Structure of Wood • Composed of small cellulose fiber units called Cells. • Held together with natural adhesive called Lignin. • New wood is formed in the Cambium Layer (near the bark) • Springwood is early growth with larger and lighter colored fibers. • Summerwood grows later and is darker and stronger. • Both of these growth bands form the Annual Rings. • Age of tree can be determined by # of annual growth rings • Wood near bark is called Sapwood which contains living cells • As tree grows sapwood becomes inactive and changes into Heartwood. • Heartwood is usually darker in color due to presence of gums and resins. • Medullary rays run perpendicular to annual growth rings and carry sap toward pith (center) of tree. • These are large in Oak, Beech and Sycamore but small in most other woods. Classification of Wood • Two main classes of wood – Softwoods & Hardwoods • Softwood trees have needles and produce cones. Also called conifers. Examples are pine, fir, spruce, redwood, cedar and cypress. • Hardwood trees have broad leaves which are shed each year. Also called deciduous. Examples are oak, walnut, birch, maple, hickory, ash and poplar. Lumbering • Two methods of harvesting trees – Clear Cutting & Selective Cutting • Clear cutting – all trees in an area are cut at one time. Seedlings are planted to replace cut trees. Used mainly for softwoods because of their rapid growth. • Selective Cutting – used mainly for hardwood. More expensive process but less disruption to ecology. Steps in Lumber Manufacturing • Logs stored in piles or ponds. • Pulled into mill by a chain device called a Jack-ladder. • Log is washed with jets of water. • Bark is removed. • The Sawyer determines best way to cut log and cuts it into a slab with the head saw. • Slab is then cut into desired lengths by Trimmer saws. • Other saws are used to cut boards to desired widths. Two most common methods of cutting logs into lumber • Plain or flat sawing – blade cuts tangent to annual rings. • Most economical – low waste, fast. • Quarter Sawing – cuts parallel to the wood rays. • More wasteful & time consuming. • Wood warps and checks less. • Good for furniture making Moisture Content • Wood must be seasoned (dried). • Greenwood (unseasoned) may have 30% to 200% moisture content (MC) • Hardwoods are usually dried to 6% or 12%. • Softwoods are usually dried to about 12%. • Air drying is natural way of drying wood outdoors (inexpensive but time consuming). More prone to warping and twisting. • Kiln drying more controlled for specific results. • Kilns range in temperature from 120 to 130 degrees F. Defects • Two major classes of defects, natural & defects after felling the tree. • Natural includes abrasions, fire damage, growth defects, insect and animal damage. • Growth defects include knots, shakes, and pitch pockets. • Knots are result of branches & limbs. • Shakes are grain separation parallel to the rings. • Pitch pockets occur along the rings and usually contain resin. (common in fir, pines & spruces. Grading • Standards vary for softwoods & hardwoods • Softwoods are divided into 3 classifications, yard lumber, factory lumber, and shop lumber. • Yard lumber is divided into select (finish) and common (utility) grades. • Select grades range from A thru D, are used for siding, partitions, & finish flooring. • Common grades range 1 thru 5 are used mainly as structural lumber. • Softwoods are usually surfaced on 4 sides to finish dimensions and are graded after surfacing. Grading • Hardwoods are graded according to minimum sizes and percentage of clear surface cuttings that can be made. • First & Seconds is the best grade for hardwoods. • No. 1 common is lowest grade for hardwoods. • Hardwoods are normally sold in rough form in random widths and lengths. (yields more lumber this way) Veneer & Plywood • Veneer – a thin piece of wood sliced from a log 1/100 to ¼ inch thick. • Three methods can be used to slice veneer, rotary cut, flat slice or quarter slice. • Rotary yields a rippled grain pattern. • Flat yields more even grain – is used for hardwoods. • Quarter yields striped grain pattern. Plywood • Made of veneers bonded to core of composites, solid lumber or cross veneers. • Cross veneers (plycore) consists of an odd # of layers, 3, 5, 7 etc. • Five grades of veneers used in construction grade (soft) plywood – A, B, C, D. A = best, B = worst. • Grade will be stamped on worst side using 2 letters. 1st. Letter represents best side, 2nd Letter represents the worst side. • Glue type must be considered during selection. OTHER PANEL TYPES • Usually made from scraps of wood which saves money and resources. • Are dimensionally stable, not affected by humidity • Types of panels are: – – – – Hardboard Particle board Wafer board Oriented strand board. – All are manufactured with various glues and resins giving them great stability and strength.