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Thread: Light bulb test of top plate

  1. #1

    Default Light bulb test of top plate

    My initial question was going to be what it is called when you shine a light behind a top plate to assess it's thickness...

    But I'd also like to know from this group exactly what it is I am looking for when I perform this test. Which parts of the top are supposed to allow light through?

  2. #2

    Default Re: Light bulb test of top plate

    not to be smart but the thin parts will show light through them

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  4. #3

    Default Re: Light bulb test of top plate

    Right. My question was more about which parts are supposed to show light through them? The entire recurve area, because that tends to be the thinnest? Only parts of the recurve? Other areas?

  5. #4
    Registered User O. Apitius's Avatar
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    Default Re: Light bulb test of top plate

    The technique you are describing is traditionally called "candling". I use it to reveal defects in the top plate such as sap pockets. Personally, I don't use it for determining thickness except as an adjunct to measuring with a thickness caliper. It can sometimes reveal some overly thin or thick areas but I will always confirm this with a caliper. Uneven pigment distribution in a spruce top, which is common, can greatly affect the amount of light showing through a given area and give false impressions of thickness. For best results, a light box that produces a very even intensity of light over its entire area must be used and even then you must have experience in "reading: what you are seeing. It's fun to mess around with and can alert you to some unseen defects within the plate but for thicknessing, I prefer calipers and flexing.
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    Registered User Rodney Riley's Avatar
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    Default Re: Light bulb test of top plate

    Think it’s called, “Neat!!” it just looks neat when you do it. Only thing it could possibly be used for is to show a hair line crack or a hidden pitch pocket. Could possibly show you planed/sanded your contour more/less on one side or the other. Others can chime in to prove me wrong if there is a real use for being lit up.

    Thanks O. Apitius. You explained it better

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    Henry Lawton hank's Avatar
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    Default Re: Light bulb test of top plate

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ID:	171706I’m finishing up on a plate with some bearclaw that turned out to be a knot shadow visible with a back lamp. Luckily I removed most of the harder knot wood in the graduating process.
    "A sudden clash of thunder, the mind doors burst open, and lo, there sits old man Buddha-nature in all his homeliness."
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  10. #7

    Default Re: Light bulb test of top plate

    Great explanations. Very helpful. Thanks, everyone.

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    Registered User Vernon Hughes's Avatar
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    Default Re: Light bulb test of top plate

    You make your tops that thin at the tailpiece end? Mine show good light around the recurve area but I tend to leave a little meat down the center of the top from the fingerboard to the tailpiece. I did carve one like your once but it was too thin and you know what happened!
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    Henry Lawton hank's Avatar
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    Default Re: Light bulb test of top plate

    Hey Vernon , thanks for the heads up. The back light is very bright making my plate look thinner than it is. The dimensions are thickish every where(Carpathian Spruce)but the recurve which is at minimum specified thickness but the diaphragm’s edge minimums were moved outboard. Everyone agrees in front of the tailpiece is very structural. This plate also has a healthy arch but will she collapse?
    "A sudden clash of thunder, the mind doors burst open, and lo, there sits old man Buddha-nature in all his homeliness."
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    Registered User Dusepo's Avatar
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    Default Re: Light bulb test of top plate

    My understanding it's called 'candling' from the days when lute makers would hold soundboards up to candles to do the same (not recommended due to fire risk!)

    You should see, as said above, some light shining through. The more light comes through, the thinner the wood.
    I am a luthier specialising in historical and world stringed instruments. You can see more info at my website.

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    Default Re: Light bulb test of top plate

    Hank, what I've experienced and seen is that rather than collapse, the area in front of the tailpiece will bulge upwards, possibly until it runs into the bottom of the tailpiece. I learned my lesson a long time ago and now always leave that area a little thicker.

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    Registered User Rodney Riley's Avatar
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    Default Re: Light bulb test of top plate

    Think the most effective use of candling is to check for bad eggs during incubation. And cleaning out your ears

  18. #13
    Henry Lawton hank's Avatar
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    Default Re: Light bulb test of top plate

    Thank you Dale. The tendency of a floating bridge to tilt forward would fit with the bulging under and in front of the tailpiece. How large an area did you thin and where was the arch or crest of the bulge? I’m at .130”in front of the tail block for about 3/8” before it starts thickening to .140” in the next 3/8” to .050 in the next 3/4”, etc to eventually .180”.
    The rear block center line .130” at the recurve starts dropping to .129” at the blocks corners continuing to thin out to it’s thinnest just below the point at 103”. The recurve is thinnest outboard very close to the rims and is very narrow before thickening to minimums or above quickly without large thin areas of the top.
    "A sudden clash of thunder, the mind doors burst open, and lo, there sits old man Buddha-nature in all his homeliness."
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    Adrian Minarovic
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    Default Re: Light bulb test of top plate

    Quote Originally Posted by hank View Post
    Thank you Dale. The tendency of a floating bridge to tilt forward would fit with the bulging under and in front of the tailpiece. How large an area did you thin and where was the arch or crest of the bulge? I’m at .130”in front of the tail block for about 3/8” before it starts thickening to .140” in the next 3/8” to .050 in the next 3/4”, etc to eventually .180”.
    The rear block center line .130” at the recurve starts dropping to .129” at the blocks corners continuing to thin out to it’s thinnest just below the point at 103”. The recurve is thinnest outboard very close to the rims and is very narrow before thickening to minimums or above quickly without large thin areas of the top.
    Those thicknesses seem reasonable. But thickness is just one part of the equation. Shape of arch in those areas makes difference as well. With more pronounced (deep) recurve and fuller arch (more curvature between tailpiece and bridge) the arch is more prone to bulging and should be left thicker. if the arch from til to bridge is "less curved" the thicknesses can be bit thinner.
    Adrian

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    Registered User Dusepo's Avatar
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    Default Re: Light bulb test of top plate

    Forgot to add above... candling is used for thinning the soundhole area to the required thickness for carving the lute rose. It's not so relevant for mandolins (other than historical ones with a carved rose), where the soundboard thickness is less varied throughout. Unless we're talking about carved top mandolins, which are a completely different thing...
    I am a luthier specialising in historical and world stringed instruments. You can see more info at my website.

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    Default Re: Light bulb test of top plate

    Hank, as Adrian said, those thicknesses seem reasonable. For me, closer to borderline. I don't have much recurve there and leave probably an inch or inch and a half wide a little thicker all the way up to the bridge. Not a ton, but some. My arch, measured from the bottom of the plate, is 5/8". My experience with this was with a mandolin I had made for a customer in England and it took about 3 years for the bulge to develop. I made him a new one. Ugh. A couple months ago I cut the back off of the failed one and am going to add a "tone bar" in that area and see how it acts. It'll be my shop beater or perhaps it will find its way to the dumpster.

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    Henry Lawton hank's Avatar
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    Default Re: Light bulb test of top plate

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ID:	171730Thanks Hogo. The recurve is very pronounced both inside and out toward the rims. I’m graduating with a steeper drop from the rim and a narrowed kerfing to move the thin wood outboard closer to the rims and also to narrow the width of this thinnest segment of the diaphragm. I also repositioned the tonebars inside the dome to center them under the bridge feet and adjust for the knot shadow stiffness behind the treble foot. The more diagonal direction of that bar caused more vibration damping until I removed its peak, stiffness and mass. Both bars tap ring at D a few cent sharp. The flexing stress area of the recurve has been narrowed in width but also increased in power in its close proximity to the rim. Dale your right. I’m pushing the tender places around and may be rewarded with cracks and bulges instead of grail from the Holy.
    "A sudden clash of thunder, the mind doors burst open, and lo, there sits old man Buddha-nature in all his homeliness."
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    Phil Goodson Philphool's Avatar
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    Default Re: Light bulb test of top plate

    Fancier name is 'transillumination'.
    Phil

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    Default Re: Light bulb test of top plate

    Hank, it's a dance with the devil. I like your satori quote, btw.

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    Adrian Minarovic
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    Default Re: Light bulb test of top plate

    Quote Originally Posted by hank View Post
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ID:	171730Thanks Hogo. The recurve is very pronounced both inside and out toward the rims. I’m graduating with a steeper drop from the rim and a narrowed kerfing to move the thin wood outboard closer to the rims and also to narrow the width of this thinnest segment of the diaphragm. I also repositioned the tonebars inside the dome to center them under the bridge feet and adjust for the knot shadow stiffness behind the treble foot. The more diagonal direction of that bar caused more vibration damping until I removed its peak, stiffness and mass. Both bars tap ring at D a few cent sharp. The flexing stress area of the recurve has been narrowed in width but also increased in power in its close proximity to the rim. Dale your right. I’m pushing the tender places around and may be rewarded with cracks and bulges instead of grail from the Holy.
    I'm a "Loar" style guy and I'll try to write what I think about this - these are mostly my gut feelings and musing based on few examples so I may be totally wrong (I would love to hear back from you when the instrument is finished to see how far I was)
    The outside recurve looks pretty normal for the lower half of top but I see you carved it up around whole top - I did that on my first as well thinking of the diaphragm and result as top that was very responsive but lacked projection. The thin region of recurve all around the top allowed for arch sinking down and later partial collapse of the recurve under tailpiece (but I carved it a tiny bit thinner than you to say the truth). My recurve was not carved inside like yours right to the lining but with smooth transition back into 3/16" thick edge. The thinning on the inside may make the recurve area weaker as the "neutral axis" will be more curved than on plate with "smooth" transition.
    From my experience the forces caused in top by string tension are twofold. One is bridge pushing the top arch down in the center and the other is tension of strings (plus some leverage of neck block, somewhat reduced if the beck is strong at the neck block) pushes th enorth and south of top towards each other and the top wants to buckle up in the center and at the same time recurve under tailpiece may have tendency to fold (if too weak or deep) but slender tonebars going right to the edge and slightly thicker area and/or shallower recurve with less s- shaped curvature to top long arch makes it stronger and less prone to this. If there is recurve under f/b support this weakness is doubled and may lead into sinking of the whole diaphragm. So for longevity and strong response you want all the forces work against each other while keeping the mass low for good responsive instrument which is IMO largely determined by top arch on second place top and back graduation. The thicker areas under extension of Loar model give some core to the tone that instruments thinner in those regions don't possess (I call them hollow, tubby, lacking that "knocking" core attack especially of a and e strings)
    I would expect your top being on bass side and the e and A may lack some body and may sound a bit tinnier than what folks expect from Loar F-5.
    (my first mandolin after re-top got some of the core but the overly thin back with recurve all around - diaphragm style - gives it more tubby kind of oval-hole flavor, with slight lack of attack or punch on higher notes but still sounds good enough that it generated interest of most local musicians) The top with very smooth long arch is as stable as it gets and after 15 years shows very little movement (if any) even with that extra flexible back.
    All depends how you combine your top with back and many other factors so just finish it and keep us posted how it comes out..
    Adrian

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    Henry Lawton hank's Avatar
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    Default Re: Light bulb test of top plate

    That’s probably an acurate Prognosis Adrian but I am still optimistic as a devil dancer! RIM SHOT! Ha Ha!
    "A sudden clash of thunder, the mind doors burst open, and lo, there sits old man Buddha-nature in all his homeliness."
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  31. #22
    Henry Lawton hank's Avatar
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    Default Re: Light bulb test of top plate

    I finally got my mandolin strung up and tested a bit for those interested. Here she is with the case I got as a birthday gift to go with it.

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    I apologize for accidentally side railing this thread with my plate graduation folly and build so I’ll start another thread to share the new subject created by this one.
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    Last edited by hank; Oct-22-2019 at 9:14am. Reason: Poor photo orientation
    "A sudden clash of thunder, the mind doors burst open, and lo, there sits old man Buddha-nature in all his homeliness."
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  32. #23
    Henry Lawton hank's Avatar
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    Default Re: Light bulb test of top plate

    I don’t know how to fix that second photo. Seems our IOS photo editing are undone when posting here.
    "A sudden clash of thunder, the mind doors burst open, and lo, there sits old man Buddha-nature in all his homeliness."
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  33. #24
    Adrian Minarovic
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    Default Re: Light bulb test of top plate

    Nice one! How does it sound?
    Adrian

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    Registered User Tom Haywood's Avatar
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    Default Re: Light bulb test of top plate

    Quote Originally Posted by hank View Post
    I don’t know how to fix that second photo. Seems our IOS photo editing are undone when posting here.
    Before posting a photo to the Cafe, I save it to my computer, run it through a photo editor (usually a free online editor or collage maker), save it on my computer and upload that file. It preserves the orientation and prevents all the frustration. Kind of sort of similar to having to post a video or sound file somewhere else first.
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