Replies

1. 
   boilerbay | Dec 23, 2009 12:24am | #1

   Will,

   If your talking rails and stiles, it's there to fill the visual gap in the stile trench when observed from the distal bottom/top edges. Also adds massive torque resistance against latent twist and poor glue up in apron/leg joints when used haunched into top of leg or many times haunched at the bottom of the apron also, the lower going into a sealed mortise. It's not so great in benefit against fracture as it is against twist. In many ways a housed tenon as used in Greene and Greene styles, is nothing more than a tennon with the last 1/8" haunched on all four sides.

   I know, clear as mud -- it's my haunch hunch. :)

   Merry Christmas

   Boiler

2. 
   SgianDubh | Dec 23, 2009 05:20am | #2

   As boilerbay mentioned, a haunch fills a groove worked in the frame members to retain a panel in a frame and panel construction. Other similar construction scenarios may use a haunch for similar reasons.

   In table frame and leg construction where a rail joins the top of the leg you need to position the end of the mortice in the leg some way in from the end. This adds strength to the top of the leg and prevents shear stresses from racking popping a plug of wood out at the top end of the leg. The rule of thumb is to divide the width of the rail into three equal parts and make the tenon about 2/3rds of the rail width-- the rule doesn't need to be followed slavishly.

   The inclusion of a haunch in this circumstance does add to the joint strength through increased long grain to long grain gluing area, but its significance from this point of view is relatively small. Where the haunch really has a role here is if the visible faces of the leg and rail are flush. The haunch, if snugly made, helps prevent the rail cupping. If the rail cups then the width that corresponds with the haunch is less likely to sit either proud or below the face of the leg. Where the face of the tenoned rail is set back from the face of the mortised member, eg the leg, then cupping of the rail after final assembly is unlikely to be much of an issue, so using a haunch becomes much less useful. Slainte.

1. 
   boilerbay | Dec 23, 2009 09:22am | #3

   Richard,

   "The haunch, if snugly made, helps prevent the rail cupping. If the rail cups then the width that corresponds with the haunch is less likely to sit either proud or below the face of the leg. Where the face of the tenoned rail is set back from the face of the mortised member, eg the leg, then cupping of the rail after final assembly is unlikely to be much of an issue, so using a haunch becomes much less useful."

   I hadn't thought of that aspect and I think probably more clear than my "twist" version. Good explanation.

   Happy Holidays

   BB

2. 
   RalphBarker | Dec 23, 2009 12:50pm | #4

   I had a haunch that might be the reason. ;-)