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Chapter 4. Lumber
Chapter 4. LumberNominal and Manufactured Lumber Sizes To avoid confusion in interpreting information in this chapter, terminology must be clarified regarding the dimensions (thickness, width, and length) of lumber. The term nominal is used for dimensions as customarily communicated in commerce, such as referring to a piece of softwood lumber as a 2x4. The term manufactured refers to the dimensions in product specifications that correspond to nominal dimensions. Manufactured dimensions depend on the state of manufacture; a surfaced-dry softwood 2x4 is required by the American Softwood Lumber Standard (USDC 1970) to be a minimum of 1.5 inches by 3.5 inches. Actual dimensions are obtained by measuring individual pieces with calipers. Actual dimensions will differ slightly from the manufactured dimensions required in specifications due to minor variations in drying shrinkage and milling accuracy. Information and examples in this chapter are based on nominal and manufactured dimensions. A mill could substitute its own actual dimensions for the manufactured dimensions required by product specifications.As discussed in Chapter 2, the manufactured sizes of North American lumber are generally different from the nominal sizes. Exceptions are special orders and some lumber exports, particularly those manufactured to metric specifications. The board foot measure of lumber volume is, however, based on the nominal size. Consequently there can be a significant discrepancy between the cubic volume of lumber implied by board foot measure and the true cubic volume of wood present in the corresponding manufactured size. In many other parts of the world lumber is measured in metric units, with negligible difference between the nominal and manufactured size. Softwood lumber is marketed by species, grade, and form of manufacture. For many products, the American Softwood Lumber Standard is a basic reference. Softwood lumber can be viewed as falling into two broad categories of use: construction lumber and lumber for remanufacture.
Most hardwood lumber is produced for remanufacture into products such as furniture and cabinets or is made directly into flooring, paneling, millwork, and so forth. The three principal market categories are factory lumber, dimension parts, and finished market products. In addition, several hardwood species can be marketed under the procedures outlined in the American Softwood Lumber Standard; currently the volume produced is quite small. This section focuses on factory lumber, which dominates hardwood production. Details on sizes required for other hardwood products are contained in the grading rules obtainable from the National Hardwood Lumber Association (NHLA 1987). Table 4-2. Standard nominal and dressed sizes for Factory and Shop lumber under Western Wood Products Association rules.
Source: WWPA (1981) and USDC (1970); metric values added by author. a4/4 is generally produced S4S in widths the same as listed for dimension in Table 4-1. All other thicknesses are generally produced S2S in random width; the average width is the nominal width plus 1/8 inch. For example, S2S 6-inch wide 8/4 actually ranges from 5.5 to 6.5 inches wide; the average width is 6.125 inches. bThe surfaced-dry thicknesses are based on the American Lumber Standard (ALS) thicknesses; in grading pieces under this rule, planer skip is not allowed. However, industry practice is to produce 5/4 and 6/4 stock as "heavy." Heavy means that these items are produced to be full-sawn (manufactured size = nominal size) when surfaced but grading this stock allows planer skip. cS4S = surfaced on all four sides. S2S = surfaced on two sides (faces). dLength in meters begins at 1.3 m and increases in 0.3 m multiples.
Table 4-3. Nominal and manufactured thicknesses of hardwood lumber.
Source: NHLA (1987). The manufactured sizes are taken from Wengert (1988). Equivalents in mm can be found by multiplying by 25.4 and rounding to the nearest whole number. aFinished size is not specified in the rules; it is subject to contract conditons. Factory Lumber Thickness. The nominal thickness of a piece of hardwood factory lumber is the specified thickness in the NHLA rules for the rough-dry state of manufacture. The standard for surfaced-dry (S2S) manufacture is also specified in the rules. The manufactured thickness for green lumber is not specified in the rules but is generally specified in contracts. For example, 2 inch thick hardwood lumber, also known as 8/4 (eight-quarter), would exceed 2 inches in manufactured green thickness. The manufactured dimension required of rough lumber expected to meet a nominal size also is commonly specified in contracts. Table 4-3 shows the standard nominal thickness of hardwood lumber in both inches and quarters along with examples of manufactured thickness (Wengert 1988) and thickness when surfaced two sides (S2S). Since hardwood factory lumber is intended for remanufacture into cuttings for furniture, thickness is always measured as the thinnest point in the cutting used for grading the board. Therefore, the thickness cannot be measured until the board is graded, since one must know where the cuttings are located in establishing grade in order to get the required thickness measure. Portions of the board not in the required cuttings may be scant in thickness. When thickness varies in a board, the thickest and thinnest locations within the grade cuttings are measured. If the difference exceeds the tolerance shown in Table 4-3, the piece is labeled miscut. But this labeling does not change the grade.Factory Lumber Width and Length. Hardwood factory lumber is not usually manufactured to standard widths, hence a piece 6-3/4 inches wide is left in that form. This is why hardwood lumber is often referred to as random width. While grades do not specify standard widths, there is a minimum width associated with each grade. When a board is tapered, width is measured one-third of the length from the narrow end. Length of hardwood lumber generally ranges from 4 to 16 feet in one-foot steps. However, standards do not permit more than 50% of pieces to be odd lengths. In measuring length, any fraction of a foot is dropped. Board Foot Lumber Volume In North America, the standard unit measure of lumber volume is the board foot (BF) calculated from the nominal dimensions. A board foot is a hypothetical piece that is 1 inch thick, 12 inches wide, and 12 inches long. Theoretically, this implies that a board foot represents 144 cubic inches of wood, that there are 12 BF per cubic foot (hence 424 BF per cubic meter), and that 1,000 BF equals 83.33 cubic feet or 2.36 cubic meters.Although these conversion factors are commonly used by statistical reporting agencies, this chapter will show that because of the difference between nominal and manufactured sizes, an assumption of 12 BF/CF may not be a correct conversion factor to use. The procedures for calculating board foot lumber volume in this chapter measure lumber as it is tallied by the sawmill. This should not be confused with the board foot when used for log scaling (Chapter 2). Many countries using the metric system saw lumber with negligible difference between nominal and manufactured size and therefore use the nominal dimensions to determine volume in cubic meters. Metric calculations and conversions are discussed below (p. 65). Softwood Board Measure. To find the BF measure of a softwood board, multiply its nominal thickness (Tn) in inches by its nominal width (Wn) in inches by its length (L) in feet, then divide by 12:
Source: Calculated
by the author. Softwood Lineal Measure. Lineal footage (Lf) is the total length in feet of a piece or all pieces in a shipment and is used largely for specialty items such as moldings: Lm = Lf / 3.23. Hardwood Board Measure. The standard measurement of volume for hardwood factory lumber is the board foot, and the same basic formula is used as for softwood lumber. In practice, there are important differences in how board foot volume is measured. If the width (W) is measured in feet rather than inches, the board foot formula becomesBF = (L * W) * T. The value in parentheses is the board's surface measure (SM), in square feet, which is rounded to the nearest whole number. If the fraction before rounding is 1/2, SM is alternately rounded up or down. In calculating board foot volume, the thickness used is always the standard nominal thickness in quarters. It is important to note that pieces thinner than one inch are considered to be one inch (4/4) for the purpose of measuring board foot volume. See Example 1.Table 4-5, column 1, presents BF volume per lineal foot for a selection of hardwood lumber thicknesses. In these calculations the width used was the midpoint of each random width range (i.e., random width 6-inch lumber represents actual widths from 5.5 to 6.4 inches, inclusive). Table 4-5. Volumes perineal foot for hardwood factory lumber.
Source: Calculated by the author. S2S-Dry
= dried and surfaced
on two sides.
SM = (8
* 6.5) / 12 = 4.33 =
4
BF =
4 * 5/4 =
5 2. An 8/4 piece is 10' 9" long and 8-1/4" wide
SM = (10
* 8.25) / 12 =
6.875 = 7
BF =
7 * 8/4 =
14 3. A 1/2 inch piece is 7' 2" long and 5-3/4" wide
SM = (7
* 5.75) / 12 = 3.35 =
3
BF =
3 * 4/4 =
3 Hardwood Surface Measure. In actual practice, board foot volume of individual pieces often is not calculated separately and summed. Instead, surface measure (SM) of all pieces of a given thickness is accumulated and the total SM is multiplied by the nominal thickness. Thus a stack of 8/4 No. 2 Common maple lumber might have a total SM of 350. The board foot volume of the stack is 350 * 8/4 = 700 BF. Hardwood Lineal Measure. As for softwoods, lineal footage measure may be used for some specialty hardwood items.
Using Sizes in Commercial Standards. The cubic foot volume of a piece of lumber, in terms of the true amount of wood present, is obtained by multiplying the manufactured or actual sizes, converting from inches to feet as needed. The following discussion is based on using manufactured dimensions. Table 4-1 shows that the manufactured dimensions of a surfaced-dry 2x4 are 1.5 by 3.5. The cubic volume of a surfaced-dry 8 foot 2x4 is
0.0365 * 8 = 0.292 ft3. The cubic foot
volume of a piece of hardwood lumber is similarly obtained.
Since hardwood factory lumber is produced random width, there
is no single value of the cubic foot content of a nominally
sized board. For example, a nominal 6-inch wide piece can range
from 5.5 to 6.4 inches in actual width. With a 1-inch rough
thickness, the cubic foot content per lineal foot will range
from 0.0310 to 0.0367, a change of 18.4%; the midpoint of this
range, corresponding to 6 inches, is 0.0339. Table 4-5 presents
such midpoint cubic foot contents per lineal foot for rough-dry,
surfaced-dry (S2S), and rough-green hardwood factory lumber.
T = (F + P) / (1 - S/100) + SV * Z T = green target thickness or width, in inches F = final thickness or width required, in inches P = planing allowance, in inches S = shrinkage percent from green to final MCod (see Chapter 1 for definitions). Tangential shrinkage values are often used, to be conservative. SV = sawing variation of the particular machine, obtained from statistical quality control procedures Z = an undersize or skip allowance factor based on the normal curve. With a 5% undersize allowed, this factor is 1.65. Contracts specify the maximum percentage of pieces allowed to be undersize. The following average and best sawkerf width, sawing variation, and planing allowance are based on Sawmill Improvement Program (SIP) studies of softwood dimension mills (USFS 1980):
Williston (1988, chap. 23) presents more detailed information for various types of headrigs and edgers. Machine type and piece size (depth of cut) are more influential on these items than type of mill per se. Column
6 of Table 4-4 and column 5 of Table 4-5 show cubic foot per lineal
foot values obtained by applying a surfacing allowance of 0.100
inch, sawing variation of 0.160 inch, and a 5% change in dimension
during drying. This represents the amount of wood fiber initially
required in each size in order to meet the surfaced-dry specifications.
Calculations for meeting any other state of manufacture are similarly
made.
Table 4-6.BFFR for softwood construction lumber.
Source: Calculated by the author. Note: Reciprocal of BFFR is the number of cubic feet of actual wood contained in a board foot of lumber of the indicated size and state of manufacture. These values are often multiplied by 1,000 to indicate the actual cubic feet present in one MBF of lumber. Thus 1 / 18.29 * 1,000 = 54.67 ft3/MBF of surfaced-dry 2x4s.
Board Foot to Cubic Foot Ratios (BFFR) Previously, it was found that a surfaced-dry 2x4 contains 5.33 BF and 0.292 cubic foot. Dividing yields 18.25 BF/ft3 for this state of manufacture. A value of 18.26 BF/ft3 is obtained by dividing 0.6667 BF per lineal foot by 0.0365 cubic foot per lineal foot. Tables 4-6 and 4-7 present the board foot fiber ratio (BFFR) for various states of manufacture. Values presented in Tables 4-4 to 4-7 were calculated on a computer and differ slightly from the examples in the text, due to rounding. Notice that for softwoods, these ratios are much higher than the theoretical 12 BF/ft3 based on nominal sizes. This theoretical value would be correct only for lumber manufactured so that there is no difference between nominal and manufactured sizes. For hardwoods, the BFFR is unusually high for thicknesses less than one inch because of the practice of considering these to be one inch thick for purposes of calculating board foot volume. Also, the BFFR for hardwood sizes one inch thick and larger is typically smaller than those for softwood counterparts, since hardwood standards require lumber to be cut thicker for a given nominal size. It should be noted that BFFR is not constant, does not change consistently across the range of sizes, and is dependent on the degree of manufacture. BFFR values for any state of manufacture or for green target sizes of a specific mill are similarly calculated. It should also be apparent that BFFRs would be different for the Shop lumber sizes shown in Table 4-2. The USFS timber assessment uses BFFR of 16.67 and 12.00 for softwood and hardwood lumber, respectively (Appendix 2). The reader should carefully note the comments at the beginning of Appendix 2. The BFFR, which measures the true amount of wood fiber contained in a given board footage, can also be used as a measure of mill recovery efficiency. If a mill tightens up its sawing variation and planing allowance, a higher BFFR can be realized, since it can cut smaller green target sizes (less cubic volume) for the same nominal board foot size. This is illustrated in Table 4-8 for 2x4s assuming the surfaced green dimensions in Table 4-1. As a mill tightens its process, the BFFR increases, thus reflecting the smaller quantity of wood fiber needed to make the same piece. As the S-Dry 2x4 example in Table 4-9 shows, in addition to not being constant for a given cross section and state of manufacture, BFFR may vary with length. This will occur when board foot volumes are rounded to the nearest whole number. While the BFFR, using the rounded BF, vary between lengths, the average is the same result obtained for each length when the BF rounding rules are ignored. Individual mills may wish to calculate BFFR with the rounding rules in order to examine trade-offs between lengths. For simplicity, this effect is ignored in Tables 4-6 and 4-7. This is not considered a major problem, since statistical data are often aggregated across lengths, and thus the average factor is appropriate. The reader may note a small difference between the values calculated in Table 4-9 and values in the text and Table 4-6. This is because the cubic foot per lineal foot factor of 0.0365 was carried to more places during computer calculations in Table 4-6. Table 4-7. BFFR for hardwood factory lumber.
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