Q:We rip a long, narrow stile
(about 72 x 3in.) and then send
it to the moulder for profiling.
We test for proper MC and casehardening stress. However, more than 50
percent of the pieces have 3/16 in. lengthwise bow, just after molding or a day or two
later. What are we are doing wrong?
any casehardening stress will cause immediate warp when machining. That is
the only manufacturing problem from
size or shape
that occur after
due to changes in
In most locations in the United
States and Canada, the in-use MC will
be around 7.0 percent MC; maybe
1 percent MC lower in the
Straight grain wintertime and 1 percent MC higher in the summer-
material will have time. To avoid warp, you need to avoid substan-
little or no risk of tial moisture changes, so it’s important that
bowing when MC incoming lumber be at 6. 8 to 7.0 percent MC.
changes. You may have to pay our kiln drying company
for extra days drying to
assure that they equalize the
lumber properly. Special note: Al-
ways check the core MC with insulated
needles to assure that it is 7.0 percent
MC. As wood is a variable material, we
would expect a small variation in MC
from piece to piece. At least two-thirds
of the pieces should be within 1 percent
MC of the 7.0 percent target.
How do you test the MC? Use insu-
lated needles and a name brand moisture
meter. Pinless moisture meters are fine
for scanning a lot of lumber, and are very
useful if some over-drying is suspected.
But any pieces that are out of range need
to be double-checked with a pin meter.
Changes in size or shape that occur
the grain) casehardening stress is
measured using a
the legs or prongs
(cut about 6 in. long
and one-quarter the
thickness of the lumber) should be parallel
with each other or nearly parallel. If there is transverse stress, the
prongs will bend inward and lumber
pieces will cup when machining.
Lengthwise warp is a different stress
and will not show up using the prong
test. Instead, I suggest that you cut a
24-in.-long piece of wood (not near
the end of the lumber) from a piece of
lumber about 8 in. wide. Then rip this
piece into two 4 in.-wide pieces. Put the
two ripped pieces back together and
they should fit with any gaps being less
than the thickness of a dollar bill.
Note that casehardening stress tests
always require the lumber to have no
moisture gradient, shell to core. Also,