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Co polymer Duplex ( The things this forum makes me do)

  • 1.  Co polymer Duplex ( The things this forum makes me do)

    Posted 11-12-2018 22:39
    So I agreed to re-string the church's Baldwin model E. No block, no bridgework, no action work, no improvements beyond new strings and pins. Okay fine. I take the strings off and find:
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    One of the bent brass duplexes has collapsed, cracked down the middle. The originals are 3/32" brass bent at a 45 degree angle. I'm thinking; (always a risky thing with me) "where do I find 3/32" brass flat stock and someone with a press brake?". Then it occurs to me that the original failed so why would I duplicate it? I imagine a couple of different ways I can fabricate an alternative out of brass and off to the local Alro metal shop I go. The staff there resisted the urge to laugh at the piano guy and showed me over to the scrap area. There I picked out $4 worth of brass flat and rod stock, odd lengths, sold by the pound. On the way back to the counter A shelf of co polymer scrap caught my eye. Acetyl co polymer is very similar to Dupont Delrin a trademarked Acetyl homopolymer which is slightly harder (+- 10%) and slightly stronger (+- 10%) and is used by Ed McMorrow in his patented duplex system and has also been written about in this forum by Jim Ialeggio. I picked up a 1/2" by 6" by 10" long piece for $3.25.

    Once back at the shop I took the wood blade off the band saw and put on the metal cutting blade. I was about to start cutting the brass stock when I looked at co polymer and decided to cut the softer material first.
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    One day I hope to have the time and a client with the money to buy a license to do a Fully Tuned Duplex Scale. When that day comes I don't want it to be the first time I've tried to work with an Acetyl material. So off we go. Having a band saw fence makes this sort of cut a lot easier. After the rough cut I cleaned up the profile with a sanding block I made with a 3/16" profile and some emery cloth.
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    I then used the 180 grit emery cloth and some 400 grit cloth to remove all the saw marks.and came up with
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    A very close approximation of the original.

    After I cut the individual pieces to length I took one of the waste pieces to the workbench. There I took a piece of music wire and laid it across the co polymer and smacked it with a hammer to see how it would respond to high pressure. There was an indentation in the co polymer to be sure but it was less than the indentation left in the brass duplex by the original string.

    In summation the Acetyl co polymer material proved to be a low cost highly effective solution to what had threatened to be a costly and time consuming side track in an otherwise pedestrian repair. I want to thank Mr. McMorrow and Mr. Ialeggio for their posts about using Acetyl materials in duplex systems. Their posts allowed me to think outside the scrap metal box while working on this one.



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    Karl Roeder
    Pompano Beach FL
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  • 2.  RE: Co polymer Duplex ( The things this forum makes me do)

    Posted 11-13-2018 09:02
    Glad to see an open mind at work.

    You are cutting some fresh tracks, so I would like to know how this all turns out when strung. When I use co-polymer, I use it as a flat profile that has a wide bearing surface. The minimum bearing surface I have used is 5/8". Usually its more like 3/4"-1" wide, depending on the plate configuration. Your fresh tracks are using a minimal radius as counterbearing contact.  I expect the wire will cut into the co-polymer more than it does when I use it...but maybe not. I'd like your feedback after stringing.

    If it does cut into the co-polymer, before you have committed your string spacing and paths, pushing the wire back and forth once, with a screwdriver, will get you a fresh bearing surface.

    Since the co-polymer has such high lubricity, the wide flat bearing, that I use, does not hang up the string with friction at all. In fact, if anything, I look to increase friction most of the time, at least a little. So, you may find, if the triangle shape presents problems, you could also just replace the triangle shape with a flat and be fine.



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    Jim Ialeggio
    grandpianosolutions.com
    Shirley, MA
    978 425-9026
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  • 3.  RE: Co polymer Duplex ( The things this forum makes me do)

    Posted 11-13-2018 09:07
    If the string does cut into the triangle shape, actually, given the lubricity of the material, it will not be an issue anyway. I think a gouge may get you a little helpful friction, with the string contacting the plastic on more of its circumference. In any case, I would love to hear how the triangle shape treats you.

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    Jim Ialeggio
    grandpianosolutions.com
    Shirley, MA
    978 425-9026
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  • 4.  RE: Co polymer Duplex ( The things this forum makes me do)

    Posted 11-16-2018 10:02
    This is very interesting. I too would like to know how it turns out.

    Pwg

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    Peter Grey
    Stratham NH
    603-686-2395
    pianodoctor57@gmail.com
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  • 5.  RE: Co polymer Duplex ( The things this forum makes me do)

    Posted 30 days ago
    Haven't had time to start stringing yet but I did take a minute to do a little comparison. I laid a piece of # 15 wire across a piece of the co polymer and hit it with a hammer to see how much it would dent. I then did the same with a piece of 1/8" inch brass I had thought of using for the same purpose. The results below.
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    This doesn't replicate exactly the forces exerted in the piano but I found it somewhat reassuring. The next hurdle will be finding something that will help to adhese the co polymer to the plate. The inherent lubricity of the co polymer makes it highly glue resistant. I'm going to try 3M 5200 and see it that works. I just need it to hold enough that the bearings not move during stringing.
    ​​

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    Karl Roeder
    Pompano Beach FL
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  • 6.  RE: Co polymer Duplex ( The things this forum makes me do)

    Posted 30 days ago
    with flat profiles, one stakes the polymer to the plate by drilling through the polymer into the plate and installing a coule 3/32 split pins. Or, one grinds the plate so the is a slight slope coming up from the capo. This method avoids staking, but I go back and forth between the two techniques depending on the plate configuration. When there is a slight slope, it really stays put amazing well as you bring up tension.

    With the triangluar shape...not sure how this aspect will work out.

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    Jim Ialeggio
    grandpianosolutions.com
    Shirley, MA
    978 425-9026
    ------------------------------