Unbending a brass hood

Discussion in 'Fettling Forum' started by kero-scene, Sep 24, 2021.

  1. kero-scene

    kero-scene Australia Subscriber

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    A recent Tilley pork pie i purchased must have been used by the previous owner to support something heavy. As a result the brass hood has squashed downwards by a cm or two, closing the vents. (I can post a photo tomorrow if that would help to describe the situation.)

    The brass sections between the open vent sections have folded over under the weight. I’m wondering how to approach straightening them without cracking the brass. Is this likely to work and would doing this at high temp be best?

    Any advice welcome.
     
  2. Fireexit1

    Fireexit1 United Kingdom Subscriber

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    Annealing the brass first will return it's malleability
     
  3. Tony Press

    Tony Press Australia Subscriber

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    With these old pork pies, and their thin brass hoods, I usually just dismantle the components and reshape the hood by hand, then use a small peening hammer and a parallel-sided adjustable spanner to get the last kinks and dents out.

    Tony
     
  4. ROBBO55

    ROBBO55 Subscriber

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    As Fireexit1 said, anneal first, let it cool then straighten the hood. After getting the worst of the damage fixed, it might be worth annealing again before finishing the repair.
     
  5. kero-scene

    kero-scene Australia Subscriber

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    Thanks for the advice. I’m looking forward to getting it working.
     
  6. James K

    James K United Kingdom Subscriber

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    After annealing the brass will be nice and soft, as you bend it it will work harden and you will feel it getting less bendy. You will need to anneal it again and bend it some more. If it is really bent you may have to anneal several times, if in doubt remember you can't anneal to many times but you can crack it if you don't.
    Hope this helps James
     
  7. podbros

    podbros United Kingdom Subscriber

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    To anneal properly you need to heat and quench.. not sure if that would make it too soft but as said the re-bending will start to stiffen/work harden

    Has someone already covered repairing these in a tutorial??
    It’s a common fault with the early caps ?
     
  8. ColinG

    ColinG United Kingdom Subscriber

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    I need to do more research to make sure I have my facts correct, but when I was a lad we were taught how to anneal brass and steel. I was told that the two metals are exactly opposite in terms of how you either soften them (anneal) or harden them.

    Brass is hardened when it is heated to 370C and allowed to cool slowly. It is softened when it is heated to the same point and cooled rapidly - quenched in water usually.

    Steel on the other hand is hardened when it is heated to cherry and quenched. It is annealed by heating it and allowing it to cool slowly... very slowly which will affect the metals crystaline structure.

    As I say, I need to check this thouroughly but I believe this is the case.
     
  9. ROBBO55

    ROBBO55 Subscriber

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    As I understand it, brass is annealed as a result of heating process. Quenching stops you burning your fingers but doesn't affect the brass.
     
  10. ColinG

    ColinG United Kingdom Subscriber

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    If there are any metalurgists out there who could clarify the correct processes involved that would be great.
     
  11. MYN

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    In practice, I have greatly soften a bunch of springy-stiff brass wires by heating it to red-hot, hold the temperature at a specific temperature range and then quenching it in water.
    To harden them again, you need to work-harden it.
     
  12. Fireexit1

    Fireexit1 United Kingdom Subscriber

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  13. Fireexit1

    Fireexit1 United Kingdom Subscriber

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    I seem to recall from metalurgy classes at college (a long time ago) when studying mechanical engineering it is about relieving internal stresses (or dislocations of molecules) caused by stretching the crystalline nature of the alloy. These metals also harden with age certain aluminium alloys especially prone to it). Heating allows the molecules in the crystalline structure to move about to a more regular arrangement (de-stressing them).
    Brass alloys can be quenched but it is not neccesary. Steel must not be quenched for annealing but cooled slowly - otherwise you are tempering it.
     
  14. kero-scene

    kero-scene Australia Subscriber

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    Thanks again for the advice.

    As it turned out the hardest part was getting the old burner out of a squashed hood - tubes jammed on tight angles and no room for tools or fingers. In the end I took a destructive approach to removing the insect screen and things got easier from there. Here are some photos, in the first one it’s just been degreased.
    0FE63CAB-F6BF-481A-BCFA-0FACBF6E14FE.jpeg
    409DF3A9-3793-46C6-AD99-6B017B4A7983.jpeg

    I was keen to try the annealling because fixing it if it cracked is beyond my tool/skill level. Its closer to straight than it looks in the photo.

    The burner dome is dented too, so that’s the next task.
     
  15. Tony Press

    Tony Press Australia Subscriber

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    It’s looking better already!

    Cheers

    Tony
     
  16. Mr cod

    Mr cod Subscriber

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    Glad to see you managed to straightened it out without breaking the three small tags, that's an achievement in itself, replacement enamelled caps are available from julian shaw at tilleylampsandstoves.
    Regards Ian.
     
  17. WimVe

    WimVe Subscriber

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    Straitening out the legs that support the top hood is always a challenge.

    If someone has a good way of how to do this i would be happy to hear it. Especially with many legs or holes like a petromax and a overhanging top you can't get a pair of pliers or clamping thingy on it.
     
  18. Tony Press

    Tony Press Australia Subscriber

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    What do you mean by “… legs that support the top hood”?

    Photos might help…

    Cheers

    Tony
     
  19. WimVe

    WimVe Subscriber

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    @Tony Press the strips of metal between the top side openings.
    Or the legs that make these holes.
    The parts that kep the top disc up. its not a simple flat piece but curves with the diameter of the hoods outside. Depending the model you have four or more. In this case you have easy acces but if you have a petromax style or the older models with round(ed) holes, rebending isn't that straight forward.

    Top legs.jpg
     
    Last edited: Dec 3, 2021
  20. Tony Press

    Tony Press Australia Subscriber

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    Thanks, Wim. I now understand.

    Tony
     
  21. Martin K.

    Martin K. Subscriber

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    I had good success several times using a glass bottle that either fits snugly in the remaining round or is only slightly smaller than the remaining opening between the bent legs. Then I carefully roll the hood with the bottle in it back and forth on a flat, hard surface, slowly straightening out the bent legs.
    It may be necessary to use several bottles of different diameters in succession so that the pressure on the bent legs is distributed over as large an area as possible.
    Obviously, this only works when the cap had been taken off.
     

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