Replies

1. 
   forestgirl | Aug 07, 2003 08:07pm | #1

   I've been reading Terri Knolls book on joinery.  Here's a quote from the "Haunched Tennon" section [emphasis added]:

   "Edge and face shoulders on a tenon increase its mechanical resistance to racking.  But each shoulder reduces the material in the tenon and replaces it with the shoulder's end grain, which lacks gluing strength.  Deep edge shoulders subtract gluing area from the tenon's cheek and make a wide part vulnerable to twisting, which easily breaks the shoulder's end-grain bond.  Wood movement and shrinking in the joint also stress the glue bond.  Haunches and multiple tenons help to overcome these difficulties."

   In the next section, he discusses sloped hanched tenons, which may be an alternative the joint seen in the article.  But looking at the drawings in the article, it appears that the steps provide additional mechanical strength in a large piece such as a bookcase (as opposed to a desk panel or door, say) to improve resistance to racking and such, while still preserving as much material as possible.

   forestgirl -- you can take the girl out of the forest, but you can't take the forest out of the girl   ;-)

   
   

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   Edited 8/7/2003 1:12:01 PM ET by forestgirl

2. 
   JohnWW | Aug 08, 2003 01:19am | #2

   Crewsr,

   I looked at the design and can't see any advantage to going through all the extra work and fussy fitting involved in making the joints that way.   

   Off hand, if I kept with the joint as designed, I'd make the mortises the same width as the grooves, 5/8 inch, for the additional strength and a simplified fit.  In any case, these are relatively small tenons, they need to be nicely fitted for the piece to have any strength at all.

   John W.

3. 
   crewsr | Aug 08, 2003 04:42pm | #3

   Well... after throwing caution to the wind I decided to chop the mortices out to the same width of the dado (a bit under 5/8").  Now that it's too late, I can see the advantage...  Keeping the mortices smaller allows for more glue contact area on the inside corner wall of the mortice before the it is interrupted by the other mortice cut perpendicular, but only about 1/8th of an inch.

   It seems like kind of a trade off... You lose 1/8" of glue surface, but you get a stouter tenon (3/8" thick tenon seems kind small to me).  You still can use all the glue surface from the dado walls and on the outer side of the mortice, so I don't think it's going to make all that much difference.

   I wonder if the author of the article reads these forums...

   Thanks for your replies!

   Rob

1. 
   forestgirl | Aug 08, 2003 08:01pm | #4

   Seems like it might be more accurate and useful to think of the glue surface lost in terms of square inches. 

   I suspect that the original design would cope better with the forces exerted when moving the bookcase around a room (racking, for instance), but whether the difference will matter in your particular situation is something only time will tell.forestgirl -- you can take the girl out of the forest, but you can't take the forest out of the girl   ;-)