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Old 05-24-08, 10:36 PM   #1
Banzai
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"Smoothing" aluminum TIG welds

I posted something similar in the framebuilders forum, but I wanted to get a few more opinions.

Is there any danger in smoothing/filing/sanding/Dremel-ing TIG welds to achieve that smooth "Cannondale look"? I mean...a number of companies do it, but do they make the welds with a load of extra material to sand down, or do they just use the weld as it is?

I'm considering doing some cosmetic work along these lines to my commuter bike and then powdercoating in a single color. My commuter is the matte black Nashbar "X" Cyclocross frame...it's a plenty "beefy" frame, and I've been told the welds look like very good, very strong welds.

Opinions appreciated before I go do something catastrophic!

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Old 05-24-08, 11:08 PM   #2
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I have to admit, a lot of welds look like they were performed by someone in their 2nd day of welding class that was being paid by the piece.
A skillful welder can reduce the amount of "finishing" needed by a great amount.

The danger in grinding etc. is removing metal from the tubing.
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Old 05-25-08, 09:17 AM   #3
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These welds are big...but they're symmetric and even. To be safe about the tubing, I don't think I'd be able to totally blend in the welds. (But I've heard that there's a powdercoat capable "Bondo"...) But I think with a Dremel and some time I could turn the welds into a smooth ring around the joint without damaging the tubing.

Luckily the frame was not expensive. If I trash it I can buy another...though Mrs Banzai will be rather upset about that!
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Old 05-25-08, 09:27 AM   #4
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Luckily the frame was not expensive. If I trash it I can buy another...though Mrs Banzai will be rather upset about that!
I don't have the technical background to know but I'd do it if it was my bike. That's how you learn. At the very worst (which I doubt) you'll have to find another frame. If that happens, chalk the cost up to "Education".
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Old 05-25-08, 11:22 AM   #5
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I was at one time a certified structural steel and pipe welder. My welds would at times be x-rayed in order to pass inspection...not trying to impress you, just letting you know I'm not pulling this stuff out of thin air.

When dealing with joints that require structural integrity, Tig welds are not usually ground down for cosmetic purposes. All hand welders try to achieve the "off-set dime stack" look that you're talking about. If my Tig welds did not come out clean, I'd either toss the metal or break the weld, grind ALL the weld off and start over again. Needless to say, this was a rare occurance after a while. On the other hand, it is not uncommon to see stick welds (arc) grounded down, but there is a reason for that. Stick welds are more prone to leave voids.

You can grind the weld down, but use caution that you don't compromise the structural integrity of the weld. If you do grind any significant amount away, you may lay a new bead over the old weld, again that's not the norm with Tig.
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Old 05-25-08, 11:28 AM   #6
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All hand welders try to achieve the "off-set dime stack" look that you're talking about.
That's a good descriptor...I like that, and thanks for the very informative post!

So. How DO manufacturers achieve that smooth look? Do you know anything about the powdercoat friendly Bondo?
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Old 05-25-08, 12:07 PM   #7
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In the automotive world they have robots doing welding.
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Old 05-25-08, 02:27 PM   #8
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I was at one time a certified structural steel and pipe welder. My welds would at times be x-rayed in order to pass inspection...not trying to impress you, just letting you know I'm not pulling this stuff out of thin air.

When dealing with joints that require structural integrity, Tig welds are not usually ground down for cosmetic purposes. All hand welders try to achieve the "off-set dime stack" look that you're talking about. If my Tig welds did not come out clean, I'd either toss the metal or break the weld, grind ALL the weld off and start over again. Needless to say, this was a rare occurance after a while. On the other hand, it is not uncommon to see stick welds (arc) grounded down, but there is a reason for that. Stick welds are more prone to leave voids.

You can grind the weld down, but use caution that you don't compromise the structural integrity of the weld. If you do grind any significant amount away, you may lay a new bead over the old weld, again that's not the norm with Tig.
Kind of comparing apples here, but in the nuclear industry, welds have to be "smooth" for decontamination purposes. Imagine a scenario where "contaminated" water drenches some piping. The joints have to be smooth enough that they can easily be wiped down to decontaminate. Porosity and other "roughness" that can "trap" radioactive particles simply isn't allowed. Grinding is very common.
Typically, aluminum isn't used for the reactor parts, but refueling equipment (staging, ladders etc.) can be.
Welds must also be smooth enough to test with dye penetrant for piping systems.
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Old 05-25-08, 02:28 PM   #9
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Grinding or "dressing" frame welds was very popular with the street rodders I used to hang with. Later, after a few thousand miles, the welds would start to crack. Since then, I've never ground any welds, ever. Yes, I get my welding done by someone who is really good at it. Yeah, aluminum welds are a little on the fat side. Your best bet is to leave them alone. bk
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Old 05-25-08, 03:08 PM   #10
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A lot will depend on the penetration. Steel tends to penetrate nicely and you can easily and safely profile the weld lines. Also it's not all that hard for a good welder to get the smooth fillet look in steel and with stainless steel.

However when welding thin walled aluminium I think you'll find that the welders or the machine setup folks don't go so much for full joint penetration. With joints of that sort the bead is needed to bridge the structural gap.

For example I've read of two stories where guys with japanese sportbikes sent in their aluminium frames for polishing. The polisher, being used to working with steel and castings more than welded assemblies, "helpfully" ground down the welding beads and then lovingly brought the metal to a mirror like finish. The frames were ruined in the process as cracks occured almost immediately when put back into use.
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Old 05-25-08, 11:30 PM   #11
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The consensus seems to be leaning towards "Don't!".

So.

Who can tell me, at least generically, how they think Cannondale does it? Or, does anyone know a powdercoat compatible Bondo type product?
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Old 05-25-08, 11:38 PM   #12
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If the welder knows their stuff they can lay the bead down in such a way and with the right penetration to form a pretty nice looking bead in the first place and in a way that will allow for some profiling. But if the bead isn't done that way from the outset then you're best off to leave it alone.

My old school R series touring Canondale has huge filleted beads. But since mine is the lower range "cheapie" of the series my frame didn't get the nice profiling done to it. So my welds look as if they were done with a caulking gun.
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Old 05-26-08, 07:59 AM   #13
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I don't have the technical background to know but I'd do it if it was my bike. That's how you learn. At the very worst (which I doubt) you'll have to find another frame. If that happens, chalk the cost up to "Education".
I'd say at the very worst you'll have to have your face surgically repaired if the joint fails at the wrong time. You're not necessarily going to know if you weakened the joint enough to cause failure until you put some stress on it - it would suck if that stress threshold was during a nice high-speed descent and your head tube decides to separate itself from the rest of your frame.
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Old 05-26-08, 09:36 AM   #14
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Who can tell me, at least generically, how they think Cannondale does it?


Sorry, I have no technical info to add to the thread. But I do remember reading an article about Cannondale once, at that time there was a guy (probably quite a few, actually) in the factory who's only job was to grind the welds smooth. My impression was that it was sort of the "new guy" position, after you did that awhile you got to move up the ladder.

Like you, I remember wondering at the time if Cannondale's welds start out with a little more material so they can afford to grind some off-
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Old 05-26-08, 12:17 PM   #15
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Who can tell me, at least generically, how they think Cannondale does it?
Check it out:
http://cgi.ebay.com/Rare-Cannondale-...QQcmdZViewItem
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Old 05-26-08, 01:51 PM   #16
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Considering that cannondales have a reputation for cracking, i'm not sure that you want to emulate them.
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Old 05-26-08, 07:52 PM   #17
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Considering that cannondales have a reputation for cracking, i'm not sure that you want to emulate them.
In all the reading I've done, I think that is a minority opinion with little to back it up. But that's not the purpose of this thread.

BikeManDan...neat picture.

I did some internet research on this subject, and it turns out that C'dale welds practically come out that way...they double weld, with the second pass laying down more metal. The few bumps that are left are sanded off, but according to the reading I did those are actually very few, and the double pass achieves higher weld penetration.

I don't weld, but it sounds plausible to me.

So. I'm not going to touch the welds on my bike. I intend to Bondo over them, and then powdercoat. Who present knows a powdercoat safe/compatible Bondo?
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Old 05-27-08, 01:50 AM   #18
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I think you should ask your powdercoater that question. I think it's probably a primer, but I'm just guessing.
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Old 05-27-08, 08:47 AM   #19
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I don't think powdercoat will adhere well on your typical plastic bondo. Youi may need to find something metallic based. Search for posts by DrDeltron. I'll bet he has answered that question before on here.
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Old 05-27-08, 04:30 PM   #20
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Dr. Deltron hasn't been heard from for quite some time, even by those of us who have been trying to reach him.
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