Mixing wood types in Mortise and Tenon?

[Robin Cruz]

I'm making a mahogany rocking chair using loose mortise and tenon joinery. I already have a boat load of red oak tenons made up from the previous oak rocking chairs and it would be a convenience to use them. Would that cause a problem to use oak tenons in mahogany mortise?. I would think since the oak surface area would be covered in TypeIII glue that it would not absorb moisture. Any thoughts?

[Chris Friesen]

Even though it would be surrounded in a thin layer of glue, it's likely that it would still absorb moisture. (Type III is water resistant, not waterproof.) Given that red oak changes dimension roughly twice as much as mahogany for a given amount of humidity change, it would probably not be ideal.

[John Keeton]

To some degree, the rate of moisture exchange impacts movement differences. There is a substantial difference in the structure of mahogany and red oak, and even if they took on similar moisture content, they will not absorb or give off moisture at the same rate. That will affect the integrity of the joint. I wouldn't do it - not when working on such a nice project with expensive wood like mahogany. If it were a shop piece and the end result was not as critical - maybe. But, the labor is a big factor and it would be a shame to have a failure on anything just because you skimped on the tenons.

[Robin Cruz]

Ok thanks. I'll mill up some mahogany tenons to be safe.
thanks for the advice