In a message dated 5/05/2001 3:32:31 AM, Tom C. wrote: <<<<"now inflate the ballon ...what happens to the line? Does the same thing happen to the shim/glue line as the board returns to its normal moisture content?">>>>JB <<"I would equate the rubber part of the balloon to the lacquer on the surface of the board and the air inside to the wood fibers of the board. The rubber is being stretched by the air and the air is being compressed by the rubber.">>TC OK....but the question was "what happens to the line"? The answer is that it gets larger both in length and width.......the same thing happens to a glue line on a dried, unsupported board and shim as the board regains moisture..........isn't that correct? It seems to me that if a board is supported/jacked/wedged, dried or not, that the resulting shim work will only be tightened as the board is allowed to relax to its natural equilibrium state. Additionally the range of movement 'before' a crack can appear is both higher and lower than if the shim is installed on a dried unsupproted board. In the last instance every movement that increases crown will increase any 'forces or pressures' (that better Del?:) acting on the shim/glue line. Further it seems to me that if an unsupported board is dried, the shim dried and installed and then the board is allowed to return to its equilibrium state that 'most of the new 'pressure' will be focused on the glue line/shim......thus causing/allowing 'cracks' to appear even after the piano is strung. I can only offer as evidence the many techs who have experienced 'cracking of shims/glue lines even after trying to do the best job possible with their shimming to include drying of board and shim. Relating back to the ballon question: if the line is drawn while the ballon is uninflated (dried board) then the line will expand when the ballon is inflated (board regains moisture)........ conversely if the line is drawn on the ballon while the ballon is inflated (supported and possibly dried board) then as the ballon is deflated (support removed from the board) the line will get smaller. The analogy ain't perfect but merely a reasonable facsimile :-) <<"If you expose the board to moisture, the wood expands and tries to lengthen the ribs, thereby putting the ribs in tension.">> Well sure the ribs will lengthen verrrry minutely in length (with the grain) but they will expand mostly against the grain (side to side) The crowning effect on compression formed boards comes more from the board pulling the ribs up rather than the ribs forcing the panel to crown......(I think that is right:) The board being held captive on the bottom, by the ribs and by the rim, is what causes the board to crown? <<"I'd be curious to know why the board failed at that particular point. Is it at a glue joint where there is a sudden change in the grain angle?">> 'Most' of the compression failures/cracks/failed joints/pressure ridges that I run across come at a junction where there is a somewhat noticeable difference in grain structure on either panel. I have noticed that it is 'almost always' the panel with the wider grain lines (more pith space:) which has the preponderence of damage with the finer lined panel having less, if any, damage. This seems to apply to all of the 4 problems I identified. <<" If you had absolutely vertical grain orientation in the entire board, would it ever crack? ">> 'Probably' sooner :-( <<"Do shims fail because you still have a mismatch of grain orientation?">> I am sure that this is among the causes of shim failure. <<"The point I was trying to make was that every inch of the board contributes to the forming of crown and if you remove part of the panel (say, an inch), it seems logical that if you dried the shim, and the board while you were at it, glued the shim on to the ribs and the exposed edges of the panel, then when the board was returned to normal EMC, **there would be some theoretical, and maybe actual, restoration of crown**">> Well "theoretically" there could possibly be but....as Del has pointed out, if the board is bad before shimming then shimming ain't gonna make it better by much if any. <<"The rim is not part of the crowning mechanism; the crown is in there before it gets glued to the rim.">> Bzzzt...wrong answer...want to call a friend? :-) On a "compression formed board the rim is a major part of the crowning mechanism. On a rib crowned board it is 'somewhat' less of a factor but still a factor. <<"But there are situations where, for example, a piano is breaking strings and the owners want to extend the life of their 100-year-old hand-me-down but don't want to pay for a new board because it sounds just fine to them. In that case, I would most likely shim the cracks because it would be the technicianly thing to do">> I agree. There are situations which call for a shim for other than cosmetic reasons and one of those is what I will call 'continuity of surface'. A board with numerous cracks in it will not be as efficient at its job as an 'amducer' (amplifier/transducer) as a board with a solid surface/body sans cracks. It seems to me that a board with numerous cracks would lose some of its power by only the ribs transferring the vibrations from panel to panel without help from the panels having solid connections to all the other panels otherwise. Said another way a board with 'continuity of surface' will perform better than one without. Now finally.......can more crown be added to a boards by shims?....probably not. Can more tension/pressure/compression be added to a board by 'well executed' shims?....definitely. Are the 'technical' effects of said shimming as long lasting as the cosmetic effects?......................??????? Del says probably not, at least in the long run. Joe says probably so, at least in the short term. Richard says he gonna redo the ribs and skirt the whole issue..hopefully. I say....if a board is otherwise sound and any needful shimming is done "properly" (whatever that means:) that the board will be, 'technically and cosmetically', a better board. Not a 'new' board or a 'recrowned' board but a "better" board. Course I have been real far off base before............ My view. Jim Bryant (FL)
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