FDM - September 2008

FDM
THE WOOD
DOCTOR’S R_x

fdmonline.com

Gene Wengert

wooddoc@uwalumni.com

Casehardening
stress, microwave
MC test

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 casehardening stress.

●
Changes in
size or shape
that occur after
machining are
due to changes in
moisture.
●
Moisture content

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 ^percentMC 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 _thatbowing when MC ^incoming ^lumber ^be ^at6. 8 to 7.0 percent MC.
changes. ^You ^may ^have ^to ^pa^your 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
continued

A:Consider your stress tests.

Transverse

(or across the grain) casehardening stress is measured using a prong-test where 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.