Lumber Characteristics
Orientation of Saw Cut Determines Appearance of Lumber
A log of uniform diameter is more or less composed of a series of nested cylinders which can be seen at the end of the log as annual growth rings. Depending on where the saw passes through a log, the grain pattern on the broad face of the resulting boards will vary because some cylinders will be cut longitudinally in half while others may have mild glancing cuts to their outer surfaces. We will explore this idea in detail below.
At the Ledyard sawmill, we typically use a sawing method that is variously called through and through, flat, or plain sawing. The log is simply held in the same orientation on the carriage and repeatedly passed through the saw.
Figure adapted from: USDA Wood Handbook: wood as an engineering material. 2010. Chapter 4. Moisture Relations and Physical Properties of Wood.
Tangentially Sawn Boards
Tangential cutting occurs when the saw cut glances on the outside of growth cylinder in the wood. The resulting pattern on the board's surface is called cathedral or crown grain, having pronounced figure as shown in the photos to the left. When such a board is viewed from the end, you can see how the annual growth cylinders are tangentially cut.
A skilled cabinet maker will glue highly figured boards edge to edge to produce a eye-pleasing large surface such as a table top. The drawback to tangentially cut boards is that they swell and shrink in width with changes in humidity. A 24" wide glued up surface may expand or contract over 1/2" with normal humidity changes in heated houses from winter to summer. When laying hardwood floors with boards that have crown figure, space must be left at the edges of the room for expansion or the floor will buckle: likewise, when building decks and hulls of wooden boats. If boards with initial high moisture content are used, spaces will develop between the boards as the wood dries.
Boards with figure can be maximized by cutting a log as indicated in the illustration to the right, producing boards with mostly tangentially cut surfaces (thanks to Richard Jones for sending this illustration). Some of the resulting would boards have bark on one edge while others will have both edges square to the face. The bark on one or both edges of a sawn board is called a live edge or wane. In a commercial mill, this edge is removed with an edging saw, essentially a large table saw, after the board is cut from the log. The resulting strip is discarded, recycled into other products (pellets or particle board) or burnt as firewood.
The center section of the log that is left in the illustration is the "dog board" and contains the pith or center of the tree. Invariably, the wood surrounding the pith will develop longitudinal splits (also called checking). The next time you are at the lumber yard, look at the building lumber, especially posts that are 4" X 4" and 6" X 6". Almost every piece is a dog board that contains the pith as will many of the 2" material such as 2" X 4" studs and other widths.
The outermost boards cut from a log, will have one broad face covered by bark. Such boards are called "slash" and are not used for lumber. In commercial operations, they may be sold for firewood or ground into chips to be used in making particle board or paper. At the Ledyard sawmill, we usually saw through and through, but on occasion will cut logs to maximize crown figure and to produce a dog board that might be used to replace a post in a building or to make a sign post.
Rift and Quarter Sawn Boards
Boards with straight grain on the broad surface are characterized as being either (1) quarter sawn or (2) rift sawn as explained below.
Rift and quarter sawn boards expand more in thickness rather than they do in width when exposed to moisture. Consequently, they are prized for flooring, wooden boat building, and other applications where lateral wood expansion and contraction could cause problems. While rift and quarter sawn boards lack figure on the face, the relatively straight grain allows easy matching of pieces when edge joining several boards to make a cabinet front or table top.
Quarter sawn boards
Also called radial sawn, straight or vertical grain boards, note the close almost straight grain on the face of the board to the left and the near vertical orientation of the annual rings in the end view. To a lumber grader, quarter sawn boards are generally defined as having an end grain orientation that is in the range 90 to 75 degrees relative the face of the board, although some suggest the definition should be a more relaxed 90 to 60 degrees.
Rift sawn boards
Also called bastard cut, the face of a rift sawn boards looks similar to quarter sawn ones, but the straight grain is more widely spaced as shown to the left. When viewed from the end, the annual growth rings are at more of an angle to the face than in quarter sawn boards. To a lumber grader, rift sawn boards have end grain that is 75 to 45 degrees to the face, although others suggest a less restrictive 60 to 30 degrees.
A log may be cut to maximize the straight, unfigured grain that characterizes quarter and rift sawn boards as shown in two illustrations prepared by Richard Jones. The first illustration shows the traditional method of quarter sawing. The log is cut into quarters and then each quarter may be sawn in either of two patterns. The second illustration below shows the modern method of sawing to maximize the width of vertical grained boards with less effort wasted in moving the log after each cut. A center "dog board" is left to be used for other applications. All boards will have wane on one edge. Both methods require reorienting the section of the log for each cut and yield narrow boards that might not be useful
Another sawing pattern that yields mostly rift sawn boards is billet sawing, as illustrated on the left by Richard Jones. The log is first cut in half longitudinally. Each half is then sawn through and through. Those boards produced by a cut that passes near the center of the half-log will be quarter sawn. Several on either side will be rift sawn with the outermost possibly having a mixed grain pattern of rift and tangential. All boards will have wane that would be trimmed off before use.
We do not use quarter sawing or billet sawing patterns at the Ledyard sawmill. However, it could be done.
How is Lumber Graded
Lumber is graded based on how many defects are in a board. These include knots, splits, insect damage, and amount of wood free of twist, cup and warp. Lumber grading rules are reasonably standardized, but no one set of rules is used for all species, for all regions of the USA, or by all countries. The links below will give you an introduction how American lumber is graded. Different rules apply to hardwood and softwood species. For more detailed information, use Google and search for hardwood (or softwood) lumber grading rules.
American Lumber Standards Committee
Northeast Lumber Manufacturer's Association Handbook. Primarily softwoods.
The Illustrated Guide to American Hardwood Grades. from American Hardwood Export Council. An excellent downloadable pdf guide with many colored photos illustrating how grading standards are applied to different cuts and species of hardwoods.
Authoritative source for detailed information about wood
Wood Handbook: Wood as an Engineering Material Centennial Edition, 2010 produced by the USDA Forest Service's Forest Products Laboratory in Madison, Wisconsin is an authoritative book about wood and its properties that is updated about once every ten years. More detailed explanations of several of the topics discussed on this webpage can be found in the corresponding chapters of the handbook.
This page prepared by Warren D. Dolphin