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  1. #1
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    Restoration of german WW1 7,6 cm mortar driving bands

    Hello,

    the friends of WW1 ordnance should know the problem with the 7,6 cm german "leichte Wurfmine" with zinc driving band: The shell is in great condition but the driving band is nothing more than white zinc oxide powder. This happens because of electrochemical reaction between the zinc and the iron especially in wet ground. The zinc is acting as an anode. Same method is used on ships to prevent rust on the hull in agressive salt water.

    So I decided to start a new project of "casting new zinc driving bands"

    So first I'll show one of the patient's:

    PS: I am not sure about some technical terms. If I am totally wrong I would be happy if a Moderator may correct them.
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    Last edited by Alpini; 27th August 2019 at 11:58 PM.

  2. #2
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    The project needed around 8 test days during the last months until we had a good molding.

    First of all I prepared a simple mold for a smooth driving band. My idea was to shape the rifling on the milling machine. But later I had not idea about how to machine the rifling because this shape is really difficult to machine without synchonising the machine feed direction with the rotation of the shell.

    The mold of the smooth band gave no problems:
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  3. #3
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    Because I had no idea about machining the rifling I started to machine an insert (placeholder) from 3 mm sheet metal and after some milling, bending and welding I got the following result. This ring insert smoothly fits into the mold and is thick enough that 1.5 mm in diameter could be turned down from the rifling after molding.
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    Last edited by Alpini; 28th August 2019 at 12:39 AM.

  4. #4
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    The following picture is showing the bottom part of a 7,6 cm Message shell (Nachrichtenmine) fitted into the mold ready for casting:
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  5. #5
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    The result was sobering because we made a beginners mistake: we did not heat up the mold - it was cold. So the zinc cooled down and became solid before it could flow around the mold:
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    Last edited by Alpini; 28th August 2019 at 12:42 AM.

  6. #6
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    For the next test we heated up the mold/shell part in our campfire and the result was much better. But it still had failures: The level of liquid zinc was to low and the capillary effect of liquid zinc is really large so there were small unfilled parts on the upper edge of the driving band (not visible in the pictures)
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    Last edited by Alpini; 28th August 2019 at 12:43 AM.

  7. #7
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    So the next logical improvement was to add a funnel to the mould. The rifling was now perfectly shaped but a new problem joined. Because we heated the mould in the campfire small parts of charcoal fell inside the mold which resulted in pitting in the driving band (no pictures taken). So I decided to make a cover for the mold (see last picture):
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  8. #8
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    After solving the problems with the mold we experimented in finding out the optimal temperature of the mold wich was at ~350°C. Then we had to find out the optimal filling level and it was also important to quickly fill in the liquid zinc. Filling in the zinc to slowly gave multiple zinc layers which don't connect well. Casting to high results in difficulties of disassembling the cold mould. With optimal filling level some hits with a hammer on the mold and it falls apart.
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    Last edited by Alpini; 28th August 2019 at 12:44 AM.

  9. #9
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    Now the remaining work were some simple operations on the lathe and some "aging" of the very bright driving band. I used a little bit of hydrochloric acid on the driving band and then neutralizing it with water+soap to give the driving band the typical dark color. Without the acid this color also builds up themself after some weeks.
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    Last edited by Alpini; 28th August 2019 at 12:46 AM.

  10. #10
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    Finally here's a picture showing two of the finished shells. The shell in the middle (a message shell = Nachrichtenmine) has an original zinc driving band, the left and right shells have new cast driving bands. There are two more message shells we cast a new driving band around.

    When I removed the white oxide from one of the shells there was a very small solid zinc part left which a small company in my village kindly examined with their X-ray fluorescence analyzer. Interestingly the original driving band had a small percentage of Titanium in the alloy. The Titanium makes the material more flexible. For our first trials we used zinc from old sanitary parts which resulted in several cracks in the driving band because this alloy was very hard. Later we used Titanium-Zinc recycled from old gutters which gave much better results with nearly no cracks.

    I hope you enjoyed this small report. It was a real challenge and was much more difficult than it looks. And without the help of a very good friend it would never have been possible because for many casting operations two people are needed.

    Feel free to ask any quesions :-)

    Regards, Alpini
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    Last edited by Alpini; 28th August 2019 at 09:28 AM.

  11. The Following 21 Users Say Thank You to Alpini For This Useful Post:

    Andysarmoury (28th August 2019), bacarnal (28th August 2019), Big Dave (28th August 2019), Depotman (28th August 2019), doppz92 (28th August 2019), Dronic69 (28th August 2019), fert (28th August 2019), Fred (30th August 2019), greif (28th August 2019), Lostround (28th August 2019), M8owner (28th August 2019), nachtwuenscher (28th August 2019), peregrinvs (28th August 2019), peteblight (28th August 2019), pysall (28th August 2019), ron3350 (28th August 2019), spotter (28th August 2019), starshell (28th August 2019), thekees (28th August 2019), TimG (28th August 2019), tnor_fr (28th August 2019)

 

 
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