I think it's the same as the difference between preload bearing and residual bearing.
If you set the bridge height to reflect 1.5 degrees of preload bearing, for example, once strung and brought to tension the board deflects some percentage and what's left is the residual bearing. You can't really tell your starting point from the residual bearing because board deflection always has some amount of unpredictability and variability.
The Baldwin accu-hitch pins allow you to target the preload bearing very precisely regardless of how accurately you set the bridge height. If you have targeted residual bearing of, say, .75 degrees you can, after the fact, still make adjustments with the accu-hitch pins, though increasing load is easier than decreasing load.
Ultimately the proof of the pudding is in the hearing and adjustments to the load can be made on that basis.
No system really allows you to 100% predict residual bearing because residual bearing across the scale depends not only on your preload settings but on how each section of the board reacts as that influences the residual bearing on adjacent sections. But some systems make it easier to achieve or, at least, adjust to your target. Accu-hitch pins and adjustable perimeter bolts are two valuable tools to do just that.
Daniel <When I got into pianos I learned crown is importantI learned this too. But what does one do, when a trend of really nice sounding flat old boards, re-worked with an appropriate protocol, different from the standard fare, keeps empirically proving that this information needs to be qualified?Yes a flat board, with a beat bridge cap, and challenged glue joints has nothing tonal to offer. However, old wood doesn't die tonally or structurally, the glue joints die. I'll repeat that... The wood does not die, even if it has experienced compression crushing. The glue joints die. Old spruce, even though compression crushed, can have a singular mature sound to it, that I really like.
So what's the deal wth crown? In my experience, crown simply is a visual indicator that the board has enough structure to resist downbearing force. If it can resist downbearing it is a visual indication that the board has adequate structure to produce nice tone. So crown, I would argue, in and of itself, does not produce tone, it is merely a structural indicator.If a board is flat, you lose the visual structural evidence that crown offers. But, lack of crown, does not definitively say there is no structure. It just says there is no visual evidence of structure. There may be sufficient structure, or there may not be. Other diagnostics, other than visual evidence of crown must be employed to know.
An apparently dead flat board will either have:
1-obviously failed glue joints. Or, in the case of hide glue joints, though not obviously failed, joints that are compromised sufficiently, to be incapable of producing sustained tone. In the case of these compromised hide glue joints, if you were to take visually acceptable, vintage hide joints apart, often, one could see at least 50% of the joint had turned to powder in the joint.
2-In the case of a compression board, where the glue joints are current and probably ok... due to the compression collapse of wood cells, the panel's dimension across the grain is reduced to such a degree, that the width of the panel, relative to the rib length, no longer forces the panel into compression. Either the rib dimension, or the near rim thinning, or probably both, at this point is too flexible to allow what structure is extant, to function tonally.
So, I've mentioned this before. On an old flat board, I assume compromised joints, and add a small fillet of either G-flex or 1:5 epoxy and colloidal silica along the entire length of all the ribs, both sides. I may add a fillet at the rim if I have concerns about that glue line, as well. I do not use any soundboard washers or any of that traditional protocol, which, from all the evidence I can see, is a complete a waste of time.
In addition to that, learn how to replace both the long bridge and bass bridge cap. It is the biggest bang-for-the-buck going in piano restoration...absolutely essential to sustained, clean piano tone. In doing the cap, add adjustable plate bolts and maybe vertical hitches. This, to minimize downbearing. The board will now have adequate structure, but its structure will be overwhelmed by superflous downbearing.
Finally, only use a cold pressed resilient low density hammer, like a Ronsen Bacon. Use of any hard dense hammer will really challenge success...thank goodness for Ray Negron.There is more to it, on a failed compression board that has been treble thinned, but this is enough for now. Also more to it in terms of tuning the impedance of the board with focused, non-centered mass loads, and a protocol to let the board tell you what it wants pre-stringing. I may save that for a class, as its a work in progress.
I think crown serves two main purposes:
One, because you have a dome shaped, non linear spring that stiffens as it is compressed you can build the assembly lighter and still achieve the requisite stiffness with downbearing.
Two, downbearing loads the assembly with potential energy You have one spring (strings) pushing down and the other (the assembly) pushing back against it. That creates a more dynamic structure than one without those two factors at play
I don't think guitars are a good comparison whether flat top or arched top. The load on a guitar top is pretty marginal. A set of light gauge strings on a guitar has about 110lbs of total tension compared with 32,000 - 40,000 lbs in a piano. The downbearing load on a guitar is nominal, In fact the force is pulling up on the back of the bridge and down on the front of the bridge. The dynamics are totally different.
Jim, I'm curious when you reuse an old panel how do you tend to modify the rib scales, or do you? Do you remove them and dry the panel in order to reestablish some compression crown reusing the original ribs? Or do you make a new rib scale? Are they crowned? How do they compare dimensionally?
I don't see a problem reusing an old panel necessarily, depending on the health of the panel or how you repair it. Old wood probably does produce something different than new wood just like different species produce somewhat different results. But there are so many variables when trying to compare them it would be difficult to tease out whet was affecting what.
The other thing is that if the board starts out flat and you set positive bearing so that you are pushing the board into a concave shape you still have a dynamic situation. Of course we consider that a no no based on tradition but is it really a problem? I'm not sure.
What I am saying is that crown and no intentional crown, both function. However, they are different physical systems, so they have to be treated differently.Compression crowning does impart a low mass stiffness. I'm not disputing that at all.
In setting up a compression crowned board, the builder will load it heavier than non-crowned board, committing to a bridge height which is appropriate for that level of crown and stiffness. They will also thin parts of the panel, particularly the treble which depend on the panel's original cross-grain dimension to function. The problem with this setup is, that when the compression goes south, that thinned portion of the panel, which was a benefit when compression was extant, becomes a serious problem. Senza the compression, the near-rim area at the tight bend of the case, becomes way too flexible, and the treble anchor to the rim provides an insufficient foundation for the panel to work off of.There may be more power in a compression board with extant compression, than in non-compression systems, but, frankly, except for concerto pianos, non compression pianos still pack a huge amount of power...more than most spaces can contain. The question is how much power is necessary for 99% of pianos out there.In a non-compression system, which I will assume all the boards I am working with are, when I'm using this protocol, they don't have the annoying race horse sound of a compression board...there is a relaxation to the sound which I find entrancing...frankly a sound I have been looking for my whole life in piano land.My system for reusing the board is rot-gut simple. I don't remove the board, or painstakingly disassemble it like Craig Hair and Richard Blais do. Although their work inspired what I am doing..I really respect what they have been doing.I will only work on boards who are not falling apart. Some, to a bunch of cracks are ok, as long as the separations at the ribs are not out to lunch. Strip the board in-situ...it stays in the piano. At the separations only, open the separated joint to new wood. This joint will get G-flex in the actual joint. The rest of the rib scale, or as much as I can get to, gets a small 3/16 to 1/4" fillet of G-flex along the entire length of each rib. That's the entire length of each rib, not just at separations.My assumption is that all the rib joints are compromised, and I proceed from that assumption. If the rim joint looks like it may have compromised joints, I probably will just do a new board. If they rim joints look reasonable, I may add a fillet at that joint as well. So, that's the bottom side.Top-side, I use a luthier's non-invasive thickness probe, Maj-ic Probe , to map where the board had been thinned, and see what the manufacturer was up to. If there is thinning at the treble bend, I will add veneer back to the panel, that the manufacturer took off, to some degree. Then I use a violin plane to taper it into the existing board. Other parts of the board, can be ok when compression goes south, but I run diagnostics to see if I need to correct those thinned areas or not. This, by mainly using a type of tap test on the bridge to see is the board, when driven, sustains , and if it sustains if it tends to break up. You can hear this tapping on the bridge itself. I mainly tap on the bridge, not the panel for this, and use a small ball peen hammer hitting a double thickness of backrail felt.Rib dimensions mostly stay original. I have reduced some very high bass ribs on some Chinese experimental offerings, and have added to rib 8 on my current project. It is interesting to see how starting with more added stock than I knew I would end up with, how the tap test at that rib location changes with as I remove millimeter by millimeter...very interesting. Ended up only adding 2mm on this one.
The rest is termination work, which is absolutely essential. Always a new cap and pins...never re-using the old cap. This protocol has gone hand in hand with a tighter front and back pin spacing (10mm) where I can get it. ..especially the high treble and all of the tenor and bass, pivot terminations at the capos and copolymer counterbearings. ..Ed Mcmorrow's FTDS almost always.
Adjustable plate bolts and often at least some vertical hitches are essential as well, to keep the load on the board minimized. Final DB angles after loading and a month or two of equalization, is .7-ish high treble, to as close to not bearing as I can get it in the bass. Vertical hitches help with this. Board is loaded when setting bridge height with my calibrated spring loaded go bars, so I can see what the board can support. Flat boards treated with this protocol, can actually take some load without oil-canning. Which means flat does not necessarily mean no structure. But one must pay attention to the load, as it is key, and load the thing to see what it can take, after the re-gluing process is complete.
Intuitively it seems there is a difference in the panel itself but I'm not sure, I'll have to let the engineers weigh in.
But the dynamics of the assembly would certainly be different. In one case the assembly is solely responsible for crown formation and the string scale applies a counterforce which compresses that crown. In the oilcan situation the downbearing force applied by the string scale is solely responsible for the "crown" formation and there is no counterforce. That seems likely a pretty important distinction.
The only addition to Jim's comments is that I now prefer solid pins as opposed to the hollow spring pin used by Baldwin. Different companies make different styles. Groov-Pin is one. Driv-Lok is another. (See the links below).
Type 2A Grooved Pins - Groov-Pin Corp.