At 13:10 -0800 24/11/07, David Love wrote: >I think the reason they didn't use taller ribs is because with compression >crowing, the taller, stiffer ribs won't bend under panel compression and you >won't get adequate crown. Isn't it true that the narrower taller ribs that >you see on some older pianos were generally radiused? Yes, but so were the others over here. I could not with certainty name a single European maker that produces the crown by compression and it's always seemed to me a pretty queer way to do things, Steinway notwithstanding. The traditional English way to crown the board is to shrink the board by drying to a certain moisture content and while it is still warm and dry to glue on the bars which are planed convex to match the ultimate concavity of the underside of the board. As the board reabsorbs moisture and expands, so the initial tension between the bars (which are maintained at normal moisture content) and the shrunken board diminishes and, provided proper procedures are followed, the result is stable and durable and quite independent of any external bending force. The inner rim may be angled to follow the crown or to tend slightly to increase it but this is a refinement. Last year I removed the board from the piano that is the subject of the project I mentioned in my last post. After 140 years it has the same crown it had when it was built and no cracks -- and most certainly none of those wonderful "compression marks"! On reflection -- and I ought to have reflected earlier! -- the reason deep bars were/are avoided is almost certainly to keep the framing members as close as possible to the line of the force they are there to withstand. Some shallow cutting out to allow passage of the bars was allowed but too much would remove strength where it was most needed -- so, I imagine the reasoning would go. I've seen perfectly functioning grand pianos with no braces at all. JD
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