Search found 60 matches

by David Malicky
Sat May 24, 2014 4:49 pm
Forum: Flat-Top Acoustic Guitars and Bass Guitars
Topic: Humidity-induced arch
Replies: 9
Views: 3486

Re: Humidity-induced arch

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...
by David Malicky
Wed May 14, 2014 2:59 pm
Forum: Flat-Top Acoustic Guitars and Bass Guitars
Topic: Humidity-induced arch
Replies: 9
Views: 3486

Re: Humidity-induced arch

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...
by David Malicky
Sun Sep 30, 2012 3:57 pm
Forum: Flat-Top Acoustic Guitars and Bass Guitars
Topic: Copying a guitar without removing the top: Is it possible?
Replies: 18
Views: 7984

Re: Copying a guitar without removing the top: Is it possible?

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 brace heigh...
by David Malicky
Thu Sep 13, 2012 2:11 am
Forum: Wood and Materials Q&A
Topic: Janka Hardness Scale
Replies: 15
Views: 7908

Re: Janka Hardness Scale

I've never tried ABW, but from those comments I'm looking forward to trying it for some saddles -- thanks!
by David Malicky
Sat Sep 08, 2012 2:59 pm
Forum: Wood and Materials Q&A
Topic: Janka Hardness Scale
Replies: 15
Views: 7908

Re: Janka Hardness Scale

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 The high oil...
by David Malicky
Thu Sep 06, 2012 4:24 am
Forum: Flat-Top Acoustic Guitars and Bass Guitars
Topic: Purpose of bridge pins?
Replies: 141
Views: 50872

Re: Purpose of bridge pins?

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...
by David Malicky
Thu Aug 30, 2012 2:56 am
Forum: Flat-Top Acoustic Guitars and Bass Guitars
Topic: Purpose of bridge pins?
Replies: 141
Views: 50872

Re: Purpose of bridge pins?

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...
by David Malicky
Tue Aug 28, 2012 3:56 am
Forum: Flat-Top Acoustic Guitars and Bass Guitars
Topic: Purpose of bridge pins?
Replies: 141
Views: 50872

Re: Purpose of bridge pins?

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 ...
by David Malicky
Tue Aug 28, 2012 3:48 am
Forum: Flat-Top Acoustic Guitars and Bass Guitars
Topic: Purpose of bridge pins?
Replies: 141
Views: 50872

Re: Purpose of bridge pins?

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...
by David Malicky
Sun Aug 26, 2012 3:26 pm
Forum: Wood and Materials Q&A
Topic: Runout
Replies: 25
Views: 12822

Re: Runout

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...
by David Malicky
Fri Aug 24, 2012 4:41 pm
Forum: Flat-Top Acoustic Guitars and Bass Guitars
Topic: Purpose of bridge pins?
Replies: 141
Views: 50872

Re: Purpose of bridge pins?

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...
by David Malicky
Fri Aug 24, 2012 4:32 pm
Forum: Flat-Top Acoustic Guitars and Bass Guitars
Topic: Purpose of bridge pins?
Replies: 141
Views: 50872

Re: Purpose of bridge pins?

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...
by David Malicky
Fri Aug 24, 2012 4:27 pm
Forum: Flat-Top Acoustic Guitars and Bass Guitars
Topic: Purpose of bridge pins?
Replies: 141
Views: 50872

Re: Purpose of bridge pins?

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 slot), st...
by David Malicky
Tue Aug 21, 2012 5:01 pm
Forum: Flat-Top Acoustic Guitars and Bass Guitars
Topic: Purpose of bridge pins?
Replies: 141
Views: 50872

Re: Purpose of bridge pins?

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...
by David Malicky
Sun Aug 19, 2012 4:56 pm
Forum: Flat-Top Acoustic Guitars and Bass Guitars
Topic: Purpose of bridge pins?
Replies: 141
Views: 50872

Re: Purpose of bridge pins?

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 ...
by David Malicky
Fri Aug 17, 2012 1:54 am
Forum: Flat-Top Acoustic Guitars and Bass Guitars
Topic: Purpose of bridge pins?
Replies: 141
Views: 50872

Re: Purpose of bridge pins?

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...
by David Malicky
Wed Aug 15, 2012 6:40 pm
Forum: Flat-Top Acoustic Guitars and Bass Guitars
Topic: Purpose of bridge pins?
Replies: 141
Views: 50872

Re: Purpose of bridge pins?

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...
by David Malicky
Wed Aug 15, 2012 4:34 am
Forum: Flat-Top Acoustic Guitars and Bass Guitars
Topic: Purpose of bridge pins?
Replies: 141
Views: 50872

Re: Purpose of bridge pins?

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...
by David Malicky
Wed Aug 15, 2012 3:47 am
Forum: Flat-Top Acoustic Guitars and Bass Guitars
Topic: Purpose of bridge pins?
Replies: 141
Views: 50872

Re: Purpose of bridge pins?

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...
by David Malicky
Tue Aug 14, 2012 4:05 am
Forum: Flat-Top Acoustic Guitars and Bass Guitars
Topic: Purpose of bridge pins?
Replies: 141
Views: 50872

Re: Purpose of bridge pins?

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" force is no...