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Thread: New bridge saddle

  1. #26
    Registered User sunburst's Avatar
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    Default Re: New bridge saddle

    Another contributing reason is the sharp inside corner, where that top is relieved for the adjuster wheels, causing a stress riser. I now cut that to round, rather than a sharp corner, on my bridges.

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  3. #27
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    Default Re: New bridge saddle

    Because I haven't figured out how to post pictures of this break, words will have to suffice. As per Sunburst, the inside corner step cuts, where the adjustment wheels are located, are radiused. The step cut itself is about 1-1/2 the thickness of the adjustment wheel. I would say almost 2mm. That said, the break/shear line runs from the step corner say 15* upwards across the compensation "scoop" of the E course and terminates at the beginning of the compensation scoop for the A course. The break is in the horizontal plane primarily.

    Thinking further about the step inside corners, wouldn't a diagonal instead of a radius or right angle corner be strongest?

  4. #28
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    Default Re: New bridge saddle

    Quote Originally Posted by dorenac View Post
    ...Thinking further about the step inside corners, wouldn't a diagonal instead of a radius or right angle corner be strongest?
    I'm not sure about that, but if you take a good look a "standard" adjustable mandolin bridge, at least to me, it begins to reveal that it really wasn't that well thought out. The aforementioned step cuts at the ends, the sharp corners, inside and out, of the compensation ramps, the proximity of the E and G ramps to the corner cuts and so forth. It looks like it was quickly designed for mechanical production with little engineering and no particular thought toward optimal use of materials for strength.

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  6. #29
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    Default Re: New bridge saddle

    I think the Brekke Traditional bridge begins to look at some of those details. The ends are not weakened by steps( steps are eliminated) and the hidden metal rod inside the saddle reinforces compression and tensile loads on the ebony. I would think compensation ramps are not as vulnerable to weakness either. But how well are string vibrations transmitted to the top?

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    Default Re: New bridge saddle

    Quote Originally Posted by sunburst View Post
    I'm not sure about that, but if you take a good look a "standard" adjustable mandolin bridge, at least to me, it begins to reveal that it really wasn't that well thought out. The aforementioned step cuts at the ends, the sharp corners, inside and out, of the compensation ramps, the proximity of the E and G ramps to the corner cuts and so forth. It looks like it was quickly designed for mechanical production with little engineering and no particular thought toward optimal use of materials for strength.
    A lot of what I do in modifying bridges takes out a bunch of the incoherence in design. Noticeable improvement in clarity. I have a couple of bridge designs that are more efficient, but haven't gotten around to making prototypes and trying to kill them!
    Stephen Perry

  8. #31
    Adrian Minarovic
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    Default Re: New bridge saddle

    Quote Originally Posted by sunburst View Post
    I'm not sure about that, but if you take a good look a "standard" adjustable mandolin bridge, at least to me, it begins to reveal that it really wasn't that well thought out. The aforementioned step cuts at the ends, the sharp corners, inside and out, of the compensation ramps, the proximity of the E and G ramps to the corner cuts and so forth. It looks like it was quickly designed for mechanical production with little engineering and no particular thought toward optimal use of materials for strength.
    Bear in mind that original patent was with aluminum saddle with thumbwheels both on top and bottom... The change to Ebony was afterthought. BUT unless made of bad material or weakened in wrong places by "well meaning" luthiers thay seem to hold quite well in standard configuration.
    Adrian

  9. #32
    Registered User sunburst's Avatar
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    Default Re: New bridge saddle

    Quote Originally Posted by HoGo View Post
    ...thay seem to hold quite well in standard configuration.
    Generally true, but when they break, they usually break in the same characteristic places.

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    Default Re: New bridge saddle

    Quote Originally Posted by HoGo View Post
    Bear in mind that original patent was with aluminum saddle with thumbwheels both on top and bottom... The change to Ebony was afterthought. BUT unless made of bad material or weakened in wrong places by "well meaning" luthiers thay seem to hold quite well in standard configuration.
    That something usually holds doesn't mean it is efficiently built.

    The tendency of bridges to crack at the stress riser suggests weakness, not strength.

    I have not seen a bridge weakened in the "wrong" spot fail. Where do you write of?
    Stephen Perry

  11. #34
    Adrian Minarovic
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    Default Re: New bridge saddle

    Quote Originally Posted by Stephen Perry View Post
    That something usually holds doesn't mean it is efficiently built.

    The tendency of bridges to crack at the stress riser suggests weakness, not strength.

    I have not seen a bridge weakened in the "wrong" spot fail. Where do you write of?
    Everything has it's weak spot (usually at a stress riser) except perfect solid steel ball. The question is just "WHEN does it break".

    Good quality mandolin bridge with ebony saddle of standard dimensions will hold just well for a loooong time.

    In every case of broken bridge saddle I came across (couple dozen) I could track the failure to (in order of most often seen, sometimes in combination)
    1. saddle height reduced by filing wood away at the thumbwheel step (right at the stress riser) sometimes cutting away so much that it hits the e string compensation cut.
    2. bridge where compensation of e strings was cut way too deep to start with (or on chinese bridges with symmetric compensations same on other side) interfering with the cut for thumb wheel
    3. wood cut at angle, especially when the grain goes from treble side stress riser up towards a strings slots (on chinese bridges also on the other side)
    4. completely unsuitable wood for bridges (typically the cheapest imports), the saddle often just bends under the load without breaking.

    When I make my own bridges I always keep the saddle above the thumbwheels offcuts high enough (approx. 5/16", never below 1/4" even close to that makes me nervous and do that only iff there is no other way to make the bridge lower). I cut the compensations either rounded or straight (for traditional look) but don't cut them as deep towards top plate as some makers do, making sure I stay in good distance from the thumbwheel cut-outs. I usually don't leave the edges of cuts at thumbwheels round, that would help a tiny bit, but not enough, IMO, to make it worthwhile.
    I've saved quite a few cracked bridges by gluing them back together and cutting right through the break (if it ws not at severe angle) along saddle. I plane the bottom flat add new nice straight grain ebony piece (with thin CA glue, 1/8" or so high to get good saddle height) plane the sides of teh new wood level and glue back the short piece of saddle between the thumbwheels (after planing off the saw marks) Then I drill through the holes for posts and fit back to instrument. Takes 15 minutes plus some setup time and so far such saddle never came back to me broken (I mostly do this to local pickers so I'm the only one go to mandolin tech for them). Kind of "backstrap" repair on bridges.
    I don't keep stock bridges or replacement parts as I'm concentrated on building mandolins and also build my own bridges so I prefer this repair over swapping for new (that may fail again unless it is expensive quality part) which is typically much less expensive and nearly invisible on ebony bridges.
    Adrian

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    Default Re: New bridge saddle

    For my own understanding please explain the term "stress riser". Thanks

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    Default Re: New bridge saddle

    [QUOTE=HoGo;1688535]
    I've saved quite a few cracked bridges by gluing them back together and cutting right through the break (if it ws not at severe angle) along saddle. I plane the bottom flat add new nice straight grain ebony piece (with thin CA glue, 1/8" or so high to get good saddle height) plane the sides of teh new wood level and glue back the short piece of saddle between the thumbwheels (after planing off the saw marks)

    HoGo are you essentially laminating a new piece of ebony to the bottom of the bridge?

  14. #37
    Registered User John Flynn's Avatar
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    Default Re: New bridge saddle

    Quote Originally Posted by sunburst View Post
    A replacement will surely sound better than the broken one.
    Weigh the old one, weigh the new one. If the new one is heavier, remove some wood until it weighs the same as the old one. That is probably the best you can do to replicate the sound of the old one (...and perhaps prevent the new one from sounding better than the old one...).
    I am sure you are right, but I will relate that I developed a chronic problem with my A course on my Old Wave oval, which had an ironwood saddle. The A strings just had a really bad tone to them. An experienced mandolin luthier in the DC area put a fairly substantial rosewood shim under the saddle, which had to have added weight to it. He also reshaped the contour of the top of the saddle. I was concerned when I first saw what he had done, but it was a great repair! My OW sounds better than when it was new.

  15. #38
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    Default Re: New bridge saddle

    Quote Originally Posted by dorenac View Post
    For my own understanding please explain the term "stress riser". Thanks
    I wish I could do that. It's one of those things that I understand just well enough to sort of know what it is, but not well enough to explain to someone else. (I learned that lesson when I worked as a tutor at a community college before I went pro as a luthier. Think you know something? Try explaining it to someone else... then you'll find out how well you know it.)

    Basically, any time there is a change of shape in an object that is under strain, bearing a load, whatever, there is an accumulation of stress where the shape changes. The more abrupt the change of shape, the more there is a stress riser. Sharp corners are prime examples. Outside corners often show chipping, inside corners are frequently the origin of cracks. In metal work, nearly all transitions are radiused, or at least curved, to help reduce stress risers.

  16. #39
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    Default Re: New bridge saddle

    Quote Originally Posted by dorenac View Post
    For my own understanding please explain the term "stress riser".
    In layman's terms, a stress riser is a shape that tends to concentrate the stress in a material into any one specific point; that point will usually be the first to break under, well, stress. And they tend to happen at angles, especially acute ones, whereas curves tend to disperse the stress along the length of the material.

    Think of a letter "V" formed out of any material (metal, plastic, stone, wood, paper), and now pull outward on the wings of the V. While the wings might bend a bit under adequate pressure, the vast majority of stress is at that inner point of the V, and that's where failure is most likely to begin, and most likely to quickly complete. Change that V to a U, and all areas share the stress more equally, resisting a higher degree of stress before failure does occur.

    There was a fairly famous case of early WWII liberty ships (quickly constructed cargo vessels) breaking up & sinking. It turned out that a reinforcing piece of steel (uspside-down T-shaped, guessing 6x12 feet, maybe 1" thick or less), at the corners where the central superstructure met the hull, failed in rough seas, with the crack quickly spreading down the side of the hull. By adding a maybe 3-foot radius to the T's inner corners, the stress riser was eliminated and the design was eventuallly considered brilliant: cheap, easy, and FAST to construct, while impervious to all but enemy attack.

    (Whoops! I was writing while John was answering, but I think the answers complement each other.)
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  17. #40
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    Default Re: New bridge saddle

    It would seem to be an easy change to do what John is doing. Drill a hole and cut to the hole and you have a nicely rounded corner. I think I will change and do that next time I make one. Thanks for the thread.
    THE WORLD IS A BETTER PLACE JUST FOR YOUR SMILE!

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    Default Re: New bridge saddle

    Looking at the old saddle and comparing to the new one, armed with info from Sunburst and EdHanrahan, I will add these observations:

    The OLD (original) saddle. At the treble step up, in the neighborhood where the break originated, the inside corner was radiused. But also there is more wood cut out, leaving about half the overall height of the bridge itself to rest on the wheels.

    The NEW saddle . The step cut is only about the height of the adjustment wheel itself. Leaving a lot more wood above, resting on the wheels. But the inside corners are NOT radiused. They are merely right angles.

    Would it be any real benefit to put a tiny radius at each inside corner instead of leaving the 90* angle?

  19. #42

    Default Re: New bridge saddle

    Just to confuse things the Liberty ship issue was primarily due to cold. There is a ductility transition point where steel becomes brittle around -40 F. Removing stress concentrations helped but the main issue was metallurgy and welding techniques.

    Putting even a small radius in corners will help significantly. I have done a lot of stress analysis using computerized models and it is amazing how even a fairly small radius will reduce stresses at a notch or corner. Going back on your existing bridge though, it is difficult to know whether removing material to add the radius would be of value. Some strength would be lost by the material you take out but you would gain by removing the sharp corner.

    John's answer is about as good as any in everyday language. It is difficult to explain without getting into math and material behavior.

  20. #43
    Registered User sunburst's Avatar
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    Default Re: New bridge saddle

    Quote Originally Posted by dorenac View Post
    Would it be any real benefit to put a tiny radius at each inside corner instead of leaving the 90* angle?
    Good question, and one I don't really know the answer.
    As I see it, a radius does no harm, and it might help, so that's why I use a radius in the corner (and also rounded compensation cuts).
    One thing: though metal has grain, it is not like wood grain. Since wood splits along the grain with less resistance than fracturing across grain, using a rounded corner in an ebony bridge probably doesn't do as much good as using a radius in a metal part that is prone to stress cracking. As I said, though, unless so much wood is removed as to weaken the part, a radius does no harm.

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    Default Re: New bridge saddle

    Could I do that with a Grobet file perhaps?

  22. #45
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    Default Re: New bridge saddle

    Speaking of metal, I saw this in the classifieds the other day......

    https://www.mandolincafe.com/ads/129963#129963

    Looks like they are pretty well machined, could be interesting to try.

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    A bunch of stuff with four strings

  23. #46
    Registered User sunburst's Avatar
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    Default Re: New bridge saddle

    I use a round jewelers file, usually, for adding a radius to an existing bridge. A small chain saw file will sometimes be about right.
    (I have a small set of Grobet files, but I usually end up using some "cheap" imported ones instead.)

  24. #47
    Adrian Minarovic
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    Default Re: New bridge saddle

    Quote Originally Posted by dorenac View Post
    Looking at the old saddle and comparing to the new one, armed with info from Sunburst and EdHanrahan, I will add these observations:

    The OLD (original) saddle. At the treble step up, in the neighborhood where the break originated, the inside corner was radiused. But also there is more wood cut out, leaving about half the overall height of the bridge itself to rest on the wheels.

    The NEW saddle . The step cut is only about the height of the adjustment wheel itself. Leaving a lot more wood above, resting on the wheels. But the inside corners are NOT radiused. They are merely right angles.

    Would it be any real benefit to put a tiny radius at each inside corner instead of leaving the 90* angle?
    If you are going to do it, don't remove material above wheel but just to the sides from wheels. Since the new saddle has more material above wheels it is also less likely to split than the old saddle.
    Adrian

  25. #48
    Registered User David Houchens's Avatar
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    Default Re: New bridge saddle

    I've been filing a radius at the inside corners of my bridges for quite a while and have had only one failure. I just leave a little meat in the corner when I cut it, then use a.056 nut file to round it out. Don't know if it has helped but I can't see any way it could hurt.

  26. #49
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    Default Re: New bridge saddle

    As of now I will wait until I change strings next so I can mark the bridge base location accurately. Also I'm going to weigh the old and new saddles as per Sunburst's comments. If the two are close enough in weight that I can sand a miniscule bit off the bottom of the new saddle I will do that and, while I'm at it, radius those inside corners. Maybe not as much as the old saddle. As I study the line of the break it appears to originate right in the middle of radius of the treble step and angles upward where it terminates at the A-course ramp.

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