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  #21  
Old 07-07-2011, 07:06 AM
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Marty Comstock Marty Comstock is offline
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There are some non scientific thoughts going on here with MIG welding.

I have successively welded with no gap, very high amperage settings, little buildup, and no peaking. I dont fuss around with grinding, i just get a bigger one.

Same rules and physics apply, when the puddle is in its molten state, outside stresses push toward the path of least resistance, the puddle, causing the shrinking. Same happens when using a shrinking disc.

The peaking happens when there is more material, more weld in a small area. Try putting less weld time into the panel. Turn up the machine, spend less time on the trigger.

When there is no gap, most people either dont turn the amperage setting up much, if at all. Now a welder has to stay on the trigger longer to get the same penetration as before without a gap. The key here is "more time on the trigger" adding more energy input, as well as more material. I do not know exactly what effect more material has other than being bad and more crap to grind, but i bet Someone has an answer.

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Last edited by Marty Comstock; 07-07-2011 at 07:23 AM. Reason: more coherent thoughts to try and explain the differences
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  #22  
Old 07-07-2011, 01:46 PM
JasonJamesWrenn JasonJamesWrenn is offline
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Quote:
Originally Posted by robert View Post
that is correct cause you have no gap and adding material
welding mig u need a gap 1 mm that way there is place for the welding material, i tack every inch or more with 1 mm gap then make a few but welds in one pass and then grind then repeat the process

i find grinding before everything cooles the metal is not restricted to shrink back to the original place if you dont grind emediatly the weld holds back the process because it is much thikker then the sheet
Thanx so much. All this info is very cool and challenges me to take what i can and apply it to what i allready know. I have read of this way before and i gave i wrote it off as overthing it and taking to much time. But, i will give it a shot and see how it feels. So 1mm huh. Would you use clecos or i also have rare earth magnets witch i love. With my work i attatch large auto panels that cant be jus help in place. Im thinkn of using my air saw more often So i can get a small gap. Almost all my panels are fitted by layn one over another then cuttin. I have had great results with this. But the gap with blade width or better. About .040 and alot of grinding.
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  #23  
Old 07-07-2011, 01:55 PM
JasonJamesWrenn JasonJamesWrenn is offline
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Quote:
Originally Posted by Marty Comstock View Post
There are some non scientific thoughts going on here with MIG welding.

I have successively welded with no gap, very high amperage settings, little buildup, and no peaking. I dont fuss around with grinding, i just get a bigger one.

Same rules and physics apply, when the puddle is in its molten state, outside stresses push toward the path of least resistance, the puddle, causing the shrinking. Same happens when using a shrinking disc.

The peaking happens when there is more material, more weld in a small area. Try putting less weld time into the panel. Turn up the machine, spend less time on the trigger.

When there is no gap, most people either dont turn the amperage setting up much, if at all. Now a welder has to stay on the trigger longer to get the same penetration as before without a gap. The key here is "more time on the trigger" adding more energy input, as well as more material. I do not know exactly what effect more material has other than being bad and more crap to grind, but i bet Someone has an answer.

Marty
Marty.... i think we are on the same page with mig. I can weld just fine with no gap and as i was taught "Hot n Fast" with out peak or panel distortion. Fast on the trigger is my mindset always. Thanx for that confirmation, Early on. I forced my donor piece in place leavn the metal no were go. Times have changed and its all good now. I realy wana find a good way to stay with my mig for now but fine tune the heck outta the process and for a lack of better words , master it... at the same time firing up the torch and begin to learn its way.
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  #24  
Old 07-07-2011, 03:56 PM
robert robert is offline
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Quote:
Originally Posted by JasonJamesWrenn View Post
Thanx so much. All this info is very cool and challenges me to take what i can and apply it to what i allready know. I have read of this way before and i gave i wrote it off as overthing it and taking to much time. But, i will give it a shot and see how it feels. So 1mm huh. Would you use clecos or i also have rare earth magnets witch i love. With my work i attatch large auto panels that cant be jus help in place. Im thinkn of using my air saw more often So i can get a small gap. Almost all my panels are fitted by layn one over another then cuttin. I have had great results with this. But the gap with blade width or better. About .040 and alot of grinding.

i also fit panels layn over the other and even tack them. after that i am using air saw and thats how i get my 1mm gap
i am not saying this is the way its just my way and it works for me
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  #25  
Old 07-25-2011, 03:22 PM
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Originally Posted by Richard K View Post
I often approach ideas a bit differently than many do. Some may see the above post as a bit sarcastic, others may see it as foolish. Others may call it "thinking outside the box".

I consider ideas and rules of thumb as points to begin. I've never allowed myself to get boxed in. I've been shut out, but not shut in.

The above post simply questions if the statement posed is valid. The statement is valid, however the application of the principal has been taken out of context.

In order to learn, we have to think. The above was written to provoke thought. Think about "Does steel shrink when it cools"

Point to ponder (1): The phrase "steel shrinks when it cools" is missing one very important word that begins with an "R". Over the years that word has been dropped from the phrase. Why? I have some ideas but do not know for sure.

Point to ponder (2): before steel can cool and possibly shrink. I must be heated and possibly expand.

Post your thoughts; in a day or two I'll add on to the discussion.
Metal does indeed shrink when it get cold, it also grows as it gets warmer. Steel is no exception. When the piece of steel changes temperature uniformly it results in even growth or shrinkage that will reverse itself as the temperature returns back to its starting point. In your example the growth from the day time high reverses itself as it cools to the nigh time low. The next day when it gets up to 120 deg F again it will be the same size as the day before.

When locally heated and then rapidly cooled, metal can only shrink in one plane if it grows in another. Otherwise there would have to be a reduction in material. The metal that is being heated, by a weld or flame, will grow. The unheated metal that surronds it will not. This surrounding metal resisists the hot metals attempts to grow out horizontally and forces the metal to expand vertically. As this hot metal cools it shrinks and pulls in on itself from all directions. In its cooled state it has shrunk by "6 millionths of an inch per inch per degree of heat rise". This new smaller dimension horizontally causes a shrink in the panel.

To illustrate this, heat a 2 foot steel rod in the middle of its length. Do it so that the cherry red only extends halfway through and the, to prevent the heat from evenly heating the rod all the way through and promote rapid cooling, throw a wet rag on it. What is happening here is the hot spot trys to grow but the metal on the other side resists its length ward growth. As a result the hot spot has to grow out from the rod instead of lengthwise. When the hot spot has cooled it shrinks in all directions and causes the bar to bend. If we heated the whole rod width of the rod evenly it would have grown and then shrunk back to it original orientation.


Wow, that sound way more concise in my head.
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  #26  
Old 07-25-2011, 09:51 PM
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Default shrinking metal

I haven't been around much lately, returned to college in Jan. majoring in Mechanical Engineering. took a break from my studies to see whats new on the site and found this post. I'm taking my ferrous metallurgy final this week so maby I can shed some light on this debate Simply put the changes in low carbon steel (1040 in this case) are dependant on the temperature you heat it to. under 300 degrees there is little change in the granular structure of the steel and it will return to it's original state. To cause shrinkage or distortion the metal must be heated above it's lower transformation temprature (about 1200 degrees) a weld puddle will get you above that temp. The distortion or shrinkage is a result of localized heat increase. Warming the area around the weld will help(but not too hot) If cooled slowly the metal will have little physical change. If the metal is quenched, the rapid cooling will cause a smaller grain structure thus the metal "shrinks". Low carbon steel doesn't harden well this is the reason it is used for bodywork. We have a Rockwell tester in our lab. mabe I could test some weld samples to see just how much the area is effected
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  #27  
Old 07-26-2011, 12:12 AM
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Ted,

I've never measured the temp the shrinking disc brings the metal up to, but I believe it to be far less than 1200 degrees. So - is it correct to say that metal heated in the range from 300 to 1200 degrees, will shrink if quenched in proportion to the amount of heat input? So if it's 500 degrees, it will shrink a little, and at 1000 degrees it will shrink more?
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  #28  
Old 07-26-2011, 12:44 PM
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well John I did the math and this is what i found, a peice of 18GA. 2' Wide x 6" long heated from 80 deg. to 500 deg. will expand .016" when heated to 1000 deg. it grows to .036 this is enough I should be able to measure the contraction after quenching in my shop. (won't be back in lab till the end of aug.) I'm going to cut a few test strips and see what happens. we know the metal shrinks the question is how much? there are so many varaiables in welding pannels the big question is how much does the weld bead contract after cooling? the math says if left to cool in still air a 6" bead will contract .109" A 1" bead only .018". I'll quench some test strips and post the results.

OK here's what happened. The 2" x 4" test strip didn't show much contraction at 500 deg. 3 heats and the shrinkage was less than .002 at about 1000 deg each heat netted .003 up to about .009 then tapered off. but here's the supprise when I heated the strip to an orange glow then let it cool slowly it contracted another .002! so much for text book learnin' From what I've found out here I'd say the sweet spot for the shrinking disk is between 800-1000 deg. I'm going to try this again in the lab under more controlled conditions and see what happens. My prof. builds race cars so he will probably like this experiment, going to send him an email now
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Last edited by 87hdrush; 07-26-2011 at 02:23 PM. Reason: results
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  #29  
Old 07-26-2011, 02:08 PM
Overkill Overkill is offline
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Default Experiment

Ted,

It will be interesting to learn the results.

You are right, there are lots of variables. Just in shrinking techniques alone. And then if you add welding to the mix, will be interesting to hear more. Because most welding is linear in nature, wonder what that adds to the discussion?
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  #30  
Old 07-26-2011, 02:13 PM
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With all the variables does scientific proof need to be pursued?

If 20 ga. has X amount of shrink at 1200 degrees, why not just pre-stretch the edge before you weld to compensate??? Oh, ya. we are not anywhere near that consistent
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