Wood Manufacturing and Finishing
Domain scope
This domain tests how wood behaves as a material and how that behavior drives machining, assembly, and finishing. Expect questions on moisture content and movement, grain and machining defects, joinery and adhesives, surface preparation, finish compatibility, and finish-defect diagnosis.
Core rules
- Moisture content (MC) controls everything. Wood is hygroscopic; its equilibrium moisture content (EMC) tracks relative humidity and temperature. Interior stock is usually kiln-dried to 6–8% MC and must acclimate before machining or finishing.
- Movement is anisotropic. Tangential movement is roughly twice radial; longitudinal movement is negligible. Flat-sawn boards move more across the width than quarter-sawn. Use slotted clips, figure-eight fasteners, or floating panels to allow cross-grain movement.
- Grain direction governs cut quality. Machining against the grain causes tearout. Reduce depth of cut, control feed, or use a spiral cutter; take lighter passes on figured stock.
- Glue needs a fresh, close-fitting surface. PVA needs clamp pressure and about 30–60 minutes before stress; full strength in 24 hours. Hide glue is reversible with heat and moisture; polyurethane needs moisture to cure and foams.
- Surface prep is a sequence. Sand 80 → 120 → 180, raise grain with a damp cloth before water-based finishes, and remove dust. Do not burnish wood so smooth that stain cannot penetrate evenly.
- Finish systems are layered. Dewaxed shellac is a universal sealer/barrier; film finishes build, penetrating oils do not. Apply thin coats and respect recoat windows.
Worked example
A 600 mm-wide flat-sawn red oak top is machined at 12% MC for a room at 45% RH (EMC about 8%). Red oak tangential shrinkage is roughly 8.6% green to oven-dry, or 0.31% per 1% MC. A 4-point MC drop gives 600 × 0.0031 × 4 ≈ 7.4 mm cross-grain shrinkage. Screwed tight to a rail, the top will split or cup. Acclimate to 8% MC before final machining and fasten with slotted clips allowing 4 mm each side.
Traps
- More coats are not more protection; films crack.
- Sanding past 220 burnishes and causes blotchy stain.
- Rigid fasteners on wide panels prevent seasonal movement.
- Oak may need pore filler; maple may blotch.