1

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:

  1. The pieces should dry assemble without the need for any persuasion by a mallet.
  2. 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.
  3. 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?

3
  • One suggestion is to watch a couple videos by good craftsmen and see how tight they work. I recently watched a youtube video from Kobeomsuk furniture with incredibly tight and hand crafted joints (not to mention elegant designs).
    – Ashlar
    Jul 7 '20 at 1:09
  • 1
    "The pieces should dry assemble without the need for any persuasion by a mallet" As I covered in the other Q, this is incorrect. The person saying it may believe it wholeheartedly, but they are fundamentally wrong. "The pieces should come apart from the above position with a single good shake" This is really much too loose. Exactly the same debates go on re. dovetails if you want to delve into that, so you can see there is a wide gulf of opinion on this. But at the end of the day, as long as joints are not excessively tight tighter is always preferable to looser. [contd]
    – Graphus
    Jul 7 '20 at 13:11
  • Re. M&T tightness in general, as I mentioned in the other Q and as @Ashlar has referred to above, watch some proper craftsmen work and you'll see theirs are much tighter than you have been led to believe is the sweet spot. Some reference to good published woodworking books (including the 2 or 3 that are specifically about joints) would be worthwhile.
    – Graphus
    Jul 7 '20 at 13:12

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Browse other questions tagged or ask your own question.