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Kerfing Questions
http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=19640		 Page 1 of 1
Author:  Darryl Young [ Wed Nov 12, 2008 10:33 pm ]
Post subject:  Kerfing Questions
I've read that using reversed kerfing substantially stiffens this brace. How does this affect the sound? It doesn't add weight so I'm curious if/how the sound is affected.

Also, does the material used for the kerfing change the sound? Do you usually match the kerfing to the material used for the top and back? Maybe lighter is better. Appreciate your thoughts.
Author:  mikebrittain [ Wed Nov 12, 2008 10:38 pm ]
Post subject:  Re: Kerfing Questions
Darryl, Yes Charles Fox proved this in one of his classes. It does help make a stiff side assembly.

Mike Brittain
gmbrittain@gmail.com
myspace.com/mikrbrittain
Author:  bluescreek [ Thu Nov 13, 2008 6:23 am ]
Post subject:  Re: Kerfing Questions
Charles Fox did show that it did at stiffness but not what you may think. What it did was ad rigidity to the bends. I like to see what these things do , and when you use reverse kerfing, you have made a type of composite truss. The lateral stiffness ( that measured accross the grain ) Showed no significant increase but with the grain did.
john hall
Author:  KiwiCraig [ Thu Nov 13, 2008 8:31 am ]
Post subject:  Re: Kerfing Questions
That's interesting John, and makes good sense. Couple those liners with good firm side braces and the sides become a fairly rigid unit .
My thinking is because there is more mass at the perimeter of the soundboard joint with a liner of larger /heavier dimensions , they offer a bigger chance of containing the soundboards energy ,and not loose so much to the sides .

I'm not sure if John was referring to the Charles Fox design two piece liners or not , which are a kind of reverse kerf liner , but I have made and installed these . Depending on the wood you use , they can add considerable weight to the finished instrument . In my case I made them out of Walnut with Sassafrass capping . Heavy liners for sure , but it was what I was after , to contain more soundboard energy.

I'll find a picture here Darryl to show you what they look like . I glued the kerfed section first , and later fitted the capping ( 2 m.m thick or 5/64 " ) which is a much thicker /firmer web than conventional reverse kerf liners. The Walnut has a rebate to accept the capping flush

Attachment:
boat31.jpg


Attachment:
boat91.JPG


I'm not convinced however , that this concept makes for a far superior instrument.
This guitar has great projection as you'd expect and is a delight to play , but because it's the type of design to throw more of it's sound out front , I added a soundport to give the player a little more feedback.
On the other hand , my Mahogany guitar is built much lighter with lightweight wedge bracing . The whole box works and pumps as a unit . It's a 'chest thumper' for sure and a very enjoyable instrument to play . Two differing styles ,both good !


Craig Lawrence
___________________
Author:  Terence Kennedy [ Thu Nov 13, 2008 9:07 am ]
Post subject:  Re: Kerfing Questions
Besides making a stiffer rimset another nice side benefit from reverse kerf is that it contains the glue a lot better when gluing the top and back so there is less squeeze out to deal with. It's a win/win situation in my mind. I'd never go back to standard lining.
Terry
Author:  Darryl Young [ Thu Nov 13, 2008 9:38 am ]
Post subject:  Re: Kerfing Questions
I assume the lining is installed to increase the glue area for attaching the side to the top/back. I'm still unsure how important it is to make this joint as stiff as possible depending on your design philosophy (per Lawrence's post). Do most here use reverse kerfs or stay traditional? My guess is there will be no concensus! <smile> Seems there should be some discussion of dampening here. Will a rigid/stiff top-to-side joint dampen less and increase sustain? If so, does reverse kerf lining help sustain? Any chance reverse kerf lining is stiff enough that it decreases the vibration area of the soundboard affecting the sound?

If there is essentially no difference in sound, would it be prudent to stay as light as possible by selecting the least dense material for the lining? Would the least dense material be basswood, spanish cedar, or what? Anyone tried balsa wood for their lining? (got it right this time Todd! :D ) It's the lightest wood I can recall.
Author:  Michael Dale Payne [ Thu Nov 13, 2008 10:00 am ]
Post subject:  Re: Kerfing Questions
Because the spine is to the outside and the individual blocks formed by the kerfs are to the inside reverse kerf linings are a tad stiffer than triangle or individual lining blocks. but not by a whole lot but yet it is still significant.

I would guess that maybe a 60/40 split on those that use reverse kerf as apposed to those that use standard triangular, and a few use individual lining blocks (mostly classical builders)

The best woods for linings are ones that will make the best glue joint. They are not required to supply a lot of tensile strength but do require to have a good shear resistance in the glue joint. Softer hardwoods like mahogany, Basswood and Aspin also Softwoods like Cedar, Spanish Cedar all make good linings. I know some even use poplar.

I don’t think the added mass or reverse kerfed lings is significant enough to concern one’s self with.

My personal favorite is a 2 piece boxed in lining (like craig showed) made from Aspin but the truth is I use mostly reverse kerfed Spanish Cedar.
Author:  Michael Dale Payne [ Thu Nov 13, 2008 11:32 am ]
Post subject:  Re: Kerfing Questions
Todd I believe you are probably right. Once the top or back is on things will equal out for the most part.
Author:  Arnt Rian [ Thu Nov 13, 2008 11:43 am ]
Post subject:  Re: Kerfing Questions
I doubt there's much real change in meaningful stiffness once the box is closed.

I agree, and to those of you who claim there is: Have you measured this somehow (I haven't, so it's an honest question)?

The way I see it, when the plates are glue on, the geometry of the kerfed lining is locked in its position. For anything to change, the 'blocks' it would have to distort, or the glue joint would have to slip, which isn't happening under normal circumstances.
Author:  Mike OMelia [ Fri Nov 14, 2008 8:38 am ]
Post subject:  Re: Kerfing Questions
Craig,

I still think you build one of the prettiest internals I've ever seen. Hope you don't mind if I follow your lead!

Mike
Author:  Mike Collins [ Fri Nov 14, 2008 1:50 pm ]
Post subject:  Re: Kerfing Questions
Craig;
I agree with Mike-that is some clean work !
What wood is that?

I've used both styles for years and agree with Todd about hearing a difference.

Solid linings seem to make more of an difference.
I use them when treble responce & loudness of the guitar are ordered from the customer.

I hope I did not open a can of worms with that statement!!!!

Mike
Author:  KiwiCraig [ Fri Nov 14, 2008 5:19 pm ]
Post subject:  Re: Kerfing Questions
Mike O'Melia wrote:
Craig,

I still think you build one of the prettiest internals I've ever seen. Hope you don't mind if I follow your lead!

Mike


Not at all Mike .That's what forums are about , to share ideas .Thanks for compliment ( Both Mikes :D ) .
Some folk may think of it as a waste of time , but I'm afraid I can't help myself make it as nice as I can manage . At least part of the interior is viewable through the soundhole and I've found a soundport becomes a peephole to many. For this reason ,I included a Paua inlay in the headblock for interest , easily
noticeable through the soundport.

Attachment:
pro41.jpg


Mike , the back and side set is Myrtle . I received two fabulous sets from L.M.I. a few years ago. They were so impressed , they have included a couple of pictures of this guitar in their latest catalogue ( page 8 ). I was wrapped to be included with all the other fine builders .

Sorry , I've drifted off ,,, back to the topic !

I'm fairly certain a perimeter of more mass has an affect on containing more of the soundboards energy , but maybe not to a large extent . I'm thinking of a speaker mounted in a flimsy box as opposed to a solid one , or ,,, A drum with a thin ply rim as opposed to a heavier one. There is definatley a difference in sound .

On the other hand , it would seem almost impossible to contain the energy to the top alone regardless of how much mass the rims have . Have your guitar touch a large heavy table whilst playing and you soon see that this energy easily excites the whole of the heavy table top , becoming an extention of the soundboard itself . This being the case , it's easy to understand that a heavier rimset must only have a limited affect on containment . Hell , if it can excite a heavy table top , what chance has half a pound of liners got of keeping the thing isolated, although as I say,and keeping in mind the drum and speaker analogy , it must have some affect at least .






Edit : Sorry this pic has shown up twice . I can't seem to edit it out ! :evil:
Author:  KiwiCraig [ Fri Nov 14, 2008 5:45 pm ]
Post subject:  Re: Kerfing Questions
Todd Stock wrote:
Thoughful work, well executed, is never wasted time! (With a nod to Craig)


Thanks so much for that Todd . It means more to me than you might think ! [:Y:]
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