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Quick advice on late-stage back graduation


Stephen Perry
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Normally I end up with pretty thin channels, but I was careful, and now I have a 120 g back.  With a good deal of graduation left.  Trying to figure out where to start.  

Channel bottom thickness in the bouts is 3.0 to 3.1.  C bout 3.4-3.5  - So there's a good deal of wood there, and the channels would tolerate a little bit of deepening without messing up the arching.  I have wood left to accommodate that.

Lungs are 2.5 to 2.7, still feeling somewhat stiff

Center is 4.9, too thick laterally still, the center line is pretty nicely established grading to 3.0 mm and then back up at the ends.  

I have the bullseye starting to come in, but it's still a bit hefty, not much though.  

Suggestion on where to start getting the weight down?  Mode 5 tap is high (I didn't measure - just BING high).  It's rather stiff in the center.  The bouts are just starting to loosen up, and I suspect the stiff edge is having an impact, but that might be a good thing.  

Thanks all.  

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i'm a believer in the m5 squared then multiplied by plate weight.  

example - 373 hz x 373 hz = whatever and then multiply that by 120. 

Try to work down to around 109 gr. and the 14,000,000 ball park for a back plate.

note - this is a suggestion for a two piece quarter sawn back, not a slab cut back. 

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22 minutes ago, DonLeister said:

Can you say how high the arch is, how thick the edges are?

Is the channel bottom thickness right next to the inside line of the purfling or is it further away from the purfling?

Arch is about 14 plus a little.  Channel bottom inside the purfling, with room to deepen and move inward a little more.  Bouts are about 2 mm inside purfling, about 1 in the waist.  Edges are 3.5 in the bouts, 3.7 or a bit more in the waist.   So it's fairly beefy (easier to remove wood than put it back on!!!)

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It's a bit difficult to use words for accurate defining of the graduation or arching.  Maps are better.

If a plate is coming in heavy and stiff, I just go back to the intended graduation pattern and take everything down by a few percent, or whatever I think will get me where I want it to be.  I suppose if you wanted to keep the stiffness distribution or the relative frequency distribution the same, then you could scale by the thickness cubed or squared... but I'm more a believer in mass distribution.

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I've gotten it mostly where I want.  To answer Leister, the arching is convex about 1/2 to 2/3 out from center line to bottom of channel, then very slightly convex, almost flat, but easily seen with grazing light or a ruler.  The wood seems to really like this.  I'm not sure how stiff it is in terms of numbers - still pretty stiff.  Graduations are across the waist, 3.5 mm to 4.6 mm, upper and lower bouts are thinnish, 2.5 to 2.7, but feel immensely strong.  This is fairly dense wood for me (although I don't see my spec sheet in the working box, so I hope that isn't disappeared).  Mode 5 is 340 hz.  Weight is 117 g.  

The back seems very strong, and doesn't really seem too thick.  I'd hoped for a bit more give, but maybe it's fine and dandy.  Final cleanup and edging and such will take it below 115 even with no additional forays into reducing thicknesses.

I'd really like another 5 grams off - I don't believe I've had a back over 110 g before, but this wood is different.  Scrapes to an almost glassy finish and sings very nicely.  Quite a bit harder than the maple I've been working with.   I cut the back out in outline form in 2015, so I'm missing scraps and so on.  

Suggestions still HIGHLY welcome.  I'm going to head back to the top - which has the arching nearly finished and is rough graduated.  

 

Thank you all.

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In recent years my backs are usually in the 100 to 105 range at about those thicknesses or a little less (and plus or minus 5 grams). It's the tops that I have to work at to get the weights down usually. 

I'd rather not make suggestions and cloud the subject but keep asking questions instead. I know about enough to cause trouble. 

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2 hours ago, DonLeister said:

In recent years my backs are usually in the 100 to 105 range at about those thicknesses or a little less (and plus or minus 5 grams). It's the tops that I have to work at to get the weights down usually. 

I'd rather not make suggestions and cloud the subject but keep asking questions instead. I know about enough to cause trouble. 

Oddly, the tops I don't generally have issues with.  I am thinking this back must be rather dense.  I had some dense wood, and this might well have been some of it.  Wish I could find my notes and such.  I started two of these, and have the top and back of the second one, but no rib garland.  Probably in a box in Chicago still!  Anyway, it's getting there, but is thinner than I am used to.  I am going very gingerly.  The back seems much more sensitive to adjustments on the outside than the inside as far as liveliness and ring.  I hadn't noticed that before. 

Thanks all

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  • 2 weeks later...
On 10/31/2020 at 9:42 PM, Stephen Perry said:

Oddly, the tops I don't generally have issues with.  I am thinking this back must be rather dense.  I had some dense wood, and this might well have been some of it.  Wish I could find my notes and such.  I started two of these, and have the top and back of the second one, but no rib garland.  Probably in a box in Chicago still!  Anyway, it's getting there, but is thinner than I am used to.  I am going very gingerly.  The back seems much more sensitive to adjustments on the outside than the inside as far as liveliness and ring.  I hadn't noticed that before. 

Thanks all

How is this turning out?

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4 hours ago, Marty Kasprzyk said:

How is this turning out?

The whole thing came together.  I ended up using fingers and sound to graduate and adjust arching, top and back, nudging into compliance.  M5 on both ended up lower than my "targets - but I have a pretty hefty edge.  Once I got things where I liked them, I had pretty much a band / bullseye in the back and a very slightly reverse graduation in the center on the top.  Pondered and bent and tapped and thought into submission.  The arching ended up a bit flatter with broader channels by a tiny bit (just seemed to want that).  Anyway, the violin (in the white) is very stiff under the fingers, responds rather fast, and has a more or less neutral and surprisingly even tone (considering it isn't varnished and is brand new settling in).  The top end is really there, all the way up, but I suspect it needs the ground and varnish.  It's a bit brash, which I've noticed with violins in the white before.  

The back goes from about 4.7 to 3.5 in the waist, and down to 2.2 in small areas of the lungs that were stiff.   The top is from 3.3 to about 2.5 in small previously stiff areas.   I spent a good deal of time feeling and working on the transition into the rib gluing surface.  Flexing, feeling, scraping.  I came to the conclusion that the arching and graduation around the corners is totally critical and somewhat difficult to pull off, but eventually the flex feels "right" and the top really seems to wake up.   I am pondering all this, and consider myself  a least confused at a slightly higher level!

Regardless, it's a much better violin than I am violinist.  Once it is varnished and pretty I'll have to find a real violinist to try it out!!!  I managed to run out of materials, so they're on the way.  Robson's balsam ground and some varnish I know will dry.  

Next project is making enough room for a garbage can UV chamber.  This going from 1400 square feet of toy space to 120 sq ft is difficult.  I have learned I can lose things very effectively in 120 sq ft.

Thanks for your interest.

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