damaged dovetail

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tippie53
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Re: damaged dovetail

Post by tippie53 »

as a mechanical engineer the mechanics of the dovetail joint , make it very secure. The lower part of the dovetail is moot if you have a good fitting dovetail as the whole joint is the mechanism. Te bottom of the V is not where the connection makes the best contact . In fact most every dovetail I have taken apart , the bottom of the V clears the bottom of the neck block. There is more surface area in the dovetail joint that secures in. In a bolt on system that is the only connection.
The video shows the mechanics of this very clearly. Martin used to use one bolt and it was at the bottom of the joint. They also glued the tenon in . This worked to a degree but the joint had a high rate of failure . 2 bolts work best and here is why . In your thinking you believe the lower end takes the most stress but the reality is , if the joint is well made and fitted , once the heel is pulled into the neck the forces are applied to a larger area.Then as the load is applied the major for is applied by compression higher up the neck above the bolt . So your bolt position becomes the fulcrum point. Most bolts loosen as the wood shrinks and need to be tightened or you use a compression washer. It is an easy fix just tighten. The lower area of the neck using an almond heel is also a weaker point so moving it up a bit gives you more meat in the joint . I have seen many bolt on designs. a 1/4 20 bolt is plenty strong. I have even seen drywall screws used.
So think of it this way , the heel of the cheeks are pulled onto the body by the bolt or the dovetail. Now as the force is applied the load is shared by the larger surface area. The bolt connection takes the stress and is being pulled on the neck block. The dovetail is most efficient in doing that as the tail of the tenon was compressed into the neck block.
John Hall
Blues Creek Guitars Inc
Authorized CF Martin Repair Center
president of Association of Stringed Instrument Artisans
http://www.bluescreekguitars.com
ruby@magpage.com
Posts: 1564
Joined: Thu May 24, 2012 8:03 am
Location: Chestertown Maryland

Re: damaged dovetail

Post by ruby@magpage.com »

John

I am a little confused. I understand that the bottom of the vee is not where the joint makes the best contact - that is where it is trying to pull away, and that is why the bottom is the only place that the joint ever opens up. So a tight fitting bottom is essential to the joint. On the other hand, the upper half of the joint never opens up (pulls away from the body) because it is in compression.

Did Martin ever figure out why their single-bolt joint necks where failing? Sounds like an engineering issue. If the neck folded up (the bolt failed) that means the bottom was in tension, and the fact that it "hinges" around the area where the body, neck, and fretboard all meet shows that area is in compression

I can understand wanting the security of a second bolt to keep the neck from being knocked out of side-to-side alignment, but if you glue the fretboard extension down (I don;t), that shouldn't happen anyway.

Sorry to hijack the thread

Ed
Ed M
tippie53
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Joined: Fri Mar 03, 2006 7:09 pm
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Re: damaged dovetail

Post by tippie53 »

the only time I have seen dovetails pull away is when the joint was done poorly. I have seen them loosen but very rarely. I have strung dovetails up without any glue to prove a point that the dovetail joint is the only self sustaining joint . Why the Martin tenon joint fails was always the bolt loosened or the neck block shrank. Simple fixes . They now redid the joint and made it a simple dovetail and no longer rely on a bolt on exclusively.
John Hall
Blues Creek Guitars Inc
Authorized CF Martin Repair Center
president of Association of Stringed Instrument Artisans
http://www.bluescreekguitars.com
ruby@magpage.com
Posts: 1564
Joined: Thu May 24, 2012 8:03 am
Location: Chestertown Maryland

Re: damaged dovetail

Post by ruby@magpage.com »

John

Where did the poorly made joint separate or loosen? Always at the bottom where you can stick a feeler gauge between the neck and body. And if the Martin bolt loosened, where did the joint open up? Always at the bottom. (while under string tension)

Yes you want the joint to be fit properly throughout - that's what craftsmanship is all about.

Ed
Ed M
tippie53
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Joined: Fri Mar 03, 2006 7:09 pm
Location: Hegins, Pa
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Re: damaged dovetail

Post by tippie53 »

the reason the bottom shows a failure is often poor workmanship to that I agree. A neck reset is a result of time and stress duration a joint failure is poor workmanship . No bolt will correct that .
John Hall
Blues Creek Guitars Inc
Authorized CF Martin Repair Center
president of Association of Stringed Instrument Artisans
http://www.bluescreekguitars.com
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