> I thought you were saying in a previous post that the pin was being pushed > up. I think you mentioned measuring an 0.011 inch change in height above > the cap over a humidity cycle. But here you're saying that the string will > be pushed up and down the pin by the bridge cap. Are both happening? If the pin is seated in the hole by tapping it down, if the MC of the bridge ever gets lower than at the point the pins were tapped in, the bottom of the hole will become the point of zero relative movement between the bridge and pin, and the shrinking bridge will push the pin up. Once the pin is clear of the hole bottom, the point of ZRM will be somewhere near the base of the cap. >> The point of zero relative movement between the pin and the bridge tends >>to be somewhere toward the bottom of the cap. > > > How did you determine that? By measurement of tested samples. If, for instance, the pin height above the bridge changed 0.011" through a 4%-12% MC cycling, the bridge root changed 0.021", and the cap changed 0.009", the point of ZRM is within a couple of thousandths of the base of the cap. In this case it's in the root. > Maybe so. Perhaps the drill bit doesn't enlarge the hole in that region as > much because of the presence of the bond. I suppose the ideal situation > would be if the point of zero relative movement between the pin and cap > were at the cap surface. I wonder if there's some way you could bond the > pin to the cap right at this point. I doubt it, but the cap could be made much denser and more dimensionally stable. Just going to a laminated cap (1.5mm laminations) lowered the 0.011" pin height difference to 0.004". An even denser cap should be even better. > One idea: I'm not sure how much interference you normally look for between > hole and pin. The supply house drills and pins tend to give about 3 - 5 > thousands interference. For the sake of discussion let's say we're going > to use 5 thousands of interference. Suppose you had a bridge pin that was > not intended to bottom in the hole. This bridge pin was also turned down > on its lower end to be 5 thousands smaller diameter than its upper > end. The upper end is sized to give 5 thousands of interference to the > hole. The pin goes into the hole smaller diameter first. It would be a > nominal fit going into the hole until it reached the larger diameter > portion of the pin, at which point it would be driven a millimeter or two > further. I'd just drive the third one I installed upside down and screw up the whole process. Given my attention span, I'd rather keep the process simple and try to improve the cap material. > What is your latest capping material? It's 8 epoxied laminations of 0.6mm (0.023") maple veneer, with another 3mm slab of maple underneath to give me enough depth to notch it. It is thoroughly impregnated with epoxy, and amounts to a fiber reinforced plastic. I expect that it won't change dimension a heck of a lot with humidity changes, that the point of ZRM will be very close to the top, and that if there is any change in pin height at all with humidity cycles, the cap is far more resistant to crushing than anything else I've tried, seen, or heard of. This ought to keep a clean termination with minimal wear for a very long time. At least that's the intent. > This brings up another topic. I know that Steinway uses a lot of maple in > their pianos, so it probably has some magical acoustical properties, and > was specifically chosen to complete the magic circle of sound. But since > you're experimenting, perhaps some other materials might be > interesting. As far as American woods go, maple is not the hardest > according to my various wood manuals. There are several which are harder: > > Hickory > Some Oaks (specifically Live Oak) > Locust > Persimmon (of which golf club heads used to be made, back in the old days > when 'woods' were > made out of wood) > Osage Orange (which I think may be one of the hardest American woods - I > would think some of this > might be found in your section of the country) > > A couple of potential problems here. There are many hickories and oaks, > some of which are harder than maple and many of which are not. So, getting > the right one from your wood merchant might not be easy. > > The woods that tend to be very hard also seem not to be very dimensionally > stable for some reason. That's a disadvantage versus maple. But, if you > put a thin top lamination on a more stable base laminate perhaps it would > hold together and provide a hard top surface. All of the above is why I'm using this new material. > I think agraffes or something like them are ultimately where we want to > go. But until such time, it seems like time well spent to improve on the > bridge pin design if possible. As the numbers we've been throwing around > show, at least for static loads, the bridge pin angles, side to side offset > of the string, choice of pin material and pin surface finish all affect the > string to pin friction. I don't know how much thought or investigation has > been put into optimizing this. Very little, I'd guess. I figure if I can virtually eliminate the relative movement between the pin and cap surface, the rest is rather a moot point. I'll see what the samples do when I run them through the MC range. > Also, there seems to be some thought that the string is trying to describe > a straight line between some point back behind the notch and the front > string termination. If that's the case it seems likely that the further > toward the pin line the edge of the notch is, the more likely it is to be > crushed. Right, but not because the string is trying to describe the straight line. The crush isn't a downbearing angle artifact, it's from friction at the pin with an expanding bridge cap pushing the string up. >We know that moving the notch back from the pin line doesn't > cause false beats. This might reduce the crushing of the notch edge. It > may, however, slightly reduce support for the pin because more wood is > taken away, so over time might lead to loose pins and flagpoling. I'm > curious as to which notching method remains most beat free over time. Too many other factors of cap density and environmental differences to "pin" it down... Sorry. > There's also the suggestion that I made to Ric, perhaps curve the top of > the bridge to describe the path that the string naturally wants to > take. This might result in less string grooving at the notch > edges. Perhaps this is impractical or provides little return on investment. > > Phil Ford It won't, because the downbearing angle isn't what crushes the notch edge. Again, with a straightened length of wire and an old bridge, determine the tangent angle of that string groove at the notch edge and you'll see the downbearing angle had nothing at all to do with it beyond the first degree or so.
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