Kiln Drying Forum
Wood Drying Topics => Impact on Planing => Topic started by: Aaron on August 24, 2011, 08:03:50 AM
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Does anyone have thoughts on what moisture content level in SYP, planer skip becomes a
"significant" problem. I have not done a shrinkage study, but am trying to get some initial
input. Thanks.
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It has been my experience that SYP does not shrink more than 1/8" when dried
to an average mc of 12%. Most skip at the planer is caused by sawmill mis-
manufacture. (knife gouges, saw run, mismatched saws on double arbor gangs, etc.) As mills went to thinner kerf saws, they leave less and less wood for the
planer to use to correct these problems. Planers now run at higher thruput in order to
be profitable at cost per thousand bf output per operating hour. In the past the
planer operator would take more off one side that the other so that he could clean
up mismanufacture. Most times now the operator does not have time to look
at the board and flip it to get the biggest cut where it needs to be. You would be
amazed at how much more money could be made if management would just
learn to slow down and correct these problems before you have millions of board
feet of mismanufactured lumber sitting on the yard, dried and ready for the planer,
which is where the problem is discovered!
PS: with mismatched saws on double-arbor gangs, the board cannot be cleaned
up because the skip will be on BOTH sides.
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The problem is not that easy.
Planer skip also depends on your target size (which should be calculated based on how much sawing variation you have), the greater the target size the more slack you have with shrinkage but the more log waste you have. If you don't have sawing variation under control you will see "planer skip" even when you run a thick target. But the other factor you have is how flat your lumber lays. We often blame "planer skip" on shrinkage but it is often due to how flat our lumber is (isn't).
Joe
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I agree that whether the board lays flat or not has great importance to quality
lumber and difficulties at the planer. Again, the single most recurring factor at my kilns causing non-flat boards would be sawmill mismanufacture. Boards that are
thicker than target cause the stacking stick to not lay flat across all boards in
that layer. This causes a gap between the layers that has no pressure created
on the thinner boards, thus allowing them to cup if board characteristics for
cupping are there. I also permits board warpage for the same reason. If boards
are thinner than target, they too, will be permitted to cup and warp because
the pressure is not on that board as the sticks are being supported by the
target boards, again permitting space for the thin board to cup and warp.
Thick and thin stacking sticks create this same problem. Thick and thin sticks
and non-target boards also cause stick warpage, which is expensive.
Ed, retired
Hello, Joe, long time no see!