>>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. Or the bridge will move down along the pin. Semantics. The result will be the same. > 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. Interesting. I wonder why? I wonder if it has something to do with the different grain angles of the bridge body and cap, so that when you run the drill bit down through both, the pin ends up being tighter in the cap than the body. > > >>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. Do you think there's a correlation between density and stability? > > >>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. It would be easy enough to mark the bottom end in some way, with a different color, etc. Not a show stopper. > > >>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. It sounds like you're making Permali. http://www.permalidehoplast.co.uk/ I think somewhere I have a sample of the Hydulignum made by this company. Maybe I could whack a piece off for you to experiment with, assuming I have anything that will cut it (which isn't a trivial assumption - I think I got this sample when visiting a waterjet cutting operation - I think the point of it was something like, we can even cut this stuff). I wouldn't think that maple would absorb the epoxy well since it's not porous. If you're essentially making a cap out of epoxy, why not make it a carbon fiber composite, rather than a wood fiber composite? It seems that it would be stronger and harder. > >>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 agree, as far as the movement of the string up and down the pin is concerned. There's still the matter of the string rendering at the pins. Some attention to pin material, surface finish, and side bearing offset might improve matters in that regard at least. While we're on that subject, I don't know how much of the resistance to rendering comes from the string friction on the bridge cap. I think the resistance of the cap to down bearing (and to humidity movement) takes place mostly near the bridge pins. So the area of the cap between the pins is really doing nothing but increasing the resistance to rendering. Perhaps the cap should have a relief between the pins. > 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. OK. But the crushing seems to be more severe at the notch edge. Because of the path that the string is taking it has a fulcrum point, if you will, at the notch edge (which I believe Ric mentioned). So, the bridge is trying to lift the string up at the edges of the two notches and they're being crushed. > > >>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. One of the things on the list of things to investigate. > > >>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. I'm not sure that I agree. Because of the present geometry, as the bridge moves up, it is essentially lifting the string up at two points - the notch edges. If the top of the bridge described the curve that the string naturally wanted to take, as the bridge tried to lift the string up, it would be lifting it along the entire length of contact with the bridge, which would lower the bearing stress considerably and perhaps prevent indenting of the cap. Phil F
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