Why is My Resawed Wood Bowing (Or Cupping)
The poster who began this discussion
was having problems with some old, 8/4 oak. It was thoroughly dry,
but it still gave him problems when he ran it through his band saw.
night, I was resawing 8/4 oak boards in half and, just when the band
saw blade was through the entire board, the two pieces popped apart.
Both sides bowed away from the direction of the cut. To make it
clearer, if I put the two pieces back together, the ends would touch,
but in the center they were about 1/2" apart over a 48"
length. Before anyone asks, these are really old, very dry pieces of
wood. I got them from Grizzly in the late 90's, and they have been
sitting in my shop, which is heated and air-conditioned, since then.
I cut apart six of them last night, and it happened on all of them. I
have another 6 to do. Could it be from the heat of the blade? I'm
using a 3/4" blade in the saw, and I'm feeding the wood at a
normal rate, not overloading the saw at all. This happened before
when I resawed some pieces from this lot. I can't understand it. Any
insight into this would be helpful. I have the boards clamped flat to
my workbench, but I hold little hope they'll straighten out. Any
advice on how to straighten them would be appreciated, too!" -
respondent had quite a bit of insight, including that what had
actually happened was "cupping," rather than "bowing."
is stress locked into this lumber. It is common with thick oak
because oak must be dried very slowly and sometimes it isn't. Because
you see it on most all of the lumber, it is a drying defect, not
stress as a result of some aspect of how the tree grew. This sort of
stress can only be relieved by steaming the lumber, a process that
will soften the fibers and let the lumber relax.
I understand correctly, Ed was in fact resawing the board into two
thinner boards and the resulting thinner boards cupped. Cup is across
the width of a board; bow is along the length. Cupping can result
from two processes that can be distinguished by how quickly it
occurs. If the resawed pieces cup immediately after resawing, it is a
result of the lumber being stressed. The most common cause of stress
is drying, (either air or kiln) the lumber too fast. When dried too
fast, the interior of the lumber is under tension. When resawed, this
tension pulls the thinner boards into a cupped configuration. It is
very easy to dry oak too fast because of how it dries and the
requirement to dry oak very slowly. Stress can also occur from how a
tree grew, but this isn't the cause of Ed's problem.
the lumber cups after a day or more, it is because the lumber is
gaining or losing moisture from one side faster than the other. There
may have been a moisture gradient across the thickness of the thick
piece. Or the resawed lumber may have been stored in a way where one
side gained or lost moisture faster than the other." - Bill T.
also received some advice on ways to go about fixing the problem. -
stacking and stickering the boards as soon as you finish resawing
them. Be sure there is some air movement through the stack. Give it
two weeks and see if there is any improvement." Howard A.
on how the wood was sawn into lumber, its drying profile (air dried?
in what conditions, kiln dried? too fast, too hot, etc.), and what's
happened since (RH cycles, storage, etc.) -- so many factors. For
older oak and a few other species, I take skim passes over the
jointer and set it aside for a couple weeks or more (ideally) before
resawing." - David B.
oak can be a lot tougher to dry than red. The cellular structure of
white oak makes it tougher for water to get out. That said, in that
amount of time, it should not still have had drying issues. It can
depend on the type of tree as well. If it was a leaner, it can have
what is called reaction wood. There are lots of stresses in reaction
wood. I would stand the boards up so air can get all around it and
see if it calms down over a week or so." - Dick C.
advice is to save this for when you need 8/4. And go buy some more
4/4, when you need it. I don't think you are going to wrestle this
wood back to flat, without reducing it to 1/2" on the jointer,
turning most of it into shavings. It probably will be fine for
projects where you need 8/4, if you use good techniques, like sawing
oversize, letting it settle, then straightening it at full thickness,
or taking equal amounts off each face. It is harder to find thick oak
here sometimes, but I can always get 4/4.
You are just buying
yourself headaches doing this." - Keith N.
some elaboration on the method of eliminating the cupping via steam.
can build a steam box really easily, just knocked together out of CD
plywood with a pipe or hose going to a steam kettle. Hose has to be
able to resist the heat of the steam, and should be fairly short to
keep from losing all the heat. If you use metal pipe you can super
heat the steam by applying a torch to the pipe (saw a neat
demonstration in the 8th grade; teacher burned paper with steam by
doing this) You also may not need to remove the bow, depending on how
bad it is and what the boards will be used for. Table tops and
casework are far more forgiving than door panels. From what I've read
about steam bending, you shoot for 1 hour per inch of thickness - -
I'll admit to having never tried it, but straightening a board would
be the same principle as steam bending, you're just clamping against
a straight caul rather than a curved one." - John
wood must become plastic to relax the stress. This will occur as soon
as it reaches a moist heat temperature of something near the boiling
point of water. Actually, it will occur before this temperature, but
I don't know exactly what temperature and it will depend on humidity.
So be sure to sock the steam to it until the wood temperature reaches
about 100 C. Then, of course, the wood needs to be carefully dried so
as to not reestablish the stress. The wood will dry quicker than it
did the first time because the cells are already largely collapsed
and the wood will not hold as much water as it once did." - Bill
Protective Finish for Plywood
Shop Floor from WoodCentral
The woodworker who began this
discussion had a question about one of the very foundations of the
shop: the floor. Specifically, whether and how he should paint or
otherwise finish it, for both practical and aesthetic reasons. -
have about 1,100 sq. ft. of bare plywood flooring that I need to do
something with before any shop equipment goes in. This needs to
happen two weeks from now when I have a week off from work. Plywood
is that crazy heavy 9/8" stuff. Walls and ceiling are drywall
and painted white. Lighting is good. The building itself is somewhat
high-end -- brick and stone outside, metal roof on top.
objectives for the floor are to (1) avoid ugly, (2) make it cleanable
(not just sweepable but to keep any tracked-in mud out of the wood),
and (3) avoid dark, light-depleting colors. I guess old-school would
be to paint it gray. I'm wondering about using a water-based poly
such as used on hardwood floors. The ply is not pretty, but perhaps
it is better looking than industrial gray. Maybe a different color
paint is advisable. Is there such a thing as "floor grade paint"
Ideas? Opinions? Any 'I did this' advice?" - Ron
did receive several "I did this" types of advice. - Editor
built a shop/shed about seven years ago and I put epoxy paint down,
and then covered it with the old carpeting and padding when we
recarpeted the house. Of course, I went around the cabinets and such
so it would be more comfortable on the feet and it was free. But if I
didn`t have the carpet, I would have stayed with just the paint. I
didn`t think it was slippery when I didn`t have the carpet." -
on your expectations. When I built my shop, I intended to paint the
plywood floor, but I only got as far as priming it, and it has held
up pretty darn well for lo these past 12 years. Sweeping is quite
easy, and the light color of the primer does brighten the shop,
although it's not as aesthetically pleasing as the medium blue-gray
paint I had originally intended. My reference to your expectations
had to do with how pristine and protected you want this floor. Most
shop floors get a lot of glue and finish dripped on them, depending
on your work habits. I only ever worried about the most egregious of
these. If you want a floor that can be mopped clean, you probably
want floor enamel or epoxy garage paint, or you could use
polyurethane over a coat of regular paint." - Ellis W.
have had very good luck using Ace Hardware polyurethane paint on my
concrete basement floor. It's very tough and they will mix any color
you want. A very light gray would be my preference for a shop floor.
Light enough to provide good reflective light without the glare of a
white floor." - Paul
floor is concrete. I used a paint with epoxy fibers. It has held up
well. My friend Mike has an 'Advantek' floor (plywood). He painted
his floor with a similar product. It worked well for him. He ended up
tinting his paint a different color." - J.L.
some research and theory. - Editor
to my lighting engineer brother in law, painting the floor a light
color will lessen the need for more lights overhead. 2. Some sort of
finish will aid sweeping. 3. Oil-based porch enamel is the hardest,
most wear resistant, of the paints. It takes a week to cure to full
hardness when it is warm. It leaves a finish slicker than I would
want on a shop floor. This paint plus sawdust could be a hazard.
There may be some epoxy product that would be better. 4. Other than
the disadvantage of color, floor varnish is a good choice. I was told
by a paint chemist that the oil-based products are more durable than
the latex porch and floor paints. I got mine at Glidden and painted
the deck of a flat bed trailer with it. It took two weeks to cure,
but it cured hard and has held up outdoors." - Bill T.