"charlie b" wrote in message
...
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.
This is where all of the discussion here about wood swelling with moisture
etc., goes a little astray. First off - yes it does swell with moisture, I
am aware. But... look at the myriad of jointery around you that has
survived decades or longer without suffering the expansion problems that are
talked about here so frequently and ask yourself what is wrong with this
picture that there is so much discussion about 1/8" of movment across grain,
or 8% humidity, etc. but there are so few broken joints in the furniture
around you. We get so focused on one aspect of things that we sometimes
overlook everything else. Sure, moisture causes swelling - to exposed and
uncontrolled wood. Wood that is secured as a tenon in a mortice does not
absorb moisture in the same way that unsecured wood does. The rates that we
see published for wood expansion and moisture absorbtion are for raw,
unsecured wood. They do not apply to sealed wood and they do not apply to
wood that is secured as in a mortice and tenon. The mortice acts to keep
the tenon compressed such that it can only absorb a certain amount of
moisture, therefore a very controlled amount of expansion. Throw it under
water if you want, but unless the glue disolves, the joint is not going to
fail due to moisture absorbtion until the entire asssembly absorbs enough
water to distort the whole piece. That would be far more than you'd ever
see in environmental changes. Joined pieces of wood (with such interactive
techniques as mortice and tenon) are simply not the same as raw wood and do
not behave the way wood charts would imply for moisture absorbtion. Once
again - look around at how few joints you can see that have actually failed,
and then look to see why they failed. Most did not fail due to the joint
being too tight and not allowing for moisture absorbtion.
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.
That does indeed make for a nice, precision fit, but it would not be correct
to suggest that a dead blow tight fit is somehow less strong or long lived.
I have put many joints together that took some pretty good persuasion to
fit, and years and years later, they are still there, just as they were when
the project was built. I have to admit, when I have to resort to a little
persuasion, it's generally because I'm rushing it along in some way and
simply did not want to spend any more time to make it that little bit more
perfect - but - the joint does not fail.
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.
I agree but I generally find that all jointery has that required amount of
slop just by the nature of the woodworking and the material at hand.
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.
Now that's a man that's talking some good stuff!
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)
All right Charlie - you've been looking over my shoulder, haven't you?
--
-Mike-