MIMF

Nate, I haven't checked how the arch changes with later carving, but I would assume it flattens some. We glue on a very flat table. Flat bottom braces. The result is really a cylindrical arch. We glue to a flat rim for simplicity, so that cylinder then makes somewhat of a dome. Our student guitars a...

A late reply but I can add to this one--we've used a humidity-induced arch for our student project guitars for about 4 years. Bob Taylor suggested it to us. We've done 32-48 guitars each Fall semester this way, roughly 140 guitars so far. At first, we used a box and a space heater or hair dryer, mea...

Al has great advice, as usual, on copying the main resonant modes -- copying geometry isn't nearly enough. Here's the light technique in action (called a "Henkogram"): http://theunofficialmartinguitarforum.yuku.com/topic/847/MARTIN-BRACING-LIBRARY#.UGihcK7X9Bk A few other ways to measure b...

I've never tried ABW, but from those comments I'm looking forward to trying it for some saddles -- thanks!

For very oily woods, I've wondered if the Janka test might not be a great indicator of the "knife edge" hardness. I've tried using Lignum Vitae for nuts and saddles and find it dents somewhat easily, despite getting a Janka score of 4500. http://en.wikipedia.org/wiki/Janka_hardness_test Th...

Trevor, you're very welcome -- glad to do it and even better if it's helpful. Those are good suggestions on the allowable stresses and bridge rotation. I would imagine wood undergoes a similar microyielding that metals do in sharp corners, although the vastly different orthotropic yield properties w...

Steve, Trevor, and Rodger, thanks for the good points, questions, and suggestions. Comments like this are very helpful because they'll either improve the model, make its limitations more clear, or both. Steve and Rodger, yes, after I made the first Iso model I tried some thinner and tapered edges on...

Here are the Von-Mises stresses on the centerline and top surface. I changed the stress scale compared to the isotropic plots, as the VM stresses peaked about twice as high. Compared to iso, the longitudinal stresses in the top propagate a long way under the bridge, probably because the top's E1 is ...

Steve, Barry, and Mario, glad you like the analysis. Barry, impedance mismatch is a vibrations issue (masses), but one could say there is a rough analog in how stresses have to negotiate the mismatch of areas at the tail. Mario, I do plan to try smaller and larger bridges and plates, but I'm not sur...

I'm no expert at all on this, but was curious so did some googling. It looks like a number of causes are supported by research, including Mario's and Alan's thoughts on strength and reacting to offset loads. And as Mario said, it appears to be another nature + nurture phenomenon, with some species p...

The shear stresses on top surface are similar to the vertical stresses, except for sign: SxyTopSurf.jpg Finally, here are the Von-Mises stresses on the centerline and top surface (V-M combines shear, normal, and all other stresses in 1 number. These plots have a different stress scale.) SvmCenterlin...

Resulting deflections (sanity check... not very meaningful with ballpark iso Es): Deflections.jpg Below are the stresses in the vertical direction (Syy), sliced through centerline. The front and rear corners show very large stress concentrations: compresion (blue) and tension (red). The middle shows...

Below are some initial results from a simplified 2.5D FEA model of the bridge, top, and plate. In FEA, it's best to start simple and then add complexity in stages -- that helps understand incremental effects and catch errors. Here's the basic model: Bridge: 1.5" x .35", integral saddle (no...

Jeff, Thanks for the suggestion, although dimensions don't help as the issues are more fundamental. It helps a lot that you brought up the retaining wall analogue -- now I see where you're coming from. I'm a mechanical engineer, but from what I know of retaining walls, I'd completely agree the metho...

Mark, Thanks for the info! That makes sense about the poor fit -- lower glue strength and maybe cracks awaiting. Peter, Good to hear, thanks for confirming that! Would you have a rough estimate of how much force it takes to get the bridge to lie down flat? Jeff, thanks for the suggestion. Yes, that ...

Mario, thanks for your feedback. I think your thoughts on the stiffness mismatch causing stress concentrations are on the money. That makes sense about the pinned bridges having less wood damage. A question for Mark S and anyone else that has seen a lot of lifted bridges, to followup on Mario's thou...

Good to see you here, too, Mario. Yes, we’ve certainly covered a lot of ground on bridges. I think the main questions that are currently being debated are (lots of ways to phrase these): 1. What are the reactions (forces and/or pressure distribution) between a pinned bridge and the top? That is, wha...

I was too late for the edit feature, but here's what I wanted to say in the 4th paragraph: Trevor, in your FBD, could you clarify your definition of the free-body? I see the dotted horizontal lines which look like joints between the bridge, top, and bridgeplate--that definition would include all str...

Nice to see you here, Trevor -- thanks for joining the discussion and for your contributions on free bodies and the peeling bridge. You've a good memory of FBDs in Statics. But actually, there is no intrinsic requirement that a free-body be rigid. It is assumed to be rigid in Statics since the equil...

Jeff, Yes, agreed with that, except... - the ballend force is carried through the plate+top to the bridge (R) -- explained below. - for a glued bridge, the vertical reaction is a complex pressure distribution with features at the front, middle, and back of the bridge. All, I know the "R" f...