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When I am cutting structural joinery, mortise and tenon joints in this case, what is the maximum difference between the mortise dimensions and the tenon for a good glue joint if I am using standard PVA wood glue (also known as carpenter's glue or yellow glue, i.e. Tightbond etc.)?

At what point would I need to consider another type of adhesive or simply recutting the joint?

  • Are you asking what tolerances or clearance is too loose to be glued? – BrownRedHawk Mar 17 '15 at 16:36
  • Absolutely. In thousands of an inch or in mm. – datUser Mar 17 '15 at 16:36
  • Matthias Wandel loves to test the strength of glue with and without gaps – RedGrittyBrick Mar 20 '15 at 0:07
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    Instead of completely recutting the joint you could convert it to a wedged mortise and tenon joint either blind or through – Sentinel Jul 22 '15 at 17:02
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    Note that if you must make a too-loise tenion fit, gluing veneer to it or otherwise building things back up and then if necessary cutting again, is better than trying to fill the gap with glue alone. – keshlam Mar 5 '16 at 6:28
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In my limited experience, I'd say that anything over a difference of 0.01"+ is getting to be 'loose'. The pieces tenon should be able to be inserted into the mortise by hand, that is to say does not need to be hammered in.

But it should have enough friction that it does when dry fitting that the tenon piece does not fall out of the tenon.

If it is looser AND it truly needs to be blind, then I would recut. If you don't mind visible dowels or similar, It would not be unheard of to convert it to a doweled/pegged mortise and tenon joint. Personally, I think they are similarly beautiful, and can add interest if you dowel/peg with a different species of wood (contrasting color like maple and oak, or poplar and mahogany are personal favorites). In the photo below, they were adding manufactured dowels, but scraps of contrasting hardwood, or simple cut dowels could be exchanged. Adding-Accent Dowels

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    0.1" is way way to loose. You should be thinking of dimensions closer to 1/64" (equivalent to about .015"). – Michael Karas Mar 18 '15 at 15:03
  • Yeah not sure that you mean 0.100" above... Is that a typo? – datUser Mar 18 '15 at 18:49
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    Yes. Thank you, that was a typo. There should have been an extra 0 in there. Much appreciated everyone. I was asleep at the wheel on this one. – BrownRedHawk Mar 18 '15 at 19:02
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Check out this page:

http://woodgears.ca/mortise/accuracy.html

He suggests that about .001 (one thou) under for the tenons is perfect, while .005 is too loose.

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