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Julian Cossmann Cooke

Clamping the top to a rib garland with blocks at...

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...uneven heights.  The Vieuxtemps has corner blocks that are higher than the upper and lower blocks -- the biggest difference being between the upper and upper corners (2.5) and then 1mm between the lower corners and the lower block, according to the poster, and even more pronounced on the upper end in the del Gesu books measurements.  I am in the habit of having uniform heights across the board.  But I am wondering whether there is any trick to getting the top plate to be flush to the garland all the way around given the height differences, particularly in this case on the upper end.  Seems like a lot to be accommodated just by plate flex.  Obviously it can be done, but again, is there a method to it?

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If you have already cut the purfling channels then find another way of gluing plates to the garland, - i.e. lose some of the taper.

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2.5 is a ton of taper. I would consider splitting the difference between the top and the back, so that neither plate is experiencing spring exceeding 1.25mm at the upper block. Other than that, use a block plane/ sanding board to ensure that the taper is perfectly gradual from the corner to the upper/lower block and you're set.

Edit: once you get everything lined up, glue the c-bouts first.

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Just flipped through the Biddulph book.  Although not every instrument in the book shows this pattern, most of them do.  I wonder if this is a product of making, e.g. always planing from corner to end block (hypothetically), or an effect of removing the plates over the years.  If we looked at a bunch of Strads (large sample size from a single maker of similar age), would we see the corner blocks higher then end blocks?  I think the question of why is important.  Nice question Julian.

-Jim

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The plate should flex just fine. But the question of top arching deformation/weakening comes in, perhaps. Assuming the top arching was even strong in the first place. 

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Jim,

You see it on Strads, other members of the Guarneri family, Stainer, and they all got it from the Amatis. Rarely is it as extreme as what Julian is reporting (I don't have the vieuxtemps scans), but damn if it isn't everywhere.

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When I glue the back to the ribs, after inserting linings and trimming blocks, the whole thing warps. I think it’s more important to have a flat surface for the belly than even block heights, so my rib corners flare up too. 

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1 hour ago, Julian Cossmann Cooke said:

...uneven heights.  The Vieuxtemps has corner blocks that are higher than the upper and lower blocks -- the biggest difference being between the upper and upper corners (2.5) and then 1mm between the lower corners and the lower block, according to the poster, and even more pronounced on the upper end in the del Gesu books measurements.  I am in the habit of having uniform heights across the board.  But I am wondering whether there is any trick to getting the top plate to be flush to the garland all the way around given the height differences, particularly in this case on the upper end.  Seems like a lot to be accommodated just by plate flex.  Obviously it can be done, but again, is there a method to it?

Julian, I wouldn't worry about it. Wood is a fairly plastic substance, at least in the thicknesses used for fiddlemaking.

Except that I do worry about stuff like that, since I'm a bit of a freak. But with the benefit of hindsight, I wouldn't recommend obsessing on this.

In my opinion,  archings and thicknesses matter a lot more.

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In my research I have hears this was done on purpose to create different tension patterns in the plate and alter sound.  And the technique was to glue the top plate on first, flexing the garland to fit.  Then glue the back plate, flexing the top plate to fit.  Material from the Chicago violin making school references this approach.

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Hi All - "pre-stressing" - building in a stress to counter the stresses that the object will encounter when in use is a common engineering practice - skis, compound archery bows, reinforced concrete - to name a few.

I always just accepted the rib taper as just that - a built-in stress to counter the string load working down through the bridge.

Much like thicknessing, the correct amount of taper would be a function of the wood properties - and - horrors of horrors - that leads one to consider if one should play the instrument in the white in order to decide how much taper to build in.

Well - while one is about it - one could also attack the outside of the body with a scraper to fine tune the response.

Maybe Anthony did carve the inside first - using a chain to establish the arching, then handed the plate over to an apprentice to rapidly carve tall was happy.he plate to a uniform thickness - using that prick-punch gadget that needs zero skill. The "appie" might even have assembled the instrument before handing it back to Anthony so that he could scrape away at the outside and test playing until all was happy. A varnishing and another masterpiece is born.

cheers edi

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10 minutes ago, edi malinaric said:

Hi All - "pre-stressing" - building in a stress to counter the stresses that the object will encounter when in use is a common engineering practice - skis, compound archery bows, reinforced concrete - to name a few.

"Static under load, not at rest." My teacher maintained that the taper was critical. When I started to take him seriously about that, my fiddles started to sound. 

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From the foregoing discussion, it sounds as though flexing the bassbar would offset any benefits accruing from the plate flexing because the latter would be stabilized.  Am I thinking about the physics correctly?

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1 hour ago, Shunyata said:

But I never heard of anything nearly so large as 2.5mm height difference between adjacent blocks!

Really? 2 mm difference between upper corners and top block seems to be fairly commonly done

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1 hour ago, Shunyata said:

In my research I have hears this was done on purpose to create different tension patterns in the plate and alter sound.  And the technique was to glue the top plate on first, flexing the garland to fit.  Then glue the back plate, flexing the top plate to fit.  Material from the Chicago violin making school references this approach.

Can't rule that out. So many theories, so little fact.

The Chicago school has recently managed to hire in a really brilliant maker and teacher. And the Boston school has had one for several years now. It remains to be seen whether students can benefit more from someone who is really good, versus someone who can just sorta assemble a basic fiddle. Information overload can sometimes be an impediment to learning.

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2 hours ago, Shunyata said:

In my research I have hears this was done on purpose to create different tension patterns in the plate and alter sound.  And the technique was to glue the top plate on first, flexing the garland to fit.  Then glue the back plate, flexing the top plate to fit.  Material from the Chicago violin making school references this approach.

Any pre stressing of the wood should disappear after a short while. Spring in bass bars tends to go away after a few weeks, doesn't it? 

So what would be any point in pre stressing joints?

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1 hour ago, JacksonMaberry said:

"Static under load, not at rest." - snip -

Hi JM - you've got me there.  Care to bring me up to speed?

cheers edi

 

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6 hours ago, Julian Cossmann Cooke said:

...uneven heights.  The Vieuxtemps has corner blocks that are higher than the upper and lower blocks -- the biggest difference being between the upper and upper corners (2.5) and then 1mm between the lower corners and the lower block, according to the poster, and even more pronounced on the upper end in the del Gesu books measurements.  I am in the habit of having uniform heights across the board.  But I am wondering whether there is any trick to getting the top plate to be flush to the garland all the way around given the height differences, particularly in this case on the upper end.  Seems like a lot to be accommodated just by plate flex.  Obviously it can be done, but again, is there a method to it?

If Roger Hargraves theory is correct, this was necessary for the neck setting in relation to the fingerboard thickness when doing it baroque style.

I personally think this is the better explanation as opposed to there plantation of building in tension on the top. The latter is in my view only the attempt to make sense of the taper from top corner blocks to top block on old (Italian) instruments without knowing where it came from.

If you want to the same  on a modern instrument, I wouldn't make 2.5 mm difference, 1.0 to 1.5 is enough and can be done by clamping the top down to the ribs. Maybe you have to make a few shavings around this area for a perfect fit.

 

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1 hour ago, edi malinaric said:

Hi JM - you've got me there.  Care to bring me up to speed?

cheers edi

 

I'm just a fiddle maker doing what my master told me to. As an engineer, you're more likely to understand what's going on with rib taper than I. As I understand it, the taper (which as I was taught is all done on the back side, for what it's worth) resists to some degree the stresses of the string tension. I wish I could do better than that, sorry.

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2 hours ago, Julian Cossmann Cooke said:

From the foregoing discussion, it sounds as though flexing the bassbar would offset any benefits accruing from the plate flexing because the latter would be stabilized.  Am I thinking about the physics correctly?

If you go into calculating such things you will probably go nuts. 

IMO the bass bar doesn't need tension for a 'better' sound anyway. The tiny little tension just helps to fit bass bars at higher speed. I think trained makers can do it in 30 minutes, Chinese factory style maybe 10 minutes. 

But yes, in theory the top can be bent lengthwise with the bass bar tension.

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5 hours ago, David Burgess said:

Julian, I wouldn't worry about it. Wood is a fairly plastic substance, at least in the thicknesses used for fiddlemaking.

Except that I do worry about stuff like that, since I'm a bit of a freak. But with the benefit of hindsight, I wouldn't recommend obsessing on this.

In my opinion,  archings and thicknesses matter a lot more.

Yes to all of that.

I just took an old top (with bass bar), put a 2.5mm shim under the neck block area, and pressed down the corners onto my surface plate.  Not a very stressful operation, I think.  Things DO move around a slight bit, so getting a good fit to the garland can be more tricky, and the outline might change slightly.

2 hours ago, JohnCockburn said:

2 mm difference between upper corners and top block seems to be fairly commonly done

The one Strad I measured was 30 at the top block and 32 at the upper corners.

 

All the blather about pre-stressing to help counteract string load or create a better sound is all just folklore IMO.  From an engineering point of view, pre-stressing is silly if you don't have serious issues with material failure or the like.  And for vibration, the theory (and tests I've done) still says it doesn't matter for things like a curved shell.

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1 hour ago, Zen Master said:

Perhaps  the rib taper is fine tuning the center of the air cavity... Any takers?

Yes: no.

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I think it's nice we see such dramatic tapers, but I think the question you want to ask is, just because you can, does that mean you should? I suppose if your doing an exact copy, well then do what it's supposed to be, but I feel in general, for our "own" work that less or no taper works just fine, simplifies things , and makes it so you don't need to add extra stress {to you} to an already stressful situation. I see no data "proving" any sonic advantage and for every plus you could give for doing it, I suppose we could come up with a minus.and reason to not go that extreme.or at all.

And no, it's not fine tuning anything imo,  it's either adding or taking away a smidgen of air volume, which in relation to the 5 million other things interacting together is most likely very minimal, a shorter rib in the upper bout region {the already stiffer of the two bouts} may add a wee more stiffness to the corpus as far as "body roll" goes but again I don't think that going to contribute to much.

Aside from that I see no predictable way to use the taper to tune anything and or no methodology that could be easily devised to do so, let alone in all this "fine tuning" it would seem that one would be gluing and popping the top off who knows how many times to "dial it in" to which of course anyone doing this would have to have a great response as to just "what they are dialing in" and what it relates to and interacts with, and the things it does interact with, one must ask are those things high on the list, a mil or two in a localized area of rib height, particularly in the front bout region is not as important as, or will not effect things as dramatically as "how thick is the top plate under my bridge" for example, as David eluded to.

I do think when we talk of instruments that may have been converted during the great period of conversion, that the possibility that some damage, or some chopping may have been needed to be done that may result in the ribs being altered from what they may have originally been does exist.  

On the other hand, maybe it's a "cutting of the ends of a roast" thing and hold over from building viola de gambas which flat backs have a definite and intentional taper. 

I wonder, do we have any defined literature from any great makers describing it and why they did it? 

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I still ask the question, is one of the surfaces flat?

10 hours ago, edi malinaric said:

 

Maybe Anthony did carve the inside first - using a chain to establish the arching, then handed the plate over to an apprentice to rapidly carve tall was happy.he plate to a uniform thickness - using that prick-punch gadget that needs zero skill. The "appie" might even have assembled the instrument before handing it back to Anthony so that he could scrape away at the outside and test playing until all was happy. A varnishing and another masterpiece is born.

cheers edi

What would this have to do with tension and the rib structure?

sadly I don’t have any apprentices. I do the outside first, but leave the edges and purfling channel til the body is complete.

i feel (and this is just speculation) that the belly vibrates better without extra tension. Take for example the sound post. If you make it 2mm too high, if it fits in the instrument you will kill the sound, not to mention damage the belly. 

So I make the surface for the back flat, and the whole thing warps after a while. When I plane/sand the belly surface flat, the rib heights are uneven. 

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