Making a tight fitting mortise and tenon joint

[Troy White]

I am making a bed with six blind mortise and tenon joints. I have the mortise and tenons cut. My problem is I cant get rid of the gap between the shoulders and the corner post. I have heard of people undercutting the shoulders but I have never tried it. Any help would be great. Thanks Troy

[John Downey]

I won't claim to have never undercut the shoulders, but it is not a good habit to get into. Some of the strength of the joint comes from the bearing area of the shoulders, so having them square is a good idea as it maximizes this area. First thing to check is that the shoulders are really out of square. Your gap could also be the result of the mortise not being square to the face of the post, though this is not as likely as the shoulders being out. Fixing the shoulders is easy enough, you can use a shoulder plane, a rabbet plane, pare them back with a chisel, or even trim them with a table saw or a router. Free hand with a chisel is obviously the most difficult method from a technique point of view, but on the other hand, trimming the shoulders with a router requires more careful set up. Maybe the most important factor in deciding how to fix them is figuring out how they got cut out of square to begin with. If you cut the shoulders with a table saw, then perhaps thats not the way to fix them, at least not without checking your set up first.

Good luck with it,
John

[David Rose]

Troy,

Is the gap just in spots here and there? Or do you mean a gap all the way around the tenoned piece? If it is "in spots", follow John's advice. If it is all the way around, you need to shorten the tenon probably. I hope this does not sound like "newby advice". We all miss stuff sometimes. ...well... some of us do!

David

[Gary Keedwell]

White,


I would undercut the shoulder. Most recent tests indicate shoulder area doesn't have much strength anyways. Most of the holding force is concentrated on a good slip fit on mortise and tenon. Not trying to start anything.....just going by controlled studies.

Gary K.

[John Downey]

For a glued joint I can agree with that, I read "bed" and assumed a dry joint, not always the case by any means. Either way its not a major issue, just a kind of sloppy solution to something that shouldn't really be a problem. I think everyone has taken that short cut.

Just out of curiosity, where is the study you refer to? I dropped my last subscription two years ago. Last MT article I read was in FWW, must have been 5 or 6 years ago. Good article, and a pretty good test in general terms. I'd like to see one where they come up with a controlled method for simulating repetitive stress failures rather than using a continuous force to failure mode. Kind of like rocking back in the dining room chair until dad notices and yells.

John

[Kirk (KC) Constable]

Look closely at the shoulders of the tenon all the way around it, and make sure the cuts line up. I know this sounds simplistic, but just a teesny bit can make a difference.

Also, check and make sure that the corners of the mortise at the bottom are clean...you can measure the 'clean part' in the middle and the depth looks fine, but the corners will stop the tenon.

KC

[Troy White]

Thanks all to the quick replies The gap is not all the way around so I will probably try the shoulder plane. I have only been woodworking for about 4 years and seem to always have a problem with mortise and tenon joints. Troy

[pat warner]

Can be routed, see pix.
No heroics , just the consequence of the method.
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