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Re: "Oilcanning" Soundboard Skeptic [Re: UnrightTooner] #2926536 12/24/19 07:06 PM
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UnrightTooner Offline OP
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FWIW

I am not an expert on soundboards and never expect to build one or even install one. Just has a question about what you fine folks considered an "oilcanned" soundboard to be because I doubted a soundboard could actually act like the bottom of an oil can.

Anyhoo, the discussion stayed with soundboards with some posters having very different views on the subject. Not knowing all that much myself, I found myself relying on some adages to try to sort out the wheat from the chaff:

If someone is focusing on themselves rather than the subject, they are probable full of it, or at least full of themselves, if there is a difference.

If someone is using typical words in untypical manners, they don't know the words to use, and nobody else can really know what they mean either.

A sure way to spot an abuser is to call them on it. If they become defensive, or offensive, there can be little doubt.

When there are aspects to a subject that can only be experienced, and not analysed, those aspects belong in la-la land.

Someone that fully understands a subject can explain it in clear terms.

A professional, humble person always points out their limits.

If you argue with a fool, people will assume you are a fool, too.


MERRY CHRISTMAS!!!


Jeff Deutschle
Part-Time Tuner
Who taught the first chicken how to peck?
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Re: "Oilcanning" Soundboard Skeptic [Re: UnrightTooner] #2926564 12/24/19 08:38 PM
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Gene Nelson Offline
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Merry Christmas
I’ll try to restate my opinion about this based on personal experience.
I have a soundboard in the shop (Out of the old piano being restored)that has as much or more negative crown as it may have positive crown when new prior to glue up in the piano belly. let’s call this an oil canned board for the sake of my explanation.
The only way I can explain that makes sense to me in relation to oil canning is:
It’s a compression board. The process of crowning involves drying the board and then gluing it to flat sawn ribs when removed from the dry box.
As the board takes up moisture it wants to expand and cannot because it is restrained by the ribs so it crowns and bows up instead.
If the moisture change from dry to ambient is extreme the cellular structure of the board can be damaged beyond repair. The fiber strength proportional limit exceeded. this can happen when new before glue up. Or later in its life after string bearing is applied as the process pushes the board very close to its FSPL.
Side note: to demo FSPL limit just start tapping on a piece of Sitka spruce with a hammer. When the dent you make will not return to undented, the FSPL has been exceeded beyond repair.
So my theory is that the board in my shop is a victim of this abuse (not pounding on it with a hammer). The cell structure is damaged beyond repair. However, the flat sawn ribs are still in tact. Initially they resist whatever crown there may have been and now they dominate the shape of the damaged board.
They bend or pull the board in the opposite direction and the board cannot resist, it’s a limp weeny. the ribs pull in the opposite direction because of cell structure and other reasons that apply to wood that warps with moisture changes.
But it’s only all a theory. I cannot prove it.


RPT
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Re: "Oilcanning" Soundboard Skeptic [Re: Gene Nelson] #2926570 12/24/19 09:11 PM
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UnrightTooner Offline OP
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Originally Posted by Gene Nelson
Merry Christmas
I’ll try to restate my opinion about this based on personal experience.
I have a soundboard in the shop (Out of the old piano being restored)that has as much or more negative crown as it may have positive crown when new prior to glue up in the piano belly. let’s call this an oil canned board for the sake of my explanation.
The only way I can explain that makes sense to me in relation to oil canning is:
It’s a compression board. The process of crowning involves drying the board and then gluing it to flat sawn ribs when removed from the dry box.
As the board takes up moisture it wants to expand and cannot because it is restrained by the ribs so it crowns and bows up instead.
If the moisture change from dry to ambient is extreme the cellular structure of the board can be damaged beyond repair. The fiber strength proportional limit exceeded. this can happen when new before glue up. Or later in its life after string bearing is applied as the process pushes the board very close to its FSPL.
Side note: to demo FSPL limit just start tapping on a piece of Sitka spruce with a hammer. When the dent you make will not return to undented, the FSPL has been exceeded beyond repair.
So my theory is that the board in my shop is a victim of this abuse (not pounding on it with a hammer). The cell structure is damaged beyond repair. However, the flat sawn ribs are still in tact. Initially they resist whatever crown there may have been and now they dominate the shape of the damaged board.
They bend or pull the board in the opposite direction and the board cannot resist, it’s a limp weeny. the ribs pull in the opposite direction because of cell structure and other reasons that apply to wood that warps with moisture changes.
But it’s only all a theory. I cannot prove it.


Thanks Gene!

OK, what you just described is an unloaded board with reverse crown and call it "oilcanned". In my mind, it is collapsed. (The bottom of an oil can pops back.) Others have described a board with inverted crown resulting in negative downbearing in part of the scale when loaded, but with regular crown when unloaded as "oilcanned". I my mind it had inappropriate downbearing and/or poor construction. (The crown came back, but did it"pop" like an oilcan?) OK, so there is no agreed upon definition of what an "oilcanned" board is - let alone what might cause the various faults that coincide with the various conditions.

But just to muse, and perhaps give you another possibility as to what happened to the board you mentioned, take a board, any board. Loaded it with ridiculous downbearing. Give it humidity cycles for, say, 100 years. I expect it would level out with nearly zero downbearing, but with negative crown. Might have sounded OK, too.

Btw, I googled "fiber strength proportional limit" and only got a few hits, all on Piano World...


Jeff Deutschle
Part-Time Tuner
Who taught the first chicken how to peck?
Re: "Oilcanning" Soundboard Skeptic [Re: UnrightTooner] #2926573 12/24/19 09:30 PM
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Gene Nelson Offline
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Maybe try fiber stress proportional limit.


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Re: "Oilcanning" Soundboard Skeptic [Re: UnrightTooner] #2926574 12/24/19 09:32 PM
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Gene Nelson Offline
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I think if I were the put my example board back in the piano and string it up the board would comply with the plane of the strings. That’s kind of popping back wouldn’t you say?


RPT
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Re: "Oilcanning" Soundboard Skeptic [Re: UnrightTooner] #2926720 12/25/19 11:40 AM
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Chernobieff Piano Offline
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If the physical weight of a board is important:

Here are some stats from 3 Steinway M's. 1) An rc&s from a well known rebuilder, 2) An Original Steinway Diaphragmatic, And one of my boards

1) 16.8 lbs., longest rib 36", Fundamental vibration frequency 79hz, stiffness factor 520
2) 15.1 lbs, longest rib 42" , Fundamental vibration frequency 51 hz, stiffness factor 282
3) 13.5 lbs, longest rib 42", Fundamental Vibration frequency 56hz, stiffness factor 300

I'm able to reduce the weight by targeting the mass where its needed, and eliminating the mass that's not needed. Usually a 1/2 -1lb from the rib stock, and 1 pound off a diaphragmatic panel, more from other panels.

I'm currently working on a 4'9" Behning Grand, as working on small grands offers unique challenges. This tiny board weighs a whopping 15 lbs. It had a very thick panel 11mm in the top treble, down to 9mm in the bass.

Overall Steinway boards are the lightest and may be a distinguishing factor of theirs.
-chris


Chernobieff Piano Restorations
Lenoir City, Tennessee U.S.A
www.chernobieffpiano.com
grandpianoman@protonmail.com
Re: "Oilcanning" Soundboard Skeptic [Re: UnrightTooner] #2928197 12/30/19 09:32 AM
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Holy cow!, with all this,--- this little Behning would be a good example, if you show the factory ribbing, why it is incorrect, and what you are going to do and why., I am very curious as to where exactly you determine the active nodal zones. "sweet spots".
Seems like some, or should I say most are confused with loud speaker applications, like a stereo.
I don't remember any discussions re; nodal placement. Major belly changes are made, and there is no input as to why the factory was so wrong. The smaller the soundboard, of course is for more difficulty, for sure. Let's see.
Richard Blais


Craig Hair
Hampshire Piano
Chesterfield, MA
Conservative Piano Restoration
Watch us on YouTube

Is it colder in the winter, or in the country?
Re: "Oilcanning" Soundboard Skeptic [Re: UnrightTooner] #2928495 12/30/19 10:50 PM
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Craig,
Contact me via PM and we'll continue this conversation privately and bypass the naysayers.
-chris


Chernobieff Piano Restorations
Lenoir City, Tennessee U.S.A
www.chernobieffpiano.com
grandpianoman@protonmail.com
Re: "Oilcanning" Soundboard Skeptic [Re: UnrightTooner] #2928644 12/31/19 10:53 AM
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Craig remembers the very first thing you said to us, was,
"what we do could not be done", We were bothered by naysayers once, but not any more, we are too busy doing the impossible.
So---why do this in private, when there is something to learn, naysayers won't learn otherwise.
Hat's off to you for your efforts. But in the past you ignored ours, no questions.
Let's keep this in the public eyes, I would be way OK with that.
Now how do we proceed, I have a suggestion.
!5,ooo comments, and this oilcan thing is still ridiculously a mystery.

If a piano never had been strung up when new, placed in a home here in New England the sound board would be just as shrunken ,and reversed crown, "without ever having strings" "give that some thought".

Just to mention, we did an 1872 Bluthner SB re-crown, and it had what must be machine crowned ribbing. "I was surprised"
They were in need of steam shaping, and they went right back in. "the piano"
Maybe we can add something positive to this, sound board construction is not only how stiff everything is.
It's like a fox hunt, it's the pursuit of the pray that is important, no more foxes, no more pursuit, "excitement" so there seem to be many that don't want reality. "end of the chase"
Richard Blais.


Craig Hair
Hampshire Piano
Chesterfield, MA
Conservative Piano Restoration
Watch us on YouTube

Is it colder in the winter, or in the country?
Re: "Oilcanning" Soundboard Skeptic [Re: UnrightTooner] #2928737 12/31/19 01:59 PM
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P W Grey Offline
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Inquiring minds want to know...😁

Pwg


Peter W. Grey, RPT
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https://www.youtube.com/watch?v=PK0T7_I_nV8
Re: "Oilcanning" Soundboard Skeptic [Re: UnrightTooner] #2928806 12/31/19 05:29 PM
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Craig Hair Offline
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.Steinway pulsators are pretty interesting., M&H had them also. they control the sweet spot. "undulation"
This behavior can be demonstrated With a table, a strip of wood a tuning fork, and a small clamp.
Oh, yes we know something about naysayers, because we have met some.

http://forum.pianoworld.com/ubbthreads.php/topics/1681779/original%20soundboard%20recrowning.html
R. Blais.


Craig Hair
Hampshire Piano
Chesterfield, MA
Conservative Piano Restoration
Watch us on YouTube

Is it colder in the winter, or in the country?
Re: "Oilcanning" Soundboard Skeptic [Re: UnrightTooner] #2928937 01/01/20 10:53 AM
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Chernobieff Piano Offline
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Craig and Richard,
Actually, I find your work fascinating, it's just not for me. I follow you guys because I am always open to learning new things. As regards to what you guys do, i'm on the fence about it to be honest. On the one hand you offer "all original" to your clients which is very valuable to some folks, but on the other hand its a fiber fatigued material that has existed under unknown conditions for 100 years. Another problem for me, is that the Original soundboard has flaws that I do not want to copy or re-use. These flaws are caused by mass production and include:

1) Original poor design. I've read many accounts of when a piano designer made a new design, it would run through the process and the design would later be tweaked and improved. That's natural. But if the designer was fired or died or whatever, the flawed design continued on. Often for decades.

2) Heavy Weight. I realized that just about every single old soundboard is overbuilt. Way overbuilt. With heavy rib scales, heavy panels, and heavy bridges. Often they are several pounds too heavy.

3) Rib scales. Again, after studying many boards, i have found only 2 that had any semblance of thought in them. The choices of lengths relative to their Height and Width was random. An engineered rib scale introduces evenness and balance which is crucial for stiffness control across the board.

4) Rib Design. The rib itself is often done without thought. The scalloping serves the purpose of aligning the "driving point" underneath the bridge. When the driving point is off then the soundboard behaves like an engine with the wrong firing order.

5) Panel grading and thinning. This too is crucial for performance and weight control. There are multiple grading plans that can be effectively applied depending on style. Wolfenden's favorite was the "wedge".

Keep doing what you enjoy doing.
-chris


Chernobieff Piano Restorations
Lenoir City, Tennessee U.S.A
www.chernobieffpiano.com
grandpianoman@protonmail.com
Re: "Oilcanning" Soundboard Skeptic [Re: Chernobieff Piano] #2928967 01/01/20 12:09 PM
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Gene Nelson Offline
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Originally Posted by Chernobieff Piano
Craig and Richard,
Actually, I find your work fascinating, it's just not for me. I follow you guys because I am always open to learning new things. As regards to what you guys do, i'm on the fence about it to be honest. On the one hand you offer "all original" to your clients which is very valuable to some folks, but on the other hand its a fiber fatigued material that has existed under unknown conditions for 100 years. Another problem for me, is that the Original soundboard has flaws that I do not want to copy or re-use. These flaws are caused by mass production and include:

Curious what new things you learned from Craig and Richard?
For anyone installing a new board every week, to me that is mass production.

1) Original poor design. I've read many accounts of when a piano designer made a new design, it would run through the process and the design would later be tweaked and improved. That's natural. But if the designer was fired or died or whatever, the flawed design continued on. Often for decades.

The implication being that at least one soundboard designer who mass produces has a flawless design to compare all others against??

2) Heavy Weight. I realized that just about every single old soundboard is overbuilt. Way overbuilt. With heavy rib scales, heavy panels, and heavy bridges. Often they are several pounds too heavy.

Too much weight and overbuilt as compared with your design criteria. You never mention scale design and tension, string bearing load - all play a roll in board response vs overall mass. Again, overbuilt as compared with your design criteria.

3) Rib scales. Again, after studying many boards, i have found only 2 that had any semblance of thought in them. The choices of lengths relative to their Height and Width was random. An engineered rib scale introduces evenness and balance which is crucial for stiffness control across the board.

The ribs on most compression boards simply hold the board together, not much engineering required. I’ll agree about controlling stiffness.

4) Rib Design. The rib itself is often done without thought. The scalloping serves the purpose of aligning the "driving point" underneath the bridge. When the driving point is off then the soundboard behaves like an engine with the wrong firing order.

Ribs are not driving members of the assembly, they are driven. A deliberate radius is very efficient at getting needed support under the bridge. And why would anyone want to do that? How about supporting the string bearing load as well as adding responsiveness to string energy? Flat sawn ribs tend to do the opposite.

5) Panel grading and thinning. This too is crucial for performance and weight control. There are multiple grading plans that can be effectively applied depending on style. Wolfenden's favorite was the "wedge".

You stated earlier that Wolfendens book was useless???
Again, soundboard mass and string scale and bearing load must be included in the decision on mass.


Keep doing what you enjoy doing.
-chris



RPT
PTG Member
Re: "Oilcanning" Soundboard Skeptic [Re: UnrightTooner] #2929020 01/01/20 01:44 PM
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Gene Nelson Offline
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Im thinking the OP is correct. I cannot come up with a scenario that can describe an oilcanned piano soundboard.


RPT
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Re: "Oilcanning" Soundboard Skeptic [Re: UnrightTooner] #2929032 01/01/20 02:31 PM
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Gene Nelson Offline
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I am however interested in flat sawn rib designs coupled with a compression designed board and could stand to be enlightened:
Lets take an imaginary rib that is 30 inches long. I induce crown by the compression technique and end up with a 60 foot radius by intentional design.
So for reasons of experiment and desire to understand, I take a 30 piece of wood on my work bench and induce a 60 foot radius into it.
How do I do this?
Simple, calculate the sag or sagitta of the arc "s" where s = r - (the square root of r squared - l squared).
Where s is the sag or sagitta or the amount of crown induced (0.156"), r is the radius (60 feet) l is half the length of the rib or (15 inches).
When you do this calc, be certain to not mix inches and feet - stick with one or the other.
So using this calculation I end up with a sag, or sagitta or available crown of about 0.156 inches. A reasonable amount of crown to work with in my opinion.
So I go to my drill index and get a jobber bit that is 0.157 (couldnt find 0.156) and put it under the rib directly in the middle .then hold the ends down against the work bench
So now for the assembly part :
The typical compression board has flat sawn ribs that simply hold the board together and at the end of the glue up/compression process lets say that this imaginary 30 inch rib ends up with a 60 foot radius and about 0.156 inch of available crown - prior to loading with string bearing. My question is how does this rib move under string bearing load if the rib is not designed to support the string bearing load?? Its the board that supports the load in this scenario and the flat sawn rib works against the crown as it wants to go flat.
Just bend a piece of wood and then let it go - it wants to return to its original shape.
Now some compression board designers will buttress the ends of this rib very snugly into the rim of the piano with the intent that the piano rim will not allow it to expand and therefore give support to the string bearing.
Great idea. I believe it has been proven that end grain oriented support like a 4x4 beam that supports a deck or bridge can be stronger than steel.
However, the assumption that the piano rim is absolutely solid and will not move and therefore will not allow this rib to expand under load and therefore retain its sagitta or crown is a little difficult to believe because all of the wood on the entire piano moves and this imaginary rib will move with the rim.
Just how much will the length of this 30 inch rib that has 60 foot radius, with 0.156 inch crown or sagitta under string load change?
My bench test shows that the length of this rib when flat as opposed to when 0.156 inch of crown or sagitta is induced using my jobber bit will change its length by an almost not measurable amount. Possibly only a couple thousandths of an inch or less. I dont have the measuring device that can accurately measure and maybe someone knows the math that can define this change in length. Possibly a variation of the sagitta calc? Im not too good at math.
The piano rim moves that much or more, I dont care how solid it is, what wood its made of or if there are Mason Hamlin restraints assisting it. This flat sawn rib may or may not retain any string bearing support or crown or sagitta assumed by buttressing the end grain into the inner rim.
Now maybe string bearing support and crown retention are not a priority in this sort of design?


What am I missing?

Last edited by Gene Nelson; 01/01/20 02:34 PM.

RPT
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Re: "Oilcanning" Soundboard Skeptic [Re: UnrightTooner] #2929053 01/01/20 03:17 PM
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Ed McMorrow, RPT Offline
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A compression crowned board structure is a pre-stressed structure. The panel is restrained by the ribs from expanding across the grain as long as the RH does not reach the RH the panel possessed at the time of rib gluing.

The ribs do not directly support the downbearing. The glue line of the ribs and bridge, (if crown conformed along the gluing line), hold the entire tensed panel and rib structure together. So one must engineer it as one engineers a tensed, arched structure to resist loads.


In a seemingly infinite universe-infinite human creativity is-seemingly possible.
According to NASA, 93% of the earth like planets possible in the known universe have yet to be formed.
Contact: Ed@LightHammerpiano.com
Re: "Oilcanning" Soundboard Skeptic [Re: Ed McMorrow, RPT] #2929068 01/01/20 03:59 PM
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Gene Nelson Offline
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Originally Posted by Ed McMorrow, RPT
A compression crowned board structure is a pre-stressed structure. The panel is restrained by the ribs from expanding across the grain as long as the RH does not reach the RH the panel possessed at the time of rib gluing.

The ribs do not directly support the downbearing. The glue line of the ribs and bridge, (if crown conformed along the gluing line), hold the entire tensed panel and rib structure together. So one must engineer it as one engineers a tensed, arched structure to resist loads.


Thank you Ed
Is there a technique where crown can be persuaded to conform to the glue line?
Seems to me that would require the use of some sort of device to help shape the board as it is taking up moisture??


RPT
PTG Member
Re: "Oilcanning" Soundboard Skeptic [Re: UnrightTooner] #2929157 01/01/20 09:12 PM
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Ed McMorrow, RPT Offline
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Your welcome Gene. I enjoy your posts too.

I have a bridge gluing caul that has a shape that mimics what I usually find in Steinway pianos. It assumes the old bridge has lost some crown conformation over time.

I also try to conform my cap stock to the crown of the bridge root. I don't like to force the cap stock down to the glue line that leaves forces twisting the bridge away from crown.


In a seemingly infinite universe-infinite human creativity is-seemingly possible.
According to NASA, 93% of the earth like planets possible in the known universe have yet to be formed.
Contact: Ed@LightHammerpiano.com
Re: "Oilcanning" Soundboard Skeptic [Re: Gene Nelson] #2929195 01/01/20 11:16 PM
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Chernobieff Piano Offline
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Originally Posted by Gene Nelson
I am however interested in flat sawn rib designs coupled with a compression designed board and could stand to be enlightened:
Lets take an imaginary rib that is 30 inches long. I induce crown by the compression technique and end up with a 60 foot radius by intentional design.
So for reasons of experiment and desire to understand, I take a 30 piece of wood on my work bench and induce a 60 foot radius into it.
How do I do this?
Simple, calculate the sag or sagitta of the arc "s" where s = r - (the square root of r squared - l squared).
Where s is the sag or sagitta or the amount of crown induced (0.156"), r is the radius (60 feet) l is half the length of the rib or (15 inches).
When you do this calc, be certain to not mix inches and feet - stick with one or the other.
So using this calculation I end up with a sag, or sagitta or available crown of about 0.156 inches. A reasonable amount of crown to work with in my opinion.
So I go to my drill index and get a jobber bit that is 0.157 (couldnt find 0.156) and put it under the rib directly in the middle .then hold the ends down against the work bench
So now for the assembly part :
The typical compression board has flat sawn ribs that simply hold the board together and at the end of the glue up/compression process lets say that this imaginary 30 inch rib ends up with a 60 foot radius and about 0.156 inch of available crown - prior to loading with string bearing. My question is how does this rib move under string bearing load if the rib is not designed to support the string bearing load?? Its the board that supports the load in this scenario and the flat sawn rib works against the crown as it wants to go flat.
Just bend a piece of wood and then let it go - it wants to return to its original shape.
Now some compression board designers will buttress the ends of this rib very snugly into the rim of the piano with the intent that the piano rim will not allow it to expand and therefore give support to the string bearing.
Great idea. I believe it has been proven that end grain oriented support like a 4x4 beam that supports a deck or bridge can be stronger than steel.
However, the assumption that the piano rim is absolutely solid and will not move and therefore will not allow this rib to expand under load and therefore retain its sagitta or crown is a little difficult to believe because all of the wood on the entire piano moves and this imaginary rib will move with the rim.
Just how much will the length of this 30 inch rib that has 60 foot radius, with 0.156 inch crown or sagitta under string load change?
My bench test shows that the length of this rib when flat as opposed to when 0.156 inch of crown or sagitta is induced using my jobber bit will change its length by an almost not measurable amount. Possibly only a couple thousandths of an inch or less. I dont have the measuring device that can accurately measure and maybe someone knows the math that can define this change in length. Possibly a variation of the sagitta calc? Im not too good at math.
The piano rim moves that much or more, I dont care how solid it is, what wood its made of or if there are Mason Hamlin restraints assisting it. This flat sawn rib may or may not retain any string bearing support or crown or sagitta assumed by buttressing the end grain into the inner rim.
Now maybe string bearing support and crown retention are not a priority in this sort of design?


What am I missing?


A lot.
How does one go from professed expert to suddenly a curious student?

I'll try an help you tho. The rib IS designed to hold the load. When Ed says it doesn't directly support the load, this is a misleading statement on his part. Because indirect support by de facto becomes direct support. It's a laminated structure that undergoes a hygroscopic process. Your bench test is a false read because it doesn't take into account woods behavior. A straight rib is not the same as a straight rib with a natural bow for instance. The assumption seems to be that flat ribs are glued on, then the board is glued in flat and expected to crown. In that scenario, you would be correct. But that would be bad bellywork.
\Hope that helps.
-chris


Chernobieff Piano Restorations
Lenoir City, Tennessee U.S.A
www.chernobieffpiano.com
grandpianoman@protonmail.com
Re: "Oilcanning" Soundboard Skeptic [Re: Chernobieff Piano] #2929201 01/01/20 11:43 PM
Joined: Sep 2004
Posts: 2,464
G
Gene Nelson Offline
2000 Post Club Member
Offline
2000 Post Club Member
G
Joined: Sep 2004
Posts: 2,464
Originally Posted by Chernobieff Piano
Originally Posted by Gene Nelson
I am however interested in flat sawn rib designs coupled with a compression designed board and could stand to be enlightened:
Lets take an imaginary rib that is 30 inches long. I induce crown by the compression technique and end up with a 60 foot radius by intentional design.
So for reasons of experiment and desire to understand, I take a 30 piece of wood on my work bench and induce a 60 foot radius into it.
How do I do this?
Simple, calculate the sag or sagitta of the arc "s" where s = r - (the square root of r squared - l squared).
Where s is the sag or sagitta or the amount of crown induced (0.156"), r is the radius (60 feet) l is half the length of the rib or (15 inches).
When you do this calc, be certain to not mix inches and feet - stick with one or the other.
So using this calculation I end up with a sag, or sagitta or available crown of about 0.156 inches. A reasonable amount of crown to work with in my opinion.
So I go to my drill index and get a jobber bit that is 0.157 (couldnt find 0.156) and put it under the rib directly in the middle .then hold the ends down against the work bench
So now for the assembly part :
The typical compression board has flat sawn ribs that simply hold the board together and at the end of the glue up/compression process lets say that this imaginary 30 inch rib ends up with a 60 foot radius and about 0.156 inch of available crown - prior to loading with string bearing. My question is how does this rib move under string bearing load if the rib is not designed to support the string bearing load?? Its the board that supports the load in this scenario and the flat sawn rib works against the crown as it wants to go flat.
Just bend a piece of wood and then let it go - it wants to return to its original shape.
Now some compression board designers will buttress the ends of this rib very snugly into the rim of the piano with the intent that the piano rim will not allow it to expand and therefore give support to the string bearing.
Great idea. I believe it has been proven that end grain oriented support like a 4x4 beam that supports a deck or bridge can be stronger than steel.
However, the assumption that the piano rim is absolutely solid and will not move and therefore will not allow this rib to expand under load and therefore retain its sagitta or crown is a little difficult to believe because all of the wood on the entire piano moves and this imaginary rib will move with the rim.
Just how much will the length of this 30 inch rib that has 60 foot radius, with 0.156 inch crown or sagitta under string load change?
My bench test shows that the length of this rib when flat as opposed to when 0.156 inch of crown or sagitta is induced using my jobber bit will change its length by an almost not measurable amount. Possibly only a couple thousandths of an inch or less. I dont have the measuring device that can accurately measure and maybe someone knows the math that can define this change in length. Possibly a variation of the sagitta calc? Im not too good at math.
The piano rim moves that much or more, I dont care how solid it is, what wood its made of or if there are Mason Hamlin restraints assisting it. This flat sawn rib may or may not retain any string bearing support or crown or sagitta assumed by buttressing the end grain into the inner rim.
Now maybe string bearing support and crown retention are not a priority in this sort of design?


What am I missing?


A lot.
How does one go from professed expert to suddenly a curious student?

I'll try an help you tho. The rib IS designed to hold the load. When Ed says it doesn't directly support the load, this is a misleading statement on his part. Because indirect support by de facto becomes direct support. It's a laminated structure that undergoes a hygroscopic process. Your bench test is a false read because it doesn't take into account woods behavior. A straight rib is not the same as a straight rib with a natural bow for instance. The assumption seems to be that flat ribs are glued on, then the board is glued in flat and expected to crown. In that scenario, you would be correct. But that would be bad bellywork.
\Hope that helps.
-chris







Iv never professed to be expert, that’s another one of your figments.
I’ll take Ed’s word over yours any day.
My bench test is indeed an accurate representation, figments cannot change wood dimensions.
The only help responses like yours give is to confirm my opinion of you.


RPT
PTG Member
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