Koma minature cracks

Discussion in '400-Day & Atmos' started by Wayne A, Jan 23, 2020.

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  1. Wayne A

    Wayne A Registered User
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    Sep 24, 2019
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    I've seen bases with cracks and splits but this clock takes it to a new level I had not seen. Base is almost faceted there's so many micro cracks in it. Curious as to how this could happen to the base.. Suspect metallurgy problem and stress from manufacturing.

    Otherwise its a nice looking clock. Anyone know of a polish that will not leave discolored residue in the tiny cracks?

    20200123_144556.jpg 20200123_144959.jpg 20200123_144853.jpg 20200123_144737.jpg
     
  2. victor miranda

    victor miranda Registered User

    Jan 13, 2017
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    that is quite curious...

    I have read that ammonia will cause cracks in military brass
    (I believe clock brass is very similar)
    perhaps the clock base was soaked in ammonia to clean it?
    or that the cleaner stayed in the underside...

    victor
     
  3. Wayne A

    Wayne A Registered User
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    Interesting point about chemical corrosion, just never thought washing would provide enough time for it to take place. Looking again at the base there is quite a large cavity that potentially could hold some fluid although the leveling feet threads probably would drain it eventually.

    20200123_173346.jpg
     
  4. Wayne A

    Wayne A Registered User
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    Little reading and it does seem likely to be stress corrosion. Cracks are only in the bell of the base where there must have been higher stresses during manufacturing. I've seen plenty of chloride stress corrosion on stainless steel working at an oil refinery but nothing on brass. Although about the only place brass was used was pneumatic tubing fittings which turned black in some areas of the plant.

    Ammonia as a brass cleaner

    The Hendrix Group > Resources > Material Property Data > Copper and Copper Alloys > Brasses > SC Cracking
     
  5. KurtinSA

    KurtinSA Registered User
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    I can add to that from another area. In the 1950s and 1960s, brass was used to form the floats in BMW motorcycle carburetors. Over time, the brass would crack and take on fuel, converting the float to a sinker...that doesn't work too well in a carbs! It was even seen in floats which had been on the shelf unused. The stresses experienced during forming were residual in the material and would later crack.

    Kurt
     
  6. MartinM

    MartinM Registered User

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    It's funny how some parts that appear to be minor stampings can end up overly work hardened while others that are really stressed in the manufacturing process are fine, years later. I have some brass tubes about 6 inched long and a half inch ID that look much like glass test tubes but, made of sheet brass and with fully rolled rims that are as solid as the day they were created.
     
  7. Wayne A

    Wayne A Registered User
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    I'm guessing that thermal stress relieving was something that clock manufactures never used.
     
  8. Snapper

    Snapper Registered User

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    I have seen this in many press formed oil fonts in some ships lamps. I consider it due to poor annealing prior to forming and subsequent failure to stree relieve.
     
  9. Wayne A

    Wayne A Registered User
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    Well it cleaned up better than I expected for all the cracks.
    Had to set the front mounted eccentric, adjust both pallets and sand down a suspension spring because I did not have the right one on hand. Called for a .003 but I think ended up closer to .0028 to get it in range. It been running about a day and turning 350deg.

    20200129_173230.jpg
     

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