Bending wire question (seriously!)

Discussion in 'The Projects Forum' started by ErnieM, Mar 21, 2012.

  1. ErnieM

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    When bending a simple piece of solid uninsulated wire what is the minimum bend radius? I can find several military specs on jacketed cable and conduit and such but nothing on wire itself.

    Does anyone know of any standard (even an in-house limit) for this operation?
     
  2. kubeek

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    Do you mean minimum achievable radius or minimum reccomended radius?
    If you apply enough force you can bend the wire in right angle without any radius..
     
  3. #12

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    No. I treat naked copper wire as if it were sheet metal. Wire is very close to being pure copper. It work hardens, it anneals, it oxidizes, it can be bent 180 degrees in its own diameter to be put under a screw clamp or in a wire nut.

    That's what I know about it, and it doesn't look like much.
    Anybody else?
     
  4. ErnieM

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    Assume if the wire breaks someone dies. That scenario may not be very far from the actual use.

    I could put the wire in a vice and get it to fold in half, essentially a 180° bend, but that is not recommended practice. There is too much risk if the wire work hardens and eventually breaks.

    Here I have a thin (.008" OD) wire and I need to dress it back on itself in production so I am looking for a recommended practice. I would imaging a bend radius of 3 to 4 times the OD would be sufficient.
     
  5. #12

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    .008" is AWG #32

    That is among the sizes that my transformer man said broke so easily that he didn't want to wind transformers with it. No wonder you're studying how to make a reliable connection!
     
  6. Wendy

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    Personally I don't think it is the angle, so much as repetition. Bend it once, and you are good. Bend it twice and scrap it.
     
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  7. THE_RB

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    Use a microscope? Overbending will form visible fractures, so if you do test bends and examine and there is no sign of fracturing it is likely to be safe, as fracturing will be visible long before it gets bad enough for the wire to fail.

    By the way, if there are safety considerations my post is entirely hypothetical and should not be taken as constituting advice of any form. :)
     
  8. Wendy

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    Chicken!

    Sounds reasonable though. Copper is a very soft metal though.
     
  9. mlog

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    If that's true, I hope you've done a safety analysis with a risk assessment. Any hazard that is potentially catastrophic needs special attention.

    With faults, you have two general means of handling them. You can avoid or tolerate.

    Avoid means to either prevent the fault (QA, procedures, etc.) or to remove the fault (repair, scrap, etc.).

    Tolerate usually involves some method of redundancy (detection or masking).

    It sounds like you're choosing the path of avoidance and prevention by employing specific wiring practices and procedures.
     
  10. wayneh

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    Some useful? comments here (see last sentence).

    Looks like most copper wire can be safely bent to radius/thickness = 0.5, or radius = 0.5 X thickness. That's much tighter than the radius of most of the cable bend tables, which show something like minimum radius = 4 x thickness.
     
  11. KJ6EAD

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    I've always been told that both NASA and IPC standard allow a minimum bend radius of one wire diameter for solid conductors but I can't give you a reference. ASTM does have defined methods for testing the minimum bend radius of wire.
     
  12. mlog

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    SAE AS50881D, paragraph 3.11.7, Radius of Bend Measured to Inside Surface, subparagraph a. says:

    "Wire, electrical cable and harness. For wiring groups, bundles or harnesses and single wires and electrical cables individually routed and supported, the minimum bend radius shall be ten times the outside diameter of the largest included wire or electrical cable...."

    The above is almost word for word identical to par. 3.11.7 of MIL-W-5088L, which was replaced in 1998 by SAE AS50881.
     
  13. ErnieM

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    Bill: "one time" is an assumption I can only hope the people in production will understand. We have good people so I am not worried there.

    RB:Of course the first sample pieces will go under a microscope for inspection. My motto has always been "trust, but verify." I may go as far as to make a simple fixture so bends are uniform as it would just be a length of "piano wire" held to the PCB with these wires, but that is probably over-thinking this.

    mlog: yes, I am looking to avoid issues. The basis of the investigation is the removal of a potential shorting condition that has yet to occur, but a customer driven improvement is being requested along with several other changes. The engineering complication arises since you can fix one thing but then break another.

    wayneh: excellent link, always good to have a reference for these obscure things.

    My actual material is an alloy of nickel and copper called cupron. It's a non standard wire, but very useful as you can both solder to it and weld to it. I solder my end and my customer welds his end, so that material is a hard requirement. The material has never appeared to be "brittle" or has a similar ductility to copper.

    The supplied link by wayneh suggests a very tight bend, essentially a (copper) wire may be dressed 180° back upon itself with a total height of 3x the wire diameter. That is tighter then I could tolerate, as there is some insulation past the bend.

    Thank you gentlemen!
     
  14. ErnieM

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    Actually that agrees with wayneh's link. One diameter.
     
  15. #12

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    referring specifically to Post #4, paragraph 2: With wire that small, you could anneal it after bending with nothing more that a candle flame. (Of course, you'd need a cleaner heat source.) Annealing temperature of copper: 761 F
    Probably in the range of some soldering irons.

    Edit. You aren't using copper. You're using cupron.
     
    Last edited: Mar 21, 2012
  16. mlog

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    I'm not trying to give advice on the design since (you) ErnieM stated "if the wire breaks someone dies," and this is the last I will say on the subject.

    However, the Fiskyalloy link was effectively a test for ductility, which means the metal will yield before it fractures. Proving in a laboratory a wire with bend radius equal to its thickness will not fracture is not the same as demonstrating reliability in the field. For example, the test does not include environmental effects such as temperature, vibration, and shock, nor does it include any factor of safety. While the SAE document might seem overly conservative, I suspect it applies a factor of safety commensurate with a catastrophic severity level.
     
  17. tedcs

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    Life-critical devices, may require consideration of factors which are usually of no consequence. Will the solid wire be used to hang a potted plant? ... Used in an implanted medical device? ... In a nuclear trigger device? ... Man-rated space equipment? ... Part of a rat trap?

    To what conditions will the wire be subjected and what characteristics of the wire are important to maintain? The exact composition of the wire and how it was manufactured might be critical ... or unimportant.

    Sometimes, one may say that it's OK if it looks OK.
    Sometimes that approach would prove unsuccessful.
     
  18. wayneh

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    I agree. The lab work even says something like "most" will pass, which can only mean that some don't. It's the edge of what can be done, a "do not exceed", not a recommendation for something to practice in the field.
     
  19. mcgyvr

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    Industry standard minimum bend radius ranges from 5 to 10 times the diameter (depending on which standards your "device" will be covered by)
     
  20. THE_RB

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    It can be common in transformer manufacture etc to bend a single core wire totally back on itself generally for termination, where it is then secured.

    The "bend radius" standards take into account lots of factors like the wire may be re-bent or subject to vibration or movement etc.

    As for what is going to survive well long term, a bit of common sense (like exactly HOW the bend will be treated over time) will go a long way.
     
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