On 5 Jun, 23:10, Andy wrote:

I thought the point of draw-boring M&Ts was that they're held together
mechanically, so glue isn't as critical.

No, they're more like putting a nut onto a bolt. It's the sides of
the bolt that take the load, the nut is just there to make sure the
bolt stays in the correct position to keep working.

A mortice and tenon shouldn't need to be glued. It's strong in shear
because of the tenon and it's rigid against racking because of the
shoulders of the tenon. It shouldn't be loaded in tension at all.

Of course for smaller work (furniture) rather than architectural
framing, these rules aren't always followed. Tenons are tensioned and
especially racked against (chairs!). They may also be cut less than
tight, so glue's needed for gap-filling too. In principle though, it's
still basically not a glued joint (they precede useful glues by
centuries). Their strongest aspect is shear sideways against the
tenon, which glue just doesn't help with. Racking can't be glued well,
because the shoulders are end-grain and so would be a weak glue joint.
The only real change glue makes is to make them resist tension, or to
resist racking by gluing the sides of the tenon.

This is why small tenons fail on chair legs - the large racking forces
are coupled into the tenon and overload the narrow neck. For framing
carpentry, a tenon should never fail, even though the forces are far
greater. The forces in that sort of structure might break a beam if
over-loaded, but agood design shouldn't fail at its tenons first.

Pegged tenons shouldn't be loaded in tension either. That peg isn't
strong, compared with the tenon in general. If framing breaks pegs,
then it's badly designed (or more usually, badly cut) and the peg was
taking a force that the tenon ought to have taken instead. In small
furniture peg break-out is a real problem (short-grain weaknesses) and
is usually caused by putting tension into an M&T joint. If you are
going to abuse this joint with tension, it needs to be glued as well.
You've now made a joint that contradicts centuries of joinery practice
and it will break when the glue fails in time. Traditional joinery
(dovetails etc.) is designed to start rattling loose after the glue
fails, but not to fall apart or to break.