do you think pulling some heat out of the joint more advantageous that shielding the inside of the weld?

    Ideally, I'd like to use both back purge and a heatsink so I have been playing with the idea of making some slip-fit heatsinks that would just sit inside of the tube as it's in the fixture. I suppose I could modify my previous set to work, they are styled like the paragon purgeable heatsinks.

    Another slightly related question: How much of a benefit is it to weld a frame in a jig? I've had good success with frames turning out straight and aligned after welding in and out of my jig. I find it a bit easier to weld the frame when its out of the jig but I have always felt that having everything held in place may help frames to be straighter more consistently.

  2. #2
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    Welding them out of the jig is the common process, so that frees you up to use whatever you want. Depending on what the frame design looks like I tend to shy away from heat sinks. For example, the Paragon head tubes are their own heat sink and you don't need to put anything in those. The only place I ever use a heat sink is in the seat tube, everything else is left alone, but then I'm not trying to build super lightweight bikes with thin headtubes or BB shells.

  3. #3
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    What material are we talking about? Unless you're using *super* thin steel, there's no need to worry about *either* of those, though you may save your cutting tools a little bit of wear and tear. If we're talking ti or stainless, obviously different story and I'll defer to someone with more expertise. If you have to choose with ti or stainless, you're going to need to choose purging, though.

    The jig is just to get the tubes where they need to be in the first place. It has basically no effect on final alignment. Good miters and weld sequence are what you need.

    -Walt

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    Thanks for the replies, I really value this input! So it seems like back-purge and heatsinks are overkill for chromoly. Do a lot of steel builders construct their frames without them? I have always thought that less heat and oxygen would produce a "better" weld. (Now that I type that, I realize that I don't know if I can quantify what would make it better...)

    I did a cursory search for weld sequence and didn't come up with much. Im sure its been well discussed in this forum, can anyone point me in the right direction with a thread? I have played around a bit with weld sequence but I have never really understood the fine balance that I feel like some really experience builders have!

  5. #5
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    Here's the story with backpurging - if you're not backpurging, the back side of the weld is also getting hot, but it's not being shielded. So some of the metal on the back side of where you're welding is ending up oxidizing, which makes things weaker by effectively making the tube wall thinner.

    As the material gets thicker, that matters less and less since you're only losing so much on the back side. On the other hand, as the material gets more and more reactive with oxygen (ie ti, higher chromium stainless) you lose more off the backside.

    So while your typical ti frame is pretty thick walls (.9mm) it's so reactive that not shielding the back of the weld results in a ton of titanium dioxide with zero strength and an unsafe bike. Steel is not nearly as reactive, so not as big of a deal.

    Your typical .9 or .8mm cromoly tubes aren't going to fail any differently if you backpurge or don't, I'd say. Look inside the head tube on any mass produced steel bike and you'll see some oxides on the back side of the weld, not shiny rainbows. And those bikes hold up just fine.

    If you're TIG welding .6mm Spirit or something, I'd probably backpurge because losing that .05mm or whatever to oxidization is maybe enough to matter.

    The biggest advantage of backpurging normal cromoly frames is that when you get rid of those oxides on the inside of the tubes, you don't have to run your expensive taps and reamers through that nasty abrasive stuff to prep the frame. For very inexperienced welders, it can also make it a bit harder to blow holes in the tubes.

    Heat sinks can also save you some effort when doing frame prep, but they're not going to have any huge effect on the end result in general. Again if you're doing, say, a very thin seat tube or head tube, a heat sink can help keep it round and easy to ream.

    If you already made the heat sinks, by all means use them.


    There was a weld sequence post or two in a thread a few months ago, but long story short you swap sides of the frame as you go. If you're producing frames that are straight enough to ride well, you're golden. Don't change a thing.

    -Walt

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