deflection jigs

[bryce]

I searched this topic a bit and found a couple of threads. If I remember, the thought was that there is around 50 lbs of pressure give or take at the bridge when strung up. I known this will vary with break angle etc, but what I was wondering, is this the same as setting 50 lbs on a seragate (sp) bridge? The jig I'm thinking about would have the weight directly over the bridge. Most I've seen had the weight canterlevered out past the body. I would rather have the weight directly over the bridge. Any thoughts would be welcome. Thanks

[sunburst]

I don't think you need 50 lb. to test deflection, just a known, repeatable amount of pressure.
I don't think a 50 lb. weight sitting on the top would be exactly the same as having the mandolin strung up, because when the mando is strung up, the strings are not only pressing the bridge down against the top, but they are "trying" to pull the tailpiece and neck together. That force might counteract some of the downward movement of the top, and it wouldn't be there in a jig that simply places a weight on the top.

[HoGo]

What John said is absolutely true. I've read some violin studies that state that the counterforce between tailpiece and neck block counteracts majority of the pressure of bridge. Without this, all the force would be severely stressing top/sides joints and even centerjoint of top.
I would advise fixing the rim (or top plate) stiff all around and use lighter load 25lb should be enough to be able to measure deflection.

[bryce]

Thanks, 50 lbs seemed an awful lot to me but I wasn't sure.

[Paul Busman]

It strikes me that Deflection Jig would be a great name for an Irish tune.

[bryce]

I'm hoping this might add another piece of info into my thoughts . I've recently had the opportunity to map out some higher priced mandolins with a magic probe (sp). Not being able to feel the stiffness of the plates on these mandolins as I could if unassembled, I thought I could get a little idea of the stiffness through the amount of defection of these mandolins at their given thickness compaired to my free plates at their thickness. I know it still won't be quite the same, but I still enjoy making the jigs and fixtures.
My thought process is one that likes to SEE reasoning. If I can see something, I can wrap my mind around it much better. Doesn't always hold water, but I've made a lot of corks in my time.

[Marty Jacobson]

David, Siminoff has a lot of good information about deflection tuning (and the physics of lutherie in general) in his books "Luthier's Handbook" and "The Art of Tap Tuning". According to his studies, the downward pressure is actually more like 37 pounds. And he does indeed fix the rim when testing, for obvious reasons.
One thing he did is put the dial indicators underneath the plate (one on each side). The plate is mounted with the inside of the plate facing upward. That way you can tune the bracing while watching the indicators.
As I am currently setting up my own "mandolin assembly line", I really appreciate Siminoff's thoughtful approach to production planning.

[bryce]

Artdeco, Thanks, Ill check on Rogers site for info.

[dunwell]

I searched this topic a bit and found a couple of threads. If I remember, the thought was that there is around 50 lbs of pressure give or take at the bridge when strung up. I known this will vary with break angle etc, but what I was wondering, is this the same as setting 50 lbs on a seragate (sp) bridge? The jig I'm thinking about would have the weight directly over the bridge. Most I've seen had the weight canterlevered out past the body. I would rather have the weight directly over the bridge. Any thoughts would be welcome. Thanks

I've got a bit of a discussion and pix on deflection jig design HERE.

I'm using ~25# downward pressure on the bridge for testing. It really doesn't matter where the weight is as long as the pressure on the bridge, or pseudo-bridge, is directly down. A lever arm system gives you the ability to sort of micro adjust by sliding a weight along the lever arm.

Alan D.