Advertise on Bikeforums.net



User Tag List

Results 1 to 10 of 10
  1. #1
    Senior Member Briareos's Avatar
    Join Date
    Oct 2006
    Location
    Illinois
    My Bikes
    No bike at the moment; In process of building it.
    Posts
    539
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)

    RE: My GT's Ugly Welds

    So I have acquired a GT Strike (I love the seatstays, so sue me), and I have noticed the welds are very large and thick, and would probably qualify for the "cottage cheese" look. Usually I wouldn't care but these welds are ridiculous, especially the ones that connect the seat-tube with the top-tube.

    I was wondering if I could slap some goggles on and go to town on the welds with a Dremel? I mostly just want to get rid of the rippling effect in the welds and make them smooth, whether or not they meet flush with the tubes is of secondary concern. Also, I am unsure of what type of attachment to grind the welds, since the bike is aluminum. I would rather have something that would grind it down slowly so I don't cut my bike in half by accident or put a hole in a critical spot.

    If this is above my head, would most welders be able to smooth them out?

    They look really, REALLY bad.

    -Rob

  2. #2
    THE Materials Oracle Falanx's Avatar
    Join Date
    Feb 2006
    Location
    Finally... home :-)
    My Bikes
    Univega Alpina 5.1 that became a 5.9, that became a road bike... DMR TrailStar custom build
    Posts
    502
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    From a purely metallurgical point of view this is a bad idea. Grinding metal tends to place tensile stresses in the skin during the operation from friction heating, and in the case of most aluminium alloys, the region at the point of contact is more than hot enough to start overaging.
    Then, as the zone ground cools, it attempts to contract and reverse the stresses. However, the surface is weakened and the material below resists this compression, yielding the ground surface. Welcome to Crack City.

    To try to remove material as gently as possible, and as gently as necessary is going to take weeks.
    "While my father fought for you, I learnt. While my father glorified your petty administration, I learnt. While he longed every day for our line, Adunís line, to be restored, I learnt. He sent me away to bring the Dark Templar back when the time was right!
    "And you tell me that I cannot do this? That I cannot feel the weight of the universe?
    "Damn you, Tellan! Aldaris killed my father!"

  3. #3
    Senior Member DannoXYZ's Avatar
    Join Date
    Jul 2005
    Location
    Saratoga, CA
    Posts
    11,507
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Heh, heh.. at first I thought MERTON was back...

    Yeah, grinding welds is usually not a good idea. In addition to the heat-treatment problems mentioned, the grinding usually leaves rough surface-stratches which are stress-risers and prone to cracking. You'll need to polish up and make the surface smooth as well.

    Rather than grinding off the welds, you want to cut instead. Use sharp carbide cutters with a high-torque die-grinder instead of the Dremel. The lower RPM heats up the metal the lowest amount for the material removed. Also have the joint freely bathed & rinsed in cutting-oil as you're cutting as well.

  4. #4
    Senior Member jacobs's Avatar
    Join Date
    Apr 2005
    Location
    Boston, MA
    My Bikes
    http://www.jacobsbicycles.com
    Posts
    787
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Bondo.

  5. #5
    Senior Member Briareos's Avatar
    Join Date
    Oct 2006
    Location
    Illinois
    My Bikes
    No bike at the moment; In process of building it.
    Posts
    539
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by DannoXYZ
    Heh, heh.. at first I thought MERTON was back...

    Yeah, grinding welds is usually not a good idea. In addition to the heat-treatment problems mentioned, the grinding usually leaves rough surface-stratches which are stress-risers and prone to cracking. You'll need to polish up and make the surface smooth as well.

    Rather than grinding off the welds, you want to cut instead. Use sharp carbide cutters with a high-torque die-grinder instead of the Dremel. The lower RPM heats up the metal the lowest amount for the material removed. Also have the joint freely bathed & rinsed in cutting-oil as you're cutting as well.
    Ok that makes sense, but I'm not clear on the process you mean. Die-grinders look like Dremel's, just industrial quality I guess. So I'm unsure what you mean by "cutting" instead of grinding.

    Thanks for the reply however!

  6. #6
    Banned
    Join Date
    Aug 2005
    Posts
    5,117
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    You don't want to grind them at all. You will probably end up running into the adjascent tubes and nicking that, you may also reduce the fillet efect. The way filelts are hidden done is by filling the weld. Bondo as mentioned, or just some 5 minute epoxy Getting the paint out of the weld is a problem, In many instances the epoxy will just adhere to it, but if you can strip it that's the best option. In other words if this is a rattle can level project then even if the epoxy does chip off you can easily get back to the starting point, and you may ride it for years without problems. If you are paying for paint though or doing this for someone else who is paying you, you need to get the paint out.

    Only problem with either bondo or epoxy is if you subsequently powder coat it you need to use a product that can deal with 400 degrees. There is such an animal. On steel they use filet brazed bronze tigged over the joint, but there is a fairing compound that takes the heat also.

    The only problem with filling is that you really need to treat everything, not just one section. But I feel strongly your choices are filling or learning to live with it.

  7. #7
    Senior Member DannoXYZ's Avatar
    Join Date
    Jul 2005
    Location
    Saratoga, CA
    Posts
    11,507
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by Briareos
    Ok that makes sense, but I'm not clear on the process you mean. Die-grinders look like Dremel's, just industrial quality I guess. So I'm unsure what you mean by "cutting" instead of grinding.
    Well, die-grinders typically run at much lower-RPMs than Dremels, around 4000-6000rpms or so. And they have a tonne more torque. This allows the bit to heat up the material less.

    And a burr-cutter looks like this:



    Cuts about 10-20x faster than a grinding-stone so you don't heat up the part as much either. Problem is it takes the precision of a neurosurgeon's hand to grind the welds away without slipping and nicking the softer tubing around it. Just one slip and you've come close to punching a hole right through the tubing. Count on 2-3 hours per joint to do a clean job. Not to mention the final polishing and painting. That's why it's easier to get a well-crafted frame to begin with. Consider what 10-15 hours of your time at current wages can buy...
    Last edited by DannoXYZ; 12-11-06 at 06:16 PM.

  8. #8
    Senior Member Briareos's Avatar
    Join Date
    Oct 2006
    Location
    Illinois
    My Bikes
    No bike at the moment; In process of building it.
    Posts
    539
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)

    Hmm

    This is going to seem silly, but could I file it down with a manual file? Those long ones that people put in cakes for their imprisoned loved ones to escape with...

  9. #9
    Banned
    Join Date
    Aug 2005
    Posts
    5,117
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Absolutely, if it was curved enough to not cut into the sides, or cut below the fillet horizon. this doesn't get past the don't grind them problems, unless you have a few peaks, there is a resistance to taking part of th ebad off, which is the art of welding bikes without leaving the intended rough fillet.

  10. #10
    THE Materials Oracle Falanx's Avatar
    Join Date
    Feb 2006
    Location
    Finally... home :-)
    My Bikes
    Univega Alpina 5.1 that became a 5.9, that became a road bike... DMR TrailStar custom build
    Posts
    502
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    It's a pity you can't get those beryllium copper ones anymore - the none sparking kind for working in gas environemnts... They're softer than steel ones and would give you much more safety margin for the removal.

    But the advice remains the same: Learn to like weld beads. Come to love the fish scales.
    "While my father fought for you, I learnt. While my father glorified your petty administration, I learnt. While he longed every day for our line, Adunís line, to be restored, I learnt. He sent me away to bring the Dark Templar back when the time was right!
    "And you tell me that I cannot do this? That I cannot feel the weight of the universe?
    "Damn you, Tellan! Aldaris killed my father!"

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •