Cycling and bicycle discussion forums. 
   Click here to join our community Log in to access your Control Panel  


Go Back   > >

Framebuilders Thinking about a custom frame? Lugged vs Fillet Brazed. Different Frame materials? Newvex or Pacenti Lugs? why get a custom Road, Mountain, or Track Frame? Got a question about framebuilding? Lets discuss framebuilding at it's finest.

User Tag List

Reply
 
Thread Tools Search this Thread
Old 12-08-06, 05:50 PM   #1
Briareos
Senior Member
Thread Starter
 
Briareos's Avatar
 
Join Date: Oct 2006
Location: Illinois
Bikes: No bike at the moment; In process of building it.
Posts: 539
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(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
Briareos is offline   Reply With Quote
Old 12-08-06, 06:13 PM   #2
Falanx
THE Materials Oracle
 
Falanx's Avatar
 
Join Date: Feb 2006
Location: Finally... home :-)
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)
Quoted: 0 Post(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.
Falanx is offline   Reply With Quote
Old 12-08-06, 07:11 PM   #3
DannoXYZ 
Senior Member
 
DannoXYZ's Avatar
 
Join Date: Jul 2005
Location: Saratoga, CA
Bikes:
Posts: 11,600
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(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.
DannoXYZ is offline   Reply With Quote
Old 12-08-06, 09:16 PM   #4
jacobs
Senior Member
 
jacobs's Avatar
 
Join Date: Apr 2005
Location: Boston, MA
Bikes: http://www.jacobsbicycles.com
Posts: 787
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(s)
Bondo.
jacobs is offline   Reply With Quote
Old 12-09-06, 02:32 AM   #5
Briareos
Senior Member
Thread Starter
 
Briareos's Avatar
 
Join Date: Oct 2006
Location: Illinois
Bikes: No bike at the moment; In process of building it.
Posts: 539
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(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!
Briareos is offline   Reply With Quote
Old 12-09-06, 03:50 PM   #6
NoReg
Banned
 
Join Date: Aug 2005
Bikes:
Posts: 5,117
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(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.
NoReg is offline   Reply With Quote
Old 12-10-06, 08:08 AM   #7
DannoXYZ 
Senior Member
 
DannoXYZ's Avatar
 
Join Date: Jul 2005
Location: Saratoga, CA
Bikes:
Posts: 11,600
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(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.
DannoXYZ is offline   Reply With Quote
Old 12-11-06, 02:46 PM   #8
Briareos
Senior Member
Thread Starter
 
Briareos's Avatar
 
Join Date: Oct 2006
Location: Illinois
Bikes: No bike at the moment; In process of building it.
Posts: 539
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(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...
Briareos is offline   Reply With Quote
Old 12-11-06, 04:11 PM   #9
NoReg
Banned
 
Join Date: Aug 2005
Bikes:
Posts: 5,117
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 0 Post(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.
NoReg is offline   Reply With Quote
Old 12-12-06, 12:39 PM   #10
Falanx
THE Materials Oracle
 
Falanx's Avatar
 
Join Date: Feb 2006
Location: Finally... home :-)
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)
Quoted: 0 Post(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.
Falanx is offline   Reply With Quote
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off



All times are GMT -6. The time now is 01:28 PM.