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PostPosted: July 24, 2014, 12:10 pm 
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esp42089 wrote:
It's a philosophical pursuit that leads to questions like why not use research grade argon while welding (which has no bearing on anything this side of NASA), but makes a great thinking exercise about why NASA does this?



Hmmm, hot Chinese chicks. Oh sorry, did you say something, I was drifting off somewhere.


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PostPosted: July 24, 2014, 9:43 pm 
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Hmmm, hot Chinese chicks. Oh sorry, did you say something, I was drifting off somewhere.


Just like Niles Crane while in the presence of Daphne? "I seem to have drifted off....."


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PostPosted: August 1, 2014, 12:15 pm 
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Has anyone taken into consideration the fact that right angle cuts on square tubing can be made easier than fishmouth cuts on round tubing. My fishmouth joints are not very good. The fitting of the parts before welding has a direct relationship to the strength and the warpage caused by the weld. The tighter the fit before the weld the better the weld.

wm


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PostPosted: August 2, 2014, 8:19 am 
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"The tighter the fit before the weld the better the weld." As a general statment relative to welding this is Wrong, but a tight fit is good for bracing i.e. braze welding.
Critical welds, such as some of the production suspenions parts, are actually fixtured with small gaps between the pieces so you get weld penetration down to the base of the metal.
With the light gauge tubes we use, I would hope most of us are getting reasonable weld penetration without the such fixturing.
Dave W


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PostPosted: August 2, 2014, 2:06 pm 
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My welding career consists of a one-semester course at my local community college and a completed Locost, but my welding instructor always insisted on tight-fitting joints. However, that was gas welding. I have heard that a slight gap works well for MIG, which, if true, is good news for the structural integrity of my car.


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PostPosted: August 2, 2014, 2:50 pm 
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From my amateur experience, tight joints minimize warp and minimize penetration. gaps can improve penetration at the risk of warpage.

Considering only MIG, since the wire is higher tensile than cold rolled, and if the penetration is good, I can't see how filling a 1/16 or even a 1/8" gap with weld can be a bad thing structurally? Shirinkage/Warping is still an issue though.

I am ready to be schooled.

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PostPosted: August 3, 2014, 2:41 am 
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davew wrote:
"The tighter the fit before the weld the better the weld." As a general statment relative to welding this is Wrong, but a tight fit is good for bracing i.e. braze welding.
Critical welds, such as some of the production suspenions parts, are actually fixtured with small gaps between the pieces so you get weld penetration down to the base of the metal.
With the light gauge tubes we use, I would hope most of us are getting reasonable weld penetration without the such fixturing.
Dave W


Yup, I see some pictures of "awesome gapless fitments" such as CNC cut fishmouthed tubes and I think some people don't quite understand the word penetration.

It's like mirror polishing a ported head, it looks nice and sells but isn't as efficient.


rx7locost wrote:
I am ready to be schooled.


Butt weld 2 pieces together without a gap and again with a gap and see which is stronger by a long way.

rx7locost wrote:
Shirinkage/Warping is still an issue though.



Allow for it.


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PostPosted: August 4, 2014, 7:15 am 
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A good butt joint for stick welding has a gap about 1/2 the rod diameter.

For mig, about 1/32 inch gives full penetration weld without burn through (as long as you keep the puddle moving)

Tig a slight gap about 1/32 is good too. Remember that you have to fill that gap with your filler rod, wire, electrode or else you're robbing material from the base metal.

Brazing a super tight fit is your friend.
:cheers:

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PostPosted: August 5, 2014, 8:55 am 
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I’m not going to start a debate about having a gap in a butt welded joint on material as thin as we are dealing with is, but I do want to comment on the original question of branch connections, and the comments on gaps. In a branch connection you have to separate the two types of weld that you have. In square tubing two of the sides are fillet welds and two of the sides are butt welds. A fish mouthed round tube is the same except there is a smooth transition from fillet weld to butt weld. Looking at the fillet weld joints the rule of thumb is that W=3/4T for a 100% strength weld. W = the leg length of the fillet weld and T = thickness of the thinner member in a joint. With our 16 BWG tube that gives us a leg length of 0.04875” If you wanted be more conservative you could make sure that your throat dimension is equal to your wall thickness and that would give you a W = 0.092 I know a actually measured all of my welds and they were all around 1/8” leg length and with my 14 BWG tubes they are only slightly over welded. Looking at allot of the welds on these projects they look to be over welded by quite a bit. So there is no need to have a weld with a gap because as long as your fillet weld is getting all the way to the bottom of the weld joint your weld is more that strong enough.

Attachment:
Fillet Weld.png


Looking at the grove weld part of the joint at least in my case the radius on the edge on the tube was greater than the wall thickness of the tube I am welding this naturally gives you a single bevel groove weld without adding any gap. If you really were worrying I would put a small bevel on the end of the tube long before I added a gap. Adding gaps in these types of welds may add strength, but it adds no required additional strength and does it at the expense of additional warping. Fish mouthed tubes follow this same logic when designed correctly so they are not fundamentally any stronger or weaker than a square tube with the proper weld design.

Attachment:
1.png


The real truth in the matter is that neither design is truly correct because in a structural weld design you should if at all possible avoid loading a material through thickness which is what both fish mouthed tubes and square tubes do. In any steel it is possible to have a lamination, and when you load a joint through a lamination you only end up with a weldment as strong at the thickness of the laminated plate. In the industry I work in you have to check any location that you design to load through thickness with 100% Ultrasonic Testing of that area to ensure that there are no laminations. Although in our world that chances of that happening are slim and not something that I would lose sleep over.

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Lamination.png


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PostPosted: March 23, 2015, 10:55 pm 
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Location: San Benito TX
The one problem I see with using round tubing is whether the person doing the welding has the skills necesary to make good welds. Square tubing with straight welds will be easier to get good welds. Good welds, or welds that require a higher degree of skill. Which is probably going to yield a stronger frame?


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PostPosted: March 24, 2015, 4:40 am 
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A subject not touched on here is the ease at which fishmouthed tubes can be joined at compound angles like on the headstock of a motorcycle frame. This just does not work with square tube without major head scratching. Also when smaller round tubes are connected to larger round tubes you get a much stronger joint , on a large square tube any converging smaller tube gets welded to the face of the larger tube without gaining any strength from the weld being near or attached to the side wall.

Bob

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PostPosted: March 24, 2015, 4:45 pm 
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Anybody use a WPS?


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PostPosted: May 18, 2015, 6:34 pm 
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Bobber wrote:
Anybody use a WPS?


I know many of the guys on here will cut and do a "macro" and or fillet weld break test sample welds. It would be something I would recomend everyone to do. Weld a sample in each position cut and check and write those parameters down, then make sure you are using the correct settings for each joint type/position. It's really the only way to know if you are getting quality welds. I wrote myself a wps with tensile and bend tests and all that jazz but mostly just as a reminder and that's what I do for a living so I had access to the equipment. I wouldn't expect anyone to go that far. The most applicable code is AWS D1.1 for what we do. I do not have a copy of that code but I bet there are prequalified weld procedures in that code that may be applicable.


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PostPosted: May 18, 2015, 7:30 pm 
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wment wrote:
Has anyone taken into consideration the fact that right angle cuts on square tubing can be made easier than fishmouth cuts on round tubing.
Doh! He's right--why have we never taken that fact into consideration? But seriously, folks, not only is it easier to cut square tubing, it's probably easier to weld, too. I know I can weld four straight inch-long seams more easily than one splendidly sine-shaped four inch seam.

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