Hardening & Tempering an E.W arbours pivot

Discussion in 'Clock Repair' started by ChrisCam, Feb 17, 2020.

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  1. ChrisCam

    ChrisCam Registered User
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    Dec 9, 2017
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    Hi,
    Having made a replacement sleeve with pivot out of silver steel for an 18th century longcase arbour that now has a broken pivot I now have to address the issue of hardening and tempering the pivot.
    I see no reason to harden and temper anything other than the pivot...but am I right?
    On the basis that I am i would like to have tried an indirect method of hardening by putting the pivot and a few mm of arbour in a hole in a piece of brass rod and heating this. I can gauge colour for hardening but here my usual practice of placing the piece in a toaster oven for tempering will fall down.
    Surely if I was to place the sleeve with its hardened only pivot in the toaster oven the whole piece would be tempered? Would this weaken the sleeve (the part not hardened) too much or should I temper another way which because of the duration the temperature has to be maintained at and the difficulty of maintaining the desired temperature seems a lot more difficult to produce the desired results.

    Lastly would the 0.055 inch pivot made from silver steel need hardening and softening or is it OK as is?
    Any suggestions as to how I could proceed gratefully received.

    Chris
     
  2. Uhralt

    Uhralt Registered User
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    I think it would be better and easier to harden and temper the complete pivot cap. When you harden the pivot only you will create a brittle transition zone between the hardened pivot and the soft arbor. I had recently a fusee mainspring breaking at just this transition zone between the annealed innermost coil and the hardened spring.
    I think the clock will probably run ok for many years if you leave the pivot unhardened. I have seen many antique clocks with soft pivots that had run for centuries without excessive wear.

    Uhralt
     
  3. ChrisCam

    ChrisCam Registered User
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    Thanks for the reply Uhralt,
    As always it's a compromise as there are issues either way.
    Here as I will be using a retaining compound to fix the sleeve to the arbour so the hole would need to really clean after being heated for a good bond.
    Starting from scratch again a better approach I would have been to machine to right external diameter and hardened the steel then machine to finish, then tempered and clean but the hole would still need cleaning well. I will leave the pivot this time as is.

    I note we anneal mainsprings where they hook on to the drum so this too must be a future weak point?

    Chris
     
  4. Uhralt

    Uhralt Registered User
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    It is, but there is much less flex at the outer diameter of the spring than at the inner coil diameter.

    Uhralt
     
  5. glenhead

    glenhead Registered User
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    Had the piece of silver steel you made the cap from been previously hardened and tempered? If not, then it is extremely unlikely that the steel was not already annealed. Most store-bought carbon steel (steel that can be hardened) is sold annealed so that it's workable and the hardness can be manipulated according to the customer's needs.

    I agree that hardening a silver-steel pivot is a bit of gilding the lily. If you leave it at its current state it will last for many years. Hardening it certainly won't hurt, though.

    If you choose to harden it the safest approach (by far!) would be to harden and temper the entire cap before installation. If you choose to harden only the pivot your toaster-oven tempering chamber will be ideal for tempering. The cap won't get any softer than it already is unless the source piece of steel was already hardened past the point you want the pivot to be, you'll just be bringing the pivot back closer to the cap's hardness. If the source piece was already hard then hardening it again will make your grain structure much larger and permanently make the pivot much more brittle. Overall, the hardness of the cap isn't particularly relevant, your focus is to protect the pivot from unnecessary internal forces.

    Regarding annealing the end of a mainspring: A spring is already tempered back to about a blue state in order to reduce its brittleness so it will coil without exploding. You're starting at that state when annealing the end of the spring, so you're further softening a piece of steel that's already not-very-brittle. Yes, there will be a transition zone, but it's a transition from not-very-brittle to less-than-not-very-brittle. Also, it's a zone, not a sharp line, so the transition doesn't particularly stress the steel.

    Hope this helps.

    Glen
     
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  6. ChrisCam

    ChrisCam Registered User
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    #6 ChrisCam, Feb 17, 2020
    Last edited: Feb 18, 2020
    Thanks Glen, really appreciate the post as you answer the point of transition states very well. Will keep this in my notes for next time.
    Chris

    Just to clarify on the back of Uhralt and gen's advice I would add for the purpose of clarity for any future reader my follow up notes, which hopefully if wrong will be amended:
    High carbon steel such as silver steel normally comes annealed. Only when such steel is hardened and a martensite structure develops within it will tempering effect it. Thus tempering an annealed piece of metal has little effect. The difference between annealing and tempering is tempering introduces a required degree of ductability (give) back into the steel as opposed to the brittle state when hardened. Different areas within a piece of metal having different ductability could impact its use dependent upon notably upon being strained / flexed.
    Chris
     
  7. kologha

    kologha Registered User

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    #7 kologha, Feb 27, 2020
    Last edited: Feb 27, 2020
    Silver steel in it's machineable state (as purchased) is altready considerably harder than mild steel so it would not be necessary to further heat treat it when making a clock arbor pivot.
     
  8. ChrisCam

    ChrisCam Registered User
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    Thanks for that Kologha

    Chris
     

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