Replies

1. 
   Pondfish | Mar 03, 2005 09:02pm | #1

   Yes, it is a reason to sometimes reduce one large tenon to two smaller ones, and to pin tenons.  PVA glue creep is another concern for large MT joints.  Also, "face grain to face grain" is misleading--you need to consider if you are glueing cross grain faces, which is often the case in MT joinery (e.g., horizontal oriented grain in a rail meeting vertically oriented grain in a leg).

2. 
   Jellyrug | Mar 03, 2005 09:17pm | #2

   When a tenon becomes too large in dimension, and you have a typical situation where the tenon will be expanding across the grain tangentially, in the mortise, where there is no mortise movement, as the grain is in longitudinal direction, you can have problems.

   (phew that was a long sentence)

   What happens if the tenon is too wide, will be expansion causing compression. Once the compression exceeds the wood yield point, either the mortise will crack, or the tenon will be compressed into a new dimension. When the humidity drops, the tenon will again shrink, but this time smaller than the original dimension. The tenon will eventually become loose in the mortise.

   This can actually be calculated for wood species and moisture content, or humidity, as all the testing work was done by US forestry and the empirical data is published.

    

    

   
   

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   Edited 3/3/2005 1:18 pm ET by Jellyrug

1. 
   gpasewicz | Mar 03, 2005 09:31pm | #3

   Alrighty then.  Are there rules of thumb one should follow to minimize wood movement in M&T?

    

   What I should've said was: What rules should one follow to compensate for wood movement?

   Edited 3/3/2005 1:32 pm ET by gj13

1. 
   Jellyrug | Mar 04, 2005 01:02am | #4

   I believe the rule of thumb is tenon thickness 1/3 of the board and across grain width below 4". Some may even say 3 1/2".

   Further I believe the rule of thumb is 1/8" expansion agross grain allowance per 10".