Q: We are experiencing raised "pimples" after machining Sycamore. It occurs on 1 to 5 in 100 pieces. It is always on glued rails and on flat sawn grain. It is very random but appears to be associated where glue has dried. The problem also appears only on the sap not the heartwood. We also run sap gum and silver leaf soft maple into this product but have never seen these random raised pimples.
A: This problem (quite common actually) of small raised spots basically results when the planer, jointer or similar surfacing machine’s exhaust system is not strong enough. Hickory, being one of the heaviest and densest U.S. species, will have chips that are heavier than most other species; heavier chips, as you can imagine, require more power to remove. If the exhaust system cannot remove the chips promptly, the chips will stay on the knife edge for a brief moment (usually one revolution) and will actually make a small indentation on the wood’s surface (which actually is where the fibers are compressed) when the knife, with the chip on the cutting edge, is cutting more wood. The defect (slight depressions; the wood is compressed)) can often be seen after planing when the surface is carefully examined. The defect is called chip marks.
At this point, the wood can be sanded smoothly with no observable flaws. However, the small spots of compressed wood are still within the piece.
When a water-based finish is used, or even exposure to high humidity, the compressed wood recovers or pops back up. This creates the pimple spots you see.
Perhaps you can make your dust system more effective when planing a heavy wood like hickory by closing off other, nonessential dust intakes. Perhaps the hickory is too dry and so there is some static on the lumber that makes chip removal difficult. Slowing the feed speed to make smaller chips and fewer chips per second will also help.
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