When making a mortise and tenon joint, it's important for maximum strength of the joint that the tenon fit inside the mortise properly: neither too tight nor too loose. If the tenon is too tight, then you run the risk of splitting the piece with the mortise, and even if it's not quite that tight, the mechanical action of sliding the joint together can act as a squeegee to remove the glue from between the two pieces of wood (reducing the bonding area between the pieces). Too loose and the glue has to bridge gaps, something it is not designed to do (unless you're using an epoxy designed to do so).
The rules of thumb that I was taught for finding the sweet spot are as follows:
- The pieces should dry assemble without the need for any persuasion by a mallet.
- When dry assembled, the pieces should hold together against gravity. I.e., if you hold one piece so that the other piece is below it with the tenon vertical, then it should still hold together.
- The pieces should come apart from the above position with a single good shake.
However, it's recently been suggested to me that these rules of thumb may actually result in a joint that is too loose.
So, my question is: How do you tell if your mortise and tenon are properly sized to each other? What rules of thumb do you use to tell you've got the right fit before starting to glue things together?