Induction heat - Paintless dent removal

Discussion in 'The VIP Lounj' started by pillatier, Oct 16, 2018.

  1. pillatier

    pillatier Well-Known Member

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    Works on steel parts not aluminum. Did not know the technique existed.
     
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  2. carlthess40

    carlthess40 Member

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    I was doing that back in the 70’s with a propane torch and a rubber mallet to pop it back in place
     
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  3. CJ

    CJ Bronze Member Admin Top Poster

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    That's actually kind of cool.
     
  4. JeffC

    JeffC Well-Known Member Donor

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    You didn't know you could shrink metal with heat? That's how folks have been getting stick bolts out for 70 years.
     
  5. pillatier

    pillatier Well-Known Member

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    I did not know of the induction heating technique/machine to take out dents. Basic physics I learned said metals expand when heated and contract when cooled, metals of different thickness will expand at different rates, similarly different kinds of metals and alloys have different expansion coefficients. I have press fitted rods through thick sheet steel by heating the pre-drilled holes in the sheet and chilling the rod in liquid CO2.
     
  6. JeffC

    JeffC Well-Known Member Donor

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    Lead sleds in the 50s were massaged by heat and seamed with lead. Sometimes sunlite will take out hail dents in a black roof when you shrink them with dry ice. Been done in the midwest for decades.
     
  7. Mike B

    Mike B Well-Known Member Donor Top Poster

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    As far as I know metals expand with heat.
     
  8. Chris Slade

    Chris Slade Well-Known Member Top Poster

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    Answer from the web (spoiler Mike is correct). The only thing I can think of that does not expand with heat is water, it expands when it freezes.
    ____________________________________________________________________________________________________________
    The answer is surprisingly simple: the bolt expands, but the nut expands more.

    What is happening here is good old thermal expansion:

    • The bolt is heated and expands outwards, its radius increasing
    • The nut is heated and... expands outwards, its radius increasing
    Now, since the nut's radius is slightly greater than the bolt's, and since the increase is proportional to the rest length, the nut expands a little more.

    Iron has a thermal coefficient in the 10-5 / K ballpark. It means that for each 1 K increase in temperature you have an increase in size of 10-5: a 1 m rod becomes 1.00001 m long.

    If your bolt has r = 1.5 mm, and the nut has R = 1.501 mm, what happens is temperature is increased of 500 K? Well:

    • r = 1.5*(1+500*10-5) mm = 1.5075 mm
    • R = 1.501*(1+500*10-5) mm = 1.508505 mm
    As you can see, before heating R - r = 1 μm, while after R - r ≈ 1.001 μm. It increased!

    Please note my numbers are quite wild and used just to make an example. I am sure that I got the starting values wrong, but I hope they help to get the message across anyway.
     
  9. pillatier

    pillatier Well-Known Member

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    Water also expands but then it evaporates forms steam - recall the steam engines? In the nut/bolt case isn't the heat applied to the nut not the whole assembly? The heated nut thus expands faster than the higher mass bolt.
     
  10. John Celardo

    John Celardo Well-Known Member Donor

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    Wow, thank you Dr. Slade! I was a history major, but I really enjoyed physics in high school. I don't remember the formulas like you engineers, but I do remember the coefficient of heat expansion. I'm glad you engineer guys know how it works. I love mechanics, and I like to figure out how things work. Unfortunately I was to dumb to be an engineer, but I'm happy we have you guys.
     
    Last edited: Nov 14, 2018

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