Replies

1. 
   Lataxe | Nov 07, 2007 02:52pm | #1

   Morty,

   In my experience, the length of a tenon is not necessarily a critical factor, assuming that it has a length sufficient to register a glue bond or other attachment mechanism.  That said, it is often a good idea to over-engineer a bit - especially with an item that will suffer various kinds of stress, such as your porch thang.  A long tenon in a well-fitting mortise will provide two "shoulders" to resist wracking.

   Part of the strength of an M&T comes from the shoulders of the tenon - the degree to which they prevent a rail skewing in a stile when the joint is under sideways pressure (eg in a door where it's own weight, not to mention being banged open and closed, tries to drag the M&T askew or even apart). The shouder of the rail must be of signficant area and be tight-held to the side of the stile where the tenon goes into the mortise.

   To keep the shoulders tight, the tenon must pull the rail close up to the stile, then.  You can do this with a big tenon and lots of glue in a well-fitting joint; and/or you can pin or wedge the tenon to provide mechanical strength other than that provided by the glue.  I like to make M&Ts mechanically strong in this way with any weighty furniture, especially if it is to get stressed.  It then doesn't matter so much if the glue fails, in whole or in part.

   In all events, once the shoulder begins to show any gap, further stress will soon part the joint entirely.

   A large M&T with all its internal surfaces glued also stands a chance of self-destructing a little.  The grain of the tenon is usually at right angles to the grain of the mortise.  If there is a large degree of expansion/contraction on the piece, any large areas of cross-grained glued surfaces may part.  Moreover, a very wide tenon may shrink more than the width of the mortise it is in, thus going sloppy and losing some of its anti-wracking strength.

   I have seen this in many a poorly-built garden gate, which is exposed to the weather and its humidity changes. 

   In the English countryside, there are many large field gates of wood, all of which use large through tenons that are pinned (draw-bored) in place with no glue at all.  This allows the M&Ts to expand and contract with the weather but keeps the shoulders of the joints tight.  Also the exposed through tenon allows any water that gets into the joint  to dry out quickly, rather than remaining inside a blind mortise where it may cause rot to develop.  These gates last for decades, despite their typical 10-12 foot width, being banged about by stock, climbed over by hikers or farmer's lads and weathered by the fine English climate.

   As your porch is outdoors (?) but also subject to stress (large people swinging on it) perhaps the M&Ts might be longish but with only that portion next to the shoulder glued (if at all) and some form of mechanical fastening added (pin through the joint or wedges).  The humidity changes will hopefuly not break any minimal gluing but the large tenon and its mechanical fastening will resist wracking or other forces travelling to the joint whilst a large lady swangs and kicks her heels.

   If the swing can get wet, I would also consider having open ended through tenons to obviate rotting. This would also allow you to wedge them more easily (internal wedging is possible but more difficult as the wedges have to be just the right length to get pushed fully home by the mortise-bottom when the tenon is knocked into the blind mortise).

   And another possibility is biscuits. Many will laugh; but biscuits provide a minimal but sufficient glue area between the parts and also provide mechanical grab through their expansion on installation; and grabby cross-hatched surfaces.  They are far less likely to fail in their glue bond than a large M&T; but you cannot pin or wedge them. (Well, you can use S6s - the large biscuits- and pin them with a bamboo or other barbecue skewer.  It does work but probably has little strength in the pin, which is only 3mm diameter).

   Apologies for rambling.

   Lataxe

    

   
   

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   Edited 11/7/2007 11:52 am ET by Lataxe

2. 
   joinerswork | Nov 07, 2007 04:22pm | #2

   morty,

   In addition to friend Lataxe's good advice, I'll add this:

   My general "rule of thumb" for tenon length is to make them 1" long, where this is practicable.  This makes for an ample amount of surface area for gluing, and additionally, reduces the possibility of errors in figuring overall length for the member, after you have determined the distance between tenons.  For instance, with the distance between tenons of 17 3/8", would you rather add two 1" tenons to that, or two 7/8" tenons?  Only of concern if one is working to a cutlist, rather than winging it as one goes.

   I'm not sure you can make simple tenons too long.  As Lataxe points out there are advantages to using thru tenons in some instances.  But in the case of tenons on aprons entering a leg (at right angles), if the tenons are over-long, they will impinge on one another at their ends if too long.  Further, if the mortises meet inside the top of the leg, it weakens the construction considerably at this area of high stress-- the web of wood remaining above the mortises (where the haunch of the tenon allows), is all that keeps the top of the leg from splitting away, if it is dragged across the floor, for instance.

   Ray 

3. 
   Spalted | Nov 07, 2007 06:39pm | #3

   For a high stress piece like this would a fox-wedged tenon be challenging enough for you. I'd like to second the other suggestions too- keep the math simple (for simple folks like me who can't add beans), and don't underestimate the effect of having good supporting shoulders.

4. 
   Routerman | Nov 08, 2007 02:29am | #4

   Tenon should penetrate ~2/3 of the stick it enters.

   Notwithstanding, for outdoor stuff I'd use some hardware. In my case, steel X-dowels and machine screws. Hardware will out last glue and provide strength many times that of wood.

    

   Routs & joins

5. 
   danmart | Nov 17, 2007 01:00am | #5

   Morty

   Don't think this sticking is exactly what you need but my plug is make the tenons long. The tenons you see here are thru tenons and things used outside are generally better off with a housed tenon. The exposed open grain can wick moisture and that is not good.

   On the other hand, every door in my uncle's house in Maine(interior and exterior) has thru tenons on the rails and they are 150 years and older. They are pinned and as far as I can tell there is no glue in the joints that I can see.

   Like others have stated load and clear- get the shoulders tight.

   
1. 
   morty | Nov 17, 2007 09:37am | #6

   Nice joints. Thanks.

   Morty