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Rib Crown

  • 1.  Rib Crown

    Registered Piano Technician
    Posted 20 days ago
      |   view attached

    Hello fellow-techs.

    When rebuilding a piano and making pre-crowned ribs: which practice is more common or a better choice, making the height match across the rib, making its bottom side crowned to match the top glue surface - or, making the bottom of the rib straight?

    From what I can tell, most people who pre-crown ribs, leave the bottom straight and allow the rib to be "taller" in the middle or centered under the bridges. They aren't likely to make the center of the rib shorter, to match the height of the rib where it terminates into the tapered sections.

    The seemingly lesser path is to shave away the middle of the rib so the top and bottom are basically parallel. This introduces minute grain run-out but nothing like the rib tapering does. Others may actually shave away at ribs after bellying the board by tapping and listening.

    Something I just learned is that altering rib height is a cubed mathematical relationship with strength whereas altering thickness is a linear relationship to strength and therefore a more forgiving dimension.

    while I have never done this, I think that if one where in an experimental situation, they could scallop material off of the side of ribs, somewhat diagonally without interfering with the glue joint to the soundboard, to open up tone.

    But the main question I have is what preference rebuilders have to cut crowned ribs parallel top and bottom - or not - and what the reasoning or experience indicates. While the piano soundboard acts as a piston where the middle moves up and down while the perimeter allows the flexibility for it to do so, there must be a limit to how rigid the soundboard should be beneath the bridges. The height of the ribs and their consistency across their length surely influences how the piston works - especially once you realize how sensitive the rib height parameter is mathematically speaking

    Thank you.

    PS. Here is my latest soundboard under construction; and I'm several days away from bellying another board.



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    Tom Wright, RPT
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  • 2.  RE: Rib Crown

    Registered Piano Technician
    Posted 19 days ago

    Bottom of rib planed or routed to match concave surface of crown board.  If using go-bar deck for gluiing ribs best to start with 4/4 spruce then plane or route sb side of rib to match crown board. After gluing ribs to board plane ribs to height and finish and/or taper to height.



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    Parker Leigh RPT
    Winchester VA
    (540) 722-3865
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  • 3.  RE: Rib Crown

    Registered Piano Technician
    Posted 19 days ago

    Before you get too far into this, can you (both) give some idea of your prior experience doing this level of rebuilding?  Thomas, I'm having some difficulty reconciling the appearance of your work (good)  with the questions you're asking. 



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    David Skolnik [RPT]
    Hastings-on-Hudson NY
    (917) 589-2625
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    Original Message


  • 4.  RE: Rib Crown

    Registered Piano Technician
    Posted 18 days ago

    I was a belleyman at Sohmer  and have been installing soundboards since 1983.  I also trained  under a veteran Steinway Belleyman, Valentine Toussaint.

    My pinblock training was with Wally Brooks.  Worked at Bechstein (1979) and Bosendorfer (1980)  and on historic pianos at Smithsonian (1975)



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    Parker Leigh RPT
    Winchester VA
    (540) 722-3865
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    Original Message


  • 5.  RE: Rib Crown

    Posted 18 days ago
    Parker wrote: "Bottom of rib planed or routed to match concave surface of crown board."

    I'm trying to understand your process here. If you are shaping a rib to match the surface of a soundboard panel - why don't you just make the rib straight? Most any panel I've ever made was pretty darn flat prior to gluing the ribs on.....



    Original Message


  • 6.  RE: Rib Crown

    Registered Piano Technician
    Posted 17 days ago

    Crown board will be machined concave,  ribs machined to be convex, panel flat. Taper panel topside after ribs are glued on, scalloped and tapered. Yes original board is a good reference. Gluing flat ribs not a good idea, I prefer a rib crown board with some mild compression, not force crowned. 



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    Parker Leigh RPT
    Winchester VA
    (540) 722-3865
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    Original Message


  • 7.  RE: Rib Crown

    Posted 17 days ago

    "Gluing flat ribs is a Bad idea"

    Why?

    As I see it, compression crowning is the best way to go. If the goal is to maximize tone quality, and to take advantage of physics (i.e. the strength to weight ratio). Before i start, i am not saying that rib crowning is a mistake, but a matter of preferences. I like Pavoratti, others may prefer Bob Dylan.

    I don't think rib crowning is actually crowning. It's more of a moulding. The purpose of an arch is to add strength by design, this equates to maximizing strength without adding weight. Floor joists for example are always crown up for that reason. When carving an arch on top of a straight board its still a straight board structurally. It doesn't take advantage of strength by design.

    Gluing a flat rib on a dried panel in an inverted deck crowns immediately. I think a lot of people incorrectly think its glued in flat, and crowns after it sits a while. Its actually the compression force that develops. The reason the compression force is valuable, is again, it adds stiffness without adding weight. And its acoustically desirable as well. When a flat rib is glued this way its no longer a flat rib, its now part of a structure. A structure is stronger than its individual parts. In some nice homes and some commercial buildings when you see curve roofs, In long spans when trusses are not used and wood is wanted for roof structure, the choice is a laminated arched beam. I've never seen them carve an arch onto a straight beam or say the laminations are working against the arch.

    Taking Steinway as an example, their boards do have flaws, and that can be taken advantage of.  Before 36' their boards were a little on the heavy side, but not too bad. Still, a better board than the diaphragmatic one. The diaphragmatic board went to the other extreme. Although i appreciate them for trying to maximize the strength to weight ratio, they went a little too far. With the too light of a structure, and the heavy downbearing, i've seen a lot of failures.  If they would add one more rib to each model, use adjustable perimeter bolts to control the downbearing,( not sure, but surely they control the factory climate now?) their pianos would fare much better. But according to a friend of mine , they don't want that.



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    Chernobieff Piano Restorations
    chrisppff@gmail.com
    Youtube@chernobieffpiano
    865-986-7720 (text only please)
    ------------------------------

    Original Message


  • 8.  RE: Rib Crown

    Registered Piano Technician
    Posted 16 days ago

    The diaphragmatic design was largely a marketing gimmick like Accelerated action.  Furthermore,  tapering the treble area is the wrong

    place to taper.  That is where you want maximum stiffness. In earlier decades I believe that Steinway Belleyman tapered as needed, mostley

    in the bass area.



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    Parker Leigh RPT
    Winchester VA
    (540) 722-3865
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    Original Message


  • 9.  RE: Rib Crown

    Posted 16 days ago
    Have you considered laminated ribs? Less chance for hidden defects, more uniform physical characteristics, efficient use of spruce, easy to make any curve radius, etc., etc.!


    Original Message


  • 10.  RE: Rib Crown

    Posted 18 days ago

    My preference is to study the original board and copy it as closely as possible and only make changes when there is a good reason to do so. Over the years i have come up with various tests to compare one board to another. I find the most important thing to do is to listen to the old board first at different stages (strings on, strings off). The reason for this, is its not hard to make a new soundboard that functions worse than the tired 100yr old board.  One problem that occurs is when the same make and model  comes in, you'll discover there is no consistency of specs. The listening test helps because you'd find that one or two sounded way better than the rest did. Finding out the why is the fun part. One testing procedure i use, is to bench test the separated ribs and record their deflection values. The scallops are interesting because they become more important in one sense than the section modulus of the rib is. The scallops are extremely sensitive and just copying the dimension of the original does not mean you'll end up with the same deflection value. The same goes for panel thicknessing as well. In this way boards do hit back.

    My method of soundboard comparison allowed me to circumnavigate the complexities of both the engineering perspective and the instrument acoustics perspective. But this takes a long time to acquire that data. My key take away from that was the importance of the stiffness to weight ratio. Make it as strong as possible and as light as possible. Backed up by a good ear. The only way to do that is to study those who made good soundboards in the previous era.

    I cannot make a comment on the practice of changing a piano that was designed in one system, and changed out for another system. It does seem analogous to driving a car someplace with a blindfold on. The nature of the questions leads me to believe there is no strategy or method in place and there should be. And there should also be a good reason for departing from the original.

    All the Best!

    -chris



    ------------------------------
    Chernobieff Piano Restorations
    chrisppff@gmail.com
    Youtube@chernobieffpiano
    865-986-7720 (text only please)
    ------------------------------



  • 11.  RE: Rib Crown

    Registered Piano Technician
    Posted 17 days ago

    Is that a picture of a go-bar deck?  What materials is it made of?



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    Parker Leigh RPT
    Winchester VA
    (540) 722-3865
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  • 12.  RE: Rib Crown

    Posted 14 days ago
    Parker wrote: "Is that a picture of a go-bar deck?  What materials is it made of?"

    Who/what are you responding to?



    Original Message