Q.Is the problem tear out or casehardening?
A: First question. Tear out is caused when the knife is unable to cut a chip from the wood as easily as it can break a chip. The break follows the grain into the piece, leaving a void space that is called tear out, chip out or torn grain. This defect will happen more often if the wood is at low MCs (even if the MC was later raised a bit) and/or was dried at temperatures over 160F. It is also worse at high feed speeds and large depth of cut. One solution is to use slower feeds and perhaps higher spindle speeds to effectively get a small cut with each knife. With this defect, the knife is acting like a wedge rather than a plow. The wedge effect dominates with large rake angles(small slender knives), while “plowing” is common (and requires more hp and sometimes will have the piece stall while feeding and increases kickback while improving quality) with small rake angles. In either case, the knives must be sharp. You could also consider making two passes when moulding, with the first cutting taking most of the wood, but then a second cut with small stock removal taking the final, quality cut. An additional comment about MC: You probably measured the interior or average MC. However, the knives are working on the outside fibers, so when using a pin-type moisture meter, try and get a surface MC reading. Hopefully, this listing will provide the information you need to determine where your process can be improved.
On question two, casehardening will not cause tear out. Casehardened lumber has stresses in it, but nothing is harder than anything else. But casehardening does cause pieces to warp immediately after machining, which you are seeing when you rip the lumber and notice immediate bow. Casehardening, which is a natural event in drying, should be zero for properly dried lumber. You might check that your supplier is properly relieving the casehardening by using a final setting in the kiln at very high MCs. However, yellow poplar has a growth defect called tension wood that causes warp when ripping; this might be your problem, especially in wide pieces.
For lengthwise (also called longitudinal) casehardening, which causes bow when ripping, my favorite test is to take an 8-inch wide piece of lumber and cut a 24-inch long piece from the lumber, avoiding the last 12-inch of the board. Then rip this piece down the center to make two 4-inch pieces. If you can put the two piece back together without a gap, then the wood is free of longitudinal casehardening. If there is a gap when you put them back together, then take one of the 4-inch pieces and rip 12-inch long fingers with a band saw. Cut about eight fingers, 1/2-inch wide. If the fingers do not move up and down and seem to bow more or less about the same amount, then you have drying stress or casehardening. If the fingers seem to jump around, then you are looking at growth stresses, which we cannot cure in the kiln.
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