Accidently sent this to Mike instead of here. So -this is sort of
like trying to hold a conversation with a 10 hour delay between
parties. Hopefully the diagrams that show the six degrees of
freedom of movement which must be resisted and which parts
of the M&T joint resists which movement.


This may clarify things a little - showing the six (yup - six) degrees
of freedom of movement that need to be fixed if two pieces of wood
are to be joined together and not come apart. A mortise and tenon
joint resists 5 of the six.

http://home.comcast.net/~charliebcz/MtPrimer4.html

As for having a "sloppy joint" - there are instances where a little
"slop" actually can be a good idea - a table apron to leg joint for
example. Having a little slop at the top of the tenon gives the
tenon a place to expand without blowing the top of the leg off.
You've got vertical grain for the legs and horizontal grain
for the apron. The cross grain in the tenon will expand more
than the vertical grain in the leg. If there's not place set
aside for that expansion it will try anyway. The tenon will
either compress and get tighter OR it will make the space
by moving some wood above it at the top of the leg.

It seems "spit tight" is what to shoot for. The tenon should
fit snug enough to go in and out with just moderate hand
pressure - no dead blow hammer, no mallet whacking etc..
But if you spit on the tenon and then seat it, it should
swell enough to make getting the joint apart difficult.

And tight also means leaving some place for a) glue inside
to go (mortise a little deeper than the tenon is long) and
b) some place for compressing the air trapped in front
of the tenon as it seats or someway for it to get out of the
joint.

The beauty of "traditional joinery" is that it lets you
dry fit things and the parts will a)self align and
b) be self supporting. That's real handy if you make
things "on the fly" - make step 1, make parts for
step 2 to fit what you have in step 1 and so on. Has
the advantage of letting you see things at full scale
each step of the way.

If you've ever worked from a "plan" and cut all your
parts BEFORE putting them together you know that
somewhere amongst all the given dimensions there's
at least one that's wrong. Working progressively
you can get dimensions off what you have. It isn't
important that a part be 22 31/32nds but rather
that if fit between the parts it's suppose to fit between.

I can "take the line", "split the line" and "leave the
line" more often than I can read a tape properly ; )
(ok - tell me you've never made a part an inch short)

charlie b