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  1. #1
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    Default Stress marks along metal fold line

    Just wondering if anyone with a metal folder has come across this sort of stress on the outside of a bend, and what might cause it, is my bend too sharp or possibly wrong grade of metal ? I don't think the strength of the metal has been altered too much - though I do think that if I was to bend it back the other way the metal would probably crack along the fold line.

    The metal is 2mm aluminium, unknown grade - folded in a home brew folder based largely on Simso's design, the width of the bend is 440mm and this is probably approaching the width limit for bending 2mm aluminium sheet as I detected a slight bow of a mm or so in the bend. I was surprised at the amount of effort needed for the bends - I initially thought my folder was not up to the task, but eventually found all I needed was to put some extra muscle into it.

    20180308_223040.jpg20180308_223051.jpg

  2. #2
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    Quote Originally Posted by familyguy View Post
    is my bend too sharp or possibly wrong grade of metal ? ]
    Yes to the bend is probably too sharp, a maybe to the material type and I would add bending along the grain line - read- in the direction of sheet rolling lines.

    I taught with a bloke who was a sheetie and recall him mentioning that the bending parallel to the " grain" line was likely to propagate a crack.

    There may be other causes .

    Grahame

  3. #3
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    Default

    I know they would sometimes preheat the aluminium at work when folding aluminium in the press brake.

  4. #4
    BobL is offline Member: Blue and white apron brigade
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    Any Al above about 1mm I would usually preheat before bending to such a hight radius.

  5. #5
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    That material is almost certainly 6061T6, meaning it was 'hardened' to a small extent - causing the stretch marks. You are right, It would break if you tried to bend it in either direction now.
    If you have access to an oxy torch and you REALLY need to bend this tight, as Com-VC said, heat the fold line. To anneal aluminium, the easiest is to light the torch without oxy and put acetylene soot on your fold line. Then add oxygen to a neutral to carborising flame and heat the fold line until the soot JUST disappears (burns off). Then let cool. The line where the soot is gone is now dead soft. Note: folding it tight will harden it again, so reversing the fold or changing it will need re-annealing .
    I'm reasonably sure you can re-anneal as often as you need to. I learnt this at an aluminium car panel making course.
    Cheers, Joe
    retired - less energy, more time to contemplate projects and more shed time....

  6. #6
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    Thanks for all of the replies.
    The Al sheet is actually an old road speed limit sign, my local salvage yard had them out for $5ea, I grabbed all of the biggest ones that were around 900mm x 600mm. I had a look at the others sheets I still have and can't detect any rolling lines probably because the material has spent some time in the weather.

    I'm quite please with the end result of the bends, all of the time taken to make the folder was well worth it, I do have another amplifier chassis to bend up that has one bend greater than 90deg and as I have an oxy I'll give Joe's annealing advice a try.

  7. #7
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    They're not so much stress marks but incipient cracks. The bend is now much weaker than the base metal, any loading that reverses the direction of the bend will likely produce a prompt failure, even a little vibration. The bend may also fail over time with no load. Reduce your bend radius significantly or anneal the sheets you're using and re-age the aluminum if necessary.
    The stone age didn't end because the cavemen ran out of stones...

  8. #8
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    Quote Originally Posted by jhovel View Post
    That material is almost certainly 6061T6 ...
    Joe out of curiosity, what makes you say this is 6061? Even before reading it was a street sign I would be very surprised if it was. 6061 is used extensively in aircraft, motor racing etc, but I'd think this is much more likely to be a 5000 series, probably 5005 or 5052.

    I don't do a lot of bending, but would like to. Indeed just took a mate to H&F to look at sheet metal equipment ... as you when a visitor hits Sydney. By the end of the 3rd hour there we were on first name basis with most of the staff Fortunately I don't currently have the room. He does, the bugger. Anyway, my limited experience with it is if you see those cracks you've overcooked the bend. Different grades/tempers aluminium can be bent at more/less inside radius without cracking. You should be safe with 1.5 -2t however. If that's 2 mm sheet, then you will need a 3-4 mm INSIDE radius bend. It looks to me as there was no bend radius provided when bending.

  9. #9
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    The al used in road signs has some additive that makes it a bit tougher than straight al. I also have some street signs I use from time to time, it always fractures along the fold line. Normal al doesn't as it is very malleable.

  10. #10
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    Quote Originally Posted by onanonanon View Post
    The al used in road signs has some additive that makes it a bit tougher than straight al. I also have some street signs I use from time to time, it always fractures along the fold line. Normal al doesn't as it is very malleable.
    What is "normal al"?

    There are dozens of different grades of aluminium commonly used also working and tempering, and all can make the materials very different from each other. I'm going to guess for something like a road sign like this they will go with one of the cheaper grades as all they need to do is not bend in the wind and also not fall down

    Edit: Indeed a quick Google search suggests the signs are indeed 5052 as suggested https://www.tccs.act.gov.au/__data/a...igns_01_00.pdf the street signs are 6061, which would make sense as they are long and narrow (6061 = more bogan proof)

    Here's the bend radii for 5052 http://www.atlassteels.com.au/docume...v_Oct_2013.pdf at H38. That's a bit larger radius than I guestimated, perhaps up to t3. My understanding is H38 is getting up there in terms of cold working but don't have time to look it up properly and it's not really my area of interest so I have no real experience with this. However I believe just like the grade, the tempering number is very specific in how the material has been treated. All covered here for a bit of light reading https://materialsdata.nist.gov/bitst...=3&isAllowed=y

    The bottom line is the material is designed to resist precisely what you're asking it to do. The answer is to increase your bend radius (or possibly re-temper the material, something I've had almost zero experience with beyond threatening material with a blowtorch from time to time with no real clue as to what I was doing!)

  11. #11
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    Not sure what adjustments your bender has, but if you can adjust the distance between the bending leaf and the radius bar, adding a thin piece of sheetmetal on the inside of the bend will increase the effective radius of your existing bar without modifying the bar itself. Ie grab a strip of thin sheetmetal, bend it tight using your exising setup, then move the radius bar back (leaving the bent piece in the bender) and slide your work underneath then clamp/bend.
    Also, if you've got the leaf to radius bar distance set too tight you could be squashing the bend and increasing the workhardening.
    Increasing the distance between bar and leaf (even without a larger radius bar on the inside) will relax the bend a bit and give a slightly larger radius. The radius won't be as perfect but its another adjustment to try.

    Out of interest what sort of amplifiers are you building. Audio? Guitar?

    Steve

  12. #12
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    Default Stress marks along metal fold line

    Slightly OT- I also don’t have the space for a sheet bender. My poor man’s solution is to cut a half thickness groove in the sheet with an aluminium cutting blade in my table saw or circular saw. I can get very tight bends with this method. Obviously the material loses a lot of its strength, but for something like a valve amp chassis it’s still plenty strong enough. That’s exactly how I made the chassis for my son’s guitar amp. I might try the oxy torch method next time.

    Looks like you have spaces for a few valves. How many watts will it be?
    Chris

  13. #13
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    Normal = normal, pure unadulterated, unalloyed. No doubt someone will come in with the chemical makeup and why there is no such thing as 'pure' aluminium. I have some nice soft aluminium sheets, nice to work with but very easily damaged and bent. Also have some harder stuff I got from Machinery Disposals in Cavan, sort of in betwen soft and road signs, but bends well and is great to work with. The al in road signs is very similar to aircraft aluminium, not quite as hard but almost.

    For anybody after some sheet aluminium, Machinery Disposals, just up the road from Paramounts, have tons and tons of sheet aluminium, cheap as.

  14. #14
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    I don't want to sound pedantic, but pretty much all aluminium you'll find being used, other than some 1000 series (useless for pretty much anything), is alloyed, it's just what it's being alloyed with that will vary. I merely mention it as it can be a critical thing to know if you want to weld it for example.

    Within the grades, the temper applied probably has as much influence on the bendability as the grade itself. Here's a data sheet for 5052 (very common as sheets) and it can be seen for precisely the same grade of aluminium, the minimum bend radius can go from 0t to 3t (@3.2 mm) based merely on the temper http://www.atlassteels.com.au/docume...v_Oct_2013.pdf So it's perfectly plausible to have two sheets, both of 5052 that will behave completely differently based purely on the temper.

    The issue with buying scrap is generally speaking you don't get to choose the grade. The scrappy by me sometimes gets aluminium in with the print still on it, and I always preserve that or remark it if that's the case, but otherwise it's a bit of a lucky dip what you get.

    I used to live next to a bauxite mine (as you do), and it was said aluminium is the closest we'll see to solid electricity!

  15. #15
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    Aluminium - Element, AL. Group - 13, Period - 3, atomic number - 13, melting point - 660.323C, boiling point - 2519C. Discovered in 1825 by Hans Oersted. Friedrich Wöhler tperfected the method of obtaining pure aluminiun, in 1827, the name aluminium is derived from the the latin 'Alumen' meaning 'bitter salt'. Arguments?

    I used to live next door to a butcher, could that be why all my work is so rough, or is it simply the reason I like sausages???

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