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Thread: Contour placement, recurve and the whole package

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    Default Contour placement, recurve and the whole package

    Hello Luthiers both pro and am,

    I have been working on the top and back of my IV A kit for a couple weeks now. Both surfaces which will be visible to the public are sanded down to 100 grit for now and looking nice. As I did not want to commit yet to specialized tools (built a dial indicator thickness gauge) and the sharpening/burnishing of said tools, I decided that for the surfaces "not for show" I would do my graduating with an orbital sander. All is well, so far so good.

    For the back I have been following the Arches F5 plan, which I comically refer to in my head as The Onion. The top I have been going back and forth between FSCOTTE's '98 A5L and the A5 Loar that has been posted often.

    My first question applies to the back, where exactly does the ~.100" for the low point of the recurve exist with respect to the gluing area? There is quite a bit of wood to the outside of where I traced out the top against the back. So I can envision where the wood will end, what do I do about making a curve from the low point of the recurve to where I'd be getting close to the flat gluing area? How much real estate would let's say from .120 down to .100 back up to .XXX take up? It's like, you're building a place for the water to flow on the sides of the street, but you can't dig under the sidewalk. I hope that makes sense.

    Second is about the top, where it seems all the magic happens. Because I don't even have a scraper and there is no way I'm putting a random orbit sander on my soft spruce top I figured I'd give the recurve a go more towards the underside. Is this doable or should I really just get close enough and leave the fancy business to my next endeavor? I suppose the same question as the first would apply if I should have a go at recurve on the uderside.

    A third question has come to mind... I have been trying to keep to the advice that I believe I first read from John Hamlett(Sunburst): It's more about the relationship between the plates than the stats of each one alone. Having said that, can the amateur look to some secret sauce guidance? Is it weight, a contour or thickness range? Of all of the grad maps I've seen on here the two I mentioned before for the top stood out as most common, yet I still read comments about one being too thick or thin, not to mention there is little consistency in the thickness mapping anyway. So let's say I'm looking at ~ .160" center for the back, moving out to .100", would logic dictate I shoot for the same on top? Also right now my back is weighing in at 170g, would my dimensions even allow for the 105-130g final weight I read about somewhere on here?

    I don't plan on this being the last mandolin I build, but I certainly would be ecstatic if this turned into a decent player.

    Thanks for any help you can lend!!

    Greg Day

  2. #2
    Registered User fscotte's Avatar
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    Default Re: Contour placement, recurve and the whole package

    -the lowest point on my recurve is about 5/8" from the outside edge. Just make it flow naturally there.

    -careful with orbital. You'll need to hand sand otherwise you'll get little swirly circle marks, especially on the maple. Shows up better when you go to stain.

    -get yourself an FFT analyzer, or audacity for the computer, and have the top plate glued to the rim with tone bars attached and f-holes cut. Tap the top in the center with the end of your finger. As a rough guide, anything around 530 hz is ok. That's my tap tuning tip.

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    Default Re: Contour placement, recurve and the whole package

    Excellent, thank you!

    I am only using the orbital on the insides, I know that swirly look and don't plan on it being part of my aesthetic

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    Henry Lawton hank's Avatar
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    Default Re: Contour placement, recurve and the whole package

    Greg have you visited Mandozine's builder questions and answers section? Michael Heiden addresses this subject of recurve placement and top/back graduation and his methods.
    Q7
    Q - On plate graduations, is there a minimum thickness in the recurve area below which you will not go? (Both for tops and backs). On topwoods, I realize it depends on species and hardness, but is there a maximum thickness in the recurve, above which performance of the plate suffers? And how far in from the rim is the thinnest point of your recurve?

    A - I have always like the tone of instruments that have enough wood to sustain and develop sweeter and sweeter tones as the years go by. I don't know if there is a rule one can adhere to, only experience with the different woods. Obviously too much wood in the recurve area will not allow the woof that is so inherently desirable in the mandolin but too thin will not allow the vibrations to get to the edge where all the power is sprung from.
    As a ball park figure I would say that for a top of Adirondack spruce the recurve should be no thinner than .060" and no thicker than .22 in the middle. I aim for around .075 and .190.
    I have never aimed for immediate volume with a thin top rather leaving enough wood for the instrument to develop tone.
    My recurve is thinnest .675" from the rim. One factor that affects this is how wide the kerfing or linings are. I round my kerfing from the side up to the plate leaving only about .125 of surface plus the side for the plate to glue to.
    "A sudden clash of thunder, the mind doors burst open, and lo, there sits old man Buddha-nature in all his homeliness."
    CHAO-PIEN

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    Registered User fscotte's Avatar
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    Default Re: Contour placement, recurve and the whole package

    There's many ways to skin a cat. The comment about "woofiness" as it relates to the thickness of the recurve doesn't always work that way. My '98 A5L which the OP refers, has a recurve above .150 thick. It has that typical Gibson woofiness.

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    Registered User sunburst's Avatar
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    Default Re: Contour placement, recurve and the whole package

    For a first build, follow the numbers.
    Loar graduations and thickness have been published and can be found "all over" the internet. Many published plans are close to Loar graduations and thicknesses. When we are building instruments that have been made for a long time, like guitars and mandolins, it is always good to copy what is successful, especially when we are learning.

    As for the recurve area, it does not have to be equidistant from the edge of the plate, and in fact I like it better when it is not. Look at some old Gibson A-style mandolins. The placement of the recurve is most easily observed on those, IMO.
    For aesthetics in general, I like to look to nature. Once while casually observing a pumpkin seed, I began to notice the shape and contours. I changed the way I carve the recurve area of my mandolins after observing that.
    Last edited by sunburst; Jul-22-2017 at 12:49pm. Reason: improve wording

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    Henry Lawton hank's Avatar
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    Default Re: Contour placement, recurve and the whole package

    I'm using woods that are less dense than Red Spruce and Maple John. This Carpathian is very light and stiff but also more tearout prone than Red Spruce. The Claro Walnut is more open pored than maple requiring either fumed silica or sawdust filler but that's not much added mass to consider. I'm leaning toward Arches nominal to slightly thicker as Chris Baird suggested to adjust for the reduced mass. Are there any of you working with these wood's that might share their experience with tuning them.

    Another question that has come up is the difference in back coupling approach to an oval aperture and an f apertured Instrument. I have been told that ovals by nature of the design cannot get the subwoofer like couplings of F hole designs. The two piece maple back of my 1923 F4 is very alive during play with a nicer more defined sound than my louder slab birch 1913 A4. This refined more complex tone is I believe because of not only the maple but it is the result of better coupling on the f4 than the A4.
    "A sudden clash of thunder, the mind doors burst open, and lo, there sits old man Buddha-nature in all his homeliness."
    CHAO-PIEN

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    Henry Lawton hank's Avatar
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    Default Re: Contour placement, recurve and the whole package

    I was talking to Mark Mario about his F5, F4 and H4 builds. He had dismal Virzi results on all three and weak bass on the F4 & H4. Mark tried further reducing his recurve thickness on the F4 during the viriectomy but couldn't tell much difference in his bass response. Has anyone cracked this short neck oval hole weak bass response difficulty? I'm beginning to wonder if raising the neck off the top plate and centering the bridge on the plate are what give hybrids F5 characteristics more than the 14th fret neck junction. Having the full plate pumping freely like this, especially with reduced aperture size may give the extra oumpf to move the back more easily. If power is lost as Micheal suggest it stands to reason power is lost not only by bridge placement but also the way the large oval cutout affects it's edge attachment in that area.
    "A sudden clash of thunder, the mind doors burst open, and lo, there sits old man Buddha-nature in all his homeliness."
    CHAO-PIEN

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    Default Re: Contour placement, recurve and the whole package

    Thank you Hank for the info from Mandozine, I've actually never been on there.

    John I definitely plan on sticking to the numbers, every time I sit down with this project I get worried I'm going to mess something up!
    The pumpkin seed analogy is great and especially appropriate since I'm doing an A.

    I went back to the parts today after I read the advice on distance from the edge to the end of the recurve. I have a lot of work to do!

    Will keep ya posted, thanks again all!

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    Default Re: Contour placement, recurve and the whole package

    Hello mandofriends,

    I wanted to post some proud Papa pics as I have made quite a bit of progress in the last few weeks. At this point, the graduating is all done. The top is around .170" down the center string line, getting down to .100 (even .090 eeeesh!) at the recurve. The back is more or less .160" out to .100" at the recurve, yet keeping to more of a teardrop pattern or as John put it naturally, a pumpkin seed. All graduating was done on the inside with an orbital sander, I am pleased yet feel like I cheated and cannot wait to work with real tools for whatever project is next!

    Here are some pics of my progress:

    My crude thickness gauge. I think I'll upgrade to a metal frame for the next project as this plywood is super flexible to the point where I had to clamp it to a table to keep the needle from moving around.
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    How the back started out, some .220" readings near the center so quite a bit of wood to turn to dust!
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    I stained the fingerboard and removed the fingernail-thin headstock. I'll be replacing it with a piece of rosewood headstock veneer I have.
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    And then cut a headstock shape. My love for stringed instruments started on the banjo and I have always loved the RB250 Bowtie head. My best effort.
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    The neck joint fits great, and I have measured/estimated a 5/8" bridge height at somewhere around the right bridge position. For anyone who doesn't do this for a living, masking the neck where it fits in the joint on the body and either chalking or using the sawdust as a guide you can save a ton of time by doing this to figure out where to file down the joint for a perfect fit. Spit and duct tape ingenuity for the win!
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    Gluing the kerfing and determining which bar fits the best to start.
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    I "built" go bar deck out of my Rubbermaid tool locker from the garage and a few bamboo shoots I had from a beat up lattice I keep on hand for garden stakes. The ghetto go bar setup is a hilarious sight in my basement shop at the moment. Note: I did add more stakes, this was just to get it to sit right for a pic.
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    So that's all for now, once the glue is dry on the tone bars I'll start messing around with tap tuning or maybe not and just shave them down and close it up.

    This is fun!! Thanks again for the insight to date, next dilemma will be finishing.

    -Greg

    PS Upon previewing this post I can't seem to figure out how to get the pics to rotate to the correct position. I apologize in advance for the chiropractor bill.

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