Re: damaged dovetail
Posted: Thu Sep 20, 2018 6:41 am
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.
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.