Mortise and Tenon Fit Tolerances

      Not too loose, not too tight — just right. April 19, 2006

I built my first set of cabinet doors using mortise and tenon joinery on a table saw. My question is how tight should a mortise tenon joint be? Should I be able to slide it together with my hands or use a hammer to gently hammer it together? Also should I be leaving a 1/16 or 1/32 gap at the end of the tenon for glue? Any input would be appreciated.

Forum Responses
(Furniture Making Forum)
From contributor A:
I think in terms of two types of mortise and tenon joints: through and blind. On both I use a shoulder. For the through joint, I cut my tenons long, about 1/4 inch to leave them proud on assembly and split to wedge and tighten up to the shoulder. For blind joints, I cut the mortise about 1/8 inch deeper than the tenon for glue squeeze to minimize glue squeeze out around the shoulders.

On both, I use the shoulder for alignment. I use the Brian Boggs criterion of getting a slight pop as I pull the tenon out of the dry mortise. Hammering to seat a tenon means that there is no room for glue on the faces of the joint.

From contributor B:
I agree with contributor A. Also, if the tenons are too tight you might bust the receiving piece or if it doesn't bust the tenon can also spread the mortised piece creating a fat spot at the joint. When you use yellow glue the water in the glue causes wood to swell a little also. Combine glue swell with a tenon that’s too tight, then sand before the moisture swell has gone. When you rub your hand across the joint a week or two later you'll have a nice dip. With the right lighting after you install this you can look at it everyday and it will say don't do that again. I’m speaking from one of my personal experiences.

From contributor C:
If I may quote a great master “It should be tight enough so it needs to be tap with a hat! Not too loose, not too tight!” What really counts is having the shoulders tight, not much on the cheeks, but well balanced.

1/16 at the bottom is enough, 1/32 is too little! It’s a balance, like anything else. The cheeks should NOT be larger than 1/3 of the thickness of the piece as a general rule. (ie: 1’’ = 5/16 tenon thickness. Ύ’’ = Ό’’ tenon. 1 Ό’’ = 3/8’’ tenon. Always better less thick than too thick. Bigger is not always better!!

From contributor A:
Our criterion is that the frames should be dry fit first, taken off the bench, then brought back on the bench for assembly. Since some of our frames are over 8' tall, it is important that the tenons be snug enough so that the top rail won't come loose and bonk anyone when stood up off the bench, but loose enough to go 95% of the way with hand pressure. Our tenons range from 3/4 x 3 x 10" in White Oak to 1/4 x 3/4 x 1 in Pine, so it is a challenge.

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