a-Wood defects due to conversion
Conversion is the process of converting raw timber to forms suitable for woodworking or construction projects. During this process, the following defects may occur:
Chip mark – Shallow depressions or indentations in the surface of a board caused by shavings or chips getting imbedded in the surface during the process of dressing. They may be formed by a planer or jointer.
Diagonal grain – Wood in which the annual growth rings are at an angle with the axis of a piece as a result of sawing at an angle. In other words, rather than running parallel to the long edge of a board, for example, the grain runs at an angle to it. Such wood is not permitted for structural applications in the American Forestry Association guidelines because it lacks the same structural strength as an equal-sized piece that has the grain running parallel to the edge.
Torn grain – An irregularity in the surface of a board where wood fibers below the level of the dressed surface have been torn or broken out by a planer.
Wane – The presence of bark or the absence of wood on the corners or along the length of a piece of lumber. Wane, in the form of bark, is more commonly associated with rough milled lumber. In the case of construction lumber (e.g., 2x4s), it can be bark or missing wood.
Machine burn – A darkening of the wood due to overheating by machine knives or rolls when pieces are stopped in the machine.
Machine bite – A depressed cut of the machine knives at the end of the piece.
Machine gouge – A groove cut by the machine below the desired line.
b-Wood defects due to seasoning
Seasoning is the process of drying lumber (either in a kiln or air drying) to an appropriate level of moisture for woodworking and other commercial uses. During this process, a board may become warped.
The term “warped” is a nonspecific term that refers to a distorted or misshapen board. More specific terms for warping include cupping, twisting, bowing, crook, and spring. Common seasoning defects, including types of warping, include:
1-Bowing: A curvature formed in the direction of the length of timber. A bowed board is flat, but bent, like a road going over a hill.
`SOLUTION Salvage a bowed board by crosscutting it into shorter sections, matching the lengths of pieces to the curve of the board.
2-Check: A check is a crack which separates the fibers of wood. It does not extend from one end to the other. It occurs across the growth rings and is usually caused by poor or improper drying processes.
`SOLUTION These cracks occur at the ends of boards, so you may simply cut off the bad areas. But don’t be too hasty. Good narrow pieces often exist on either side of a check.
Shakes, because of their orientation, usually have to be cut off. Be leery of boards with excessive shake. This may be a result of the board simply being dropped on one end, but shakes also can be a sign of improper drying.
3-Crook: Where the board remains flat, but the ends move away from the center. Another type of warp.
`SOLUTION How you straighten the edge of a crooked board depends on the severity of the defect. If the crook is mild, run the concave edge over your jointer to straighten it. Use caution to prevent the leading end from catching on the outfeed table.
For boards with severe crook, options exist. You can crosscut the board into shorter pieces, then joint each, as discussed above. You also can rip off the crooked edge at the table saw using a long carrier board, as shown in photo above. Or snap a straight line on the board, cut it with a handheld circular saw, then joint the edge smooth.
4-Twisting: Where the board curves in length and width like a propeller.
`SOLUTION A severely twisted board is difficult to save. You may salvage short pieces, though, by using a combination of the methods previously described.
5-Cupping: Where the face of a board warps up across its width such that if one looks at the end of the board, it will look like a shallow letter “U.” Is common with plain-sawn lumber.
`SOLUTION Rip a wide, cupped board into narrow flat sections, as shown in Photo B. Rip each piece slightly wider than you need, then rerip or joint the edges square to the face. You even can glue these sections back together to create a wide board.
6-Knots: These are remnants of branches.
SOLUTION If they’re tightly held in the wood, knots usually pose just appearance problems. Use these boards in inconspicuous places where the knots won’t show. Loose knots, on the other hand, may fall out or be pulled free by cutting bits and blades. Cut out and discard areas with loose knots.
c-Moisture-related defects
Wood is a hygroscopic material. This means it naturally absorbs and releases water (moisture) to balance its internal moisture content with the surrounding environment.
Because wood retains its hygroscopic nature after it is put into use, it is subjected to fluctuating humidity. Shrinkage and swelling may occur in wood when there are changes in humidity and temperature. This may eventually result in cracks, gaps, and weak joints.
One major problem that occurs when drying wood is the tendency of its outer layers to dry out more rapidly than the interior ones. If these layers are allowed to dry much below the fiber saturation point while the interior is still saturated, drying stresses are set up because the shrinkage of the outer layer is restricted by the wet interior. Rupture in the wood tissues occurs, and consequently splits and cracks occur if these stresses across the grain exceed the strength across the grain.
Since wood shrinks and changes shape as it dries, the bulk of that shrinkage and change of shape should occur before a woodworker starts working with it.
The amount of shrinkage varies from species to species, but generally wood shrinks 8 to 10 percent tangentially, 4 to 5 percent radially, and close to zero percent lengthwise. In other words, the surface of the board where the grain intersects it perpendicularly, or close to perpendicularly, shrinks the most. This means woods of different shapes will shrink differently based on how they’re cut from the tree.
Some kiln-dried wood can change 1/8” to ¼” in width for every foot. They may not seem like much, but when you add it up, a 4-foot-wide table can vary in width by as much as an inch from dry season to wet season.
Moisture, therefore, is certainly the most important factor affecting the performance and service life of wood and wood products. Because moisture affects the dimensional movement of wood and wood products, under certain conditions moisture change can lead to major dimensional change.
Depending on where wood is stored and under what conditions, wood can easily absorb or release moisture. For that reason, all wood should be monitored regularly and certainly before use in order to avoid moisture-related problems
Solutions for defects
Many woodworkers prefer to avoid wood with defects because they detract from the beauty or value of the finished product. For others, though, defects in their works are often highly prized.
Many woodworkers rely on suppliers to ship them the wood they want. Most suppliers are reputable and they are careful to send their customers wood that is generally free of deformed or irregular wood.
On the other hand, woodworkers who buy their wood from a lumberyard or big box store should carefully check the wood and select only those pieces that are usable. They may have to get permission to sort through the lumber and also promise to restack everything when done.