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Re: Mandolin Bridge Form
I think the gap makes a conformal fit easier, especially as the soundboard can change shape. Other than that, there’s not much engineering. Consider the thumbwheel adjustable bridge - seems unlikely to be a good transducer, but apparently is good enough. Flat top one piece bridges are so thin that they conform easily directly under the strings, and that’s fine too. Dense, less-compressible main body should help, and the saddle should allow for smallest string contact area without being crushed, and be machinable with fine grain, so ebony, metals, bone.
Top radius:
No single radius; a compound curve, so fitting a bridge to it involves directly lapping using sandpaper and some shaped holder to avoid rounding the thing in the fore and aft direction. I suppose one could get a crude shape using one of those comb-like woodworking profile things. Or if the soundboard was shaped by a CNC, program it to cut the negative into a bridge. I’ve used the (presumably now extinct) flexible lead-in-plastic draftsman’s rules for other contours. The only thing with sufficient final accuracy, though, is lapping the bridge to the individual top.
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Re: Mandolin Bridge Form
The two-legged design most likely originated from violin connection that Gibson arch topped mandolins were looking up to. Most classical mandolins had single foot bridge. Difference in fitting two-foot and full contact bridge IMO is negligible (some 20% more surface to fit but the process is same) unless the top curvature is weird.
Sound transmission is red herring. The whole thing moves in zillion of tiny movements in complicated patterns from headstock to tailpiece and singeling out some "sound" transmission though bridge feet is completely off grid IMO. The sound is where periodic air pressure changes meet your eardrums. Until then it is just vibrations of solid or gas medium.
I prefer full contact bridges for one single reason - they do keep the shape of top healthy and prevent inevitable buckling of arch under the center of bridge. I've not noticed any significant change in tone when I replaced base with full contact or filled the cutout of existing bridge with new wood and fitted as full contact.
Regarding the "pre-tensioning" of bridge curvature. I fit the bridge so there is tiny gap between top and bridge center (something like 0.2-0.3 mm) I don't measure it but I get so much of it that it will go away with just slight pressure on the bridge with my hand. I pre bend the base with my fingers as I hold it against the top during fitting (pushing the posts against each other with my thumb and middle finger and aplying slight pressure in the center with index) and it naturally produces as much gap as given piece of wood needs. This prevents any chance of bridge ends not sit firmly on the top surface. You may not see a gap in bridge that is not pre-tensioned but as the seasons change (especially humid months) the ends may even lift from the surface as the arch rises and cause buzzes or weirs effects. I often see mandoins where I can literally feel the bridge base ends wiggle forward and back as they are floating invisibly above the surcface and most of the bridge pressure is concentrated near the center of bridge and on two footers on the inside corners of the feet leaving sometimes ugly scars on top, but most often you can just feel depressions in the area. Even violin makers use devices to slightly spread legs of newly fitted bridge for cellos or basses at the feet are flexible.
If you want to fit your 3D printed bridge you may need scanning of your top arch as they will not be all same and often not even symmetric and you'll need to do some final fitting as well no matter what you do. You may try to do what I would call "epoxy" fitting once the gap is minimal since your bridge will be plastic anyway...
Adrian
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Re: Mandolin Bridge Form
Glad Adrian said “red herring”! instead of me.
Anyway, since I really couldn’t describe the two contour-measuring devices, here they are fresh and dirty from my bench. The steel version might be tough on soft wood, but it also comes in plastic. Didn’t mention use of things like putty or clay, which also works and can have very high resolution, perhaps useful for fitting inlays.
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Re: Mandolin Bridge Form
Thank you Adrian and Richard for you inputs! It's now much clearer to me.
And yeah that's a nice idea with these tools, Richard. Maybe I can lend it somewhere or form it myself via clay or some compound materials from the shop.
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