Abstract:
A technique for end-grain creation is employed for obtaining rapid and uniform drying of lumber while simultaneously reducing warp. The stability-kerfing responsible for the improved drying of the lumber decreases the edgewise bending strength by less than ten percent, a loss readily recovered due to the ability of stability-kerfing to achieve lower and more uniform moisture contents than those realized in the contemporary drying of lumber. 
     The improved moisture condition provided by the stability-kerfing also fosters future dimensional stability at the time of entry into the marketing stream compared to that for contemporary lumber. The required stability-kerfing is easily accomplished by the specialized implementation of existing saw equipment and associated technology into the contemporary processing lines.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to the lumber industry, and particularly to cutting and/or shaping of lumber as part of the drying process and to minimize warpage. 
   Dimension lumber is defined in the US as lumber with a nominal thickness of from 2 inches up to 4 inches and a nominal width of 2 inches or more. Most of such lumber is of nominal 2 inch thickness. In the U.S., softwood dimension lumber in excess of 19% average moisture content (“MC”) is defined as “unseasoned”. Framing lumber of nominal 2 inch thickness must not exceed 19% MC to be grade stamped “S-DRY.” S-DRY lumber is generally more dimensionally stable and stronger than unseasoned or green lumber and therefore commands a higher price, and significant cost and equipment has been used to attempt to rapidly and efficiently dry lumber to the S-DRY grade. 
   One of the primary factors hindering rapid and quality drying of softwood dimension lumber is the inherent lack of permeability of the wood. It is well accepted that moisture moves within the board parallel to the grain of the wood markedly easier than perpendicular to the grain. Moisture moving a given distance parallel to the grain encounters only a fraction of the cell wall substance encountered over the same distance perpendicular to the grain. It is stated in the literature that moisture travels about 15 to 20 times faster through end grain than side grain. For example, in an 8 foot long 2×4 board, the two ends quickly dry for some distance along the grain. In the remainder of the board, drying must occur by transmission of moisture through the side grain, i.e. perpendicular to board length. In a green 8 foot nominal 2×4 board, there is less than 13 in 2  of exposed end grain, but nearly 1100 in 2  of exposed side grain. Consequently, in spite of fast drying through the end grain, most of the overall drying must occur through side grain. 
   Most drying of nominal 2 inch thick dimension lumber occurs in a kiln to an average of 14 to 15% MC prior to being “surfaced four sides” (S4S) and then grade stamped. The resulting range in MC for the thousands of boards in a single kiln run is about 4% to 19%, or often higher than 19%. The pieces in the 4% to 8% range are over dried and thus have warped excessively, principally in the forms of crook, bow, and twist. With strict limits on the allowable amount of warp for a given grade of the lumber, the warp degrade translates into an immediate loss in value. The severe warp also adversely affects the ability to S4S the lumber. Pieces of higher MC, in the range of 13% to 19% or higher, can undergo post drying during storage and transport or in the context of structural incorporation. The post drying and associated warp fuels further economic loss and depreciates overall customer acceptance of the product. Drying to a lower average MC and narrower range in MC, while minimizing warp, should produce both higher economic return and customer satisfaction. 
   In the drying of contemporary lumber, essentially all moisture movement must take place perpendicular to the grain. This causes steep MC gradients within the boards that result in severe drying stresses. The increased drying stresses typically result in increased warpage. 
   Most of the dimension lumber produced is utilized for framing in which loading is perpendicular to a narrow edge. For softwood dimension lumber used as floor joists, rafters, door headers, etc. the major strength requirement is bending strength for loading perpendicular to the narrow edge. The use of wider pieces, e.g. the nominal 10 and 12 inch widths for floor joists, headers etc., has decreased rather dramatically over the past 2 or more decades. One factor contributing to the decreased use of wide dimension lumber is the harvesting of smaller trees. A second and equally important reason is the unreliable dimensional stability of the currently produced solid lumber. Recent commentary states that nearly 90 percent of floors for new homes in California use engineered I-Joists rather than solid lumber and then goes on to say that in a survey of U.S. building contractors lack of “straightness” was what made them least satisfied with solid lumber. 
   Bending strength is understood to be highly dependent on the moment of inertia, commonly designated as “I”. For a rectangular cross section, the I value is determined as:
 
 I=bd   3 /12
 
in which b=breadth and d=depth. For a seasoned, nominal S4S 2×12, the I value is:
 
 I= 1.5 inches×(11.25 inches) 3 /12=178 inch 4 
 
When used as a floor joist e.g. the stress in bending equals the bending moment times d/2 divided by the I value. The dominating effect of I value upon stress is quite apparent.
 
   The cross section of a selected engineered wood I-joist has the following dimensions: depth=11 inches, top and bottom flanges each 2.5 inches wide by 1.4 inches deep, and the web member of 3 layer plywood is 0.35 inches thick with a clear span depth of 8.2 inches. Its numerical I value is 178 inches 4 . As shown above, the numerical I value for a seasoned nominal 2×12 is 178 inches 4 . The engineered I-joist thus appears designed to replace the 2×12, doing so with only 60% of the cross sectional area of the 2×12. 
   Improved drying both within and between individual lumber pieces has been long desired. Some pretreatments, such as presteaming or prefreezing, have proved beneficial for certain species. However, these are difficult and expensive for incorporation into the contemporary production lines common for construction lumber. 
   SUMMARY OF THE INVENTION 
   The invention is a new and unique processing technique for framing lumber that significantly improves its drying while simultaneously enhancing its structural capability. The technique involves placing stability-kerfs perpendicular to the length of the green board, preferably on both wide faces, in a way that does not significantly alter the edge-wise bending strength of the board but so as to expose significant end grain throughout the length of the board, so that the majority of drying can substantially occur through the end-grain exposed by the stability-kerfs rather than nearly only through the side grain. The invention amplifies end grain contribution in a manner that greatly improves the drying behavior of the lumber while enhancing its future performance as a structural component. After drying, the lumber can be S4S, with the stability-kerfs visible after the S4S treatment. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of a nominal 2×4 board (prior to S4S) showing a preferred stability-kerfing profile of the present invention. 
       FIG. 2  is a cross-sectional view of the board of  FIG. 1  taken along lines  2 - 2 . 
       FIG. 3  is a cross-sectional view of the 2×4 board of  FIGS. 1 and 2  taken along lines  3 - 3 . 
       FIG. 4  is an end view of the board of  FIGS. 1-3  after S4S. 
       FIG. 5  is a perspective view depicting the method of the present invention. 
       FIG. 6  is a cross-sectional view similar to  FIG. 2  but of a 2×10 stud (after S4S) showing an alternative preferred stability-kerfing profile of the present invention. 
       FIG. 7  is a cross-sectional view of a second alternative preferred stability-kerfing profile. 
       FIG. 8  is an end view of a third alternative preferred stability-kerfing profile. 
       FIG. 9  is an elevational view of an alternative method of forming stability-kerfs of the present invention. 
       FIG. 10  is a graph of moisture content versus drying time for studs stability-kerfed in accordance with the preferred stability-kerfing profile of  FIGS. 1-4 , shown relative to standard 2×4 control boards. 
   

   While the above-identified figures set forth preferred embodiments, other embodiments of the present invention are also contemplated, some of which are noted in the discussion. In all cases, this disclosure presents the illustrated embodiments of the present invention by way of representation and not limitation. Numerous other minor modifications and embodiments can be devised by those skilled in the art which fall within the scope and spirit of the principles of this invention. 
   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIGS. 1-4  depict the present invention embodied in a 2×4 board  10 . The board  10  has a length l, a green thickness b g  and a green width d g . As depicted in  FIG. 1 , the board  10  has a length l which is ten or more times its green thickness b g . Various lengths of such framing lumber, e.g. 8′, 10′, 12′, etc. are marketed and used in construction. The board  10  depicted in  FIG. 1  is particularly shown such at a length l of about 100 inches, but the invention is equally applicable to all board lengths in which the length of the board is significantly greater than its thickness. As depicted in  FIGS. 1-3 , green width d g  and thickness b g  for the board  10  is about 3.75 inches and 1.65 inches respectively. This green thickness b g  and width d g  compensates for shrinkage during drying plus an allowance for the final S4S of  FIG. 4  to a final width d of 3.5 inches and a final thickness b of 1.5 inches, represented by the dashed outline in  FIGS. 1-3 . Stability-kerfs  12  are added along the wide faces  14  of the board  10 . 
   The spacing s between adjacent stability-kerfs  12  should be selected based upon the relative permeabilities of the board  10  along the grain versus across the grain. For a board  10  of 1.65 inches in thickness b g , the maximum cross-grain distance that moisture has to travel to dry the board  10  is about 0.82 inches. The stability-kerfs  12  should be spaced commensurately. For instance, if moisture in the type of wood (such as red pine) travels 15 to 20 times faster with the grain than across the grain, the stability kerfs  12  should be spaced no more than 30 to 40 times 0.82 inches, i.e., the maximum spacing s between adjacent stability-kerfs  12  should be less than 32.8 inches, so the longest distance moisture need travel with the grain to exit the board is 16.4 inches. Such a spacing ensures that moisture has generally has a quicker route of travel leaving the board  10  through the end grain exposed by the stability-kerf  12  than through the face  14  of the board  10 . In fact, the direction of moisture travel depends upon permeabilities in both directions (along grain versus across grain) and moisture level gradients in both directions at each location within the board  10 , and is thus not easily modeled. The intent of the stability-kerfs  12  is to expose as much end grain as possible for air flow and drying through the stability-kerfs  12  while not significantly reducing the strength of the board  10 . Because the stability-kerfs  12  do not extend all the way through the board  10  but rather expose only part of the end grain, spacing stability-kerfs  12  a distance significantly less than 32.8 inches apart provides significant drying advantages. A preferred value for the spacing s of the stability-kerfs  12  is in the range of 2 to 18 inches, with a more preferred spacing range being from 3 to 6 inches. For instance, adjacent stability-kerfs  12  can be longitudinally positions with a spacing s of about 6 inches from one another, so the greatest distance moisture need travel with the grain to exit the board  10  is 3 inches. 
   The width w of each stability-kerf  12  in the longitudinal direction of the board  10  need not be great. However, each stability-kerf  12  should be sufficiently wide to permit air flow within the stability-kerf  12  during the drying process, so moisture can be readily removed through the stability-kerf  12 . So long as moisture removal through the stability-kerf  12  occurs readily, the stability-kerf  12  should be as thin as possible in accordance with the method of forming the stability-kerf  12 . The preferred embodiment, the width w of each stability-kerf  12  in the longitudinal direction is the thickness of a saw-blade, about 1/10 of an inch. Using thin stability-kerfs  12  is helpful when the board  10  is used in construction, as the remainder of the board  10  provides a flat surface for nailing or screwing into, supporting overlying sheet material, etc. 
   The preferred stability-kerfs  12  are cut at intervals along each wide face  14 , with stability-kerfs  12  on one face  14  interposed mid-length to those on the opposite face  14 . For instance, with adjacent stability-kerfs  12  on one side  14  of the board  10  longitudinally spaced about 6 inches from one another, each stability-kerf  12  is spaced about 3 inches from the closest stability-kerfs  12  on the opposite face  14  of the board  10 . By offsetting stability-kerfs  12  on one side  14  of the board  10  from the stability-kerfs  12  on the opposite side  14  of the board  10 , the decrease in board strength caused by the stability-kerfs  12  is minimized. 
   To be effective, the stability-kerfs  12  must expose significant end grain for drying. For instance the stability-kerfs  12  should expose at least 10% of the end grain of the board  10 . The stability-kerfs  12  can be formed, for instance, by penetration of a circular saw blade (3⅝ inch diameter) to the maximum midpoint penetration p g  of ½ inch. This leaves a band of unpenetrated wood ⅝ inches thick and 1.65 inches wide along each narrow edge  16  of the board  10 , with this unpenetrated wood providing the majority of the strength of the board  10 . The length k g  of the exposed saw stability-kerf  12  on each wide face  14  of the green board  10  is thereby 2.5 inches. The area of the end grain exposed by each stability-kerf  12  of this size is about 0.86 sq. in., compared to the 6.19 sq. in. cross-sectional area of the green board  10 . That is, each stability-kerf  12  exposes about 14% of the end grain of the green board  10 , with the stability-kerfs  12  from both sides  14  exposing about 28% of the end grain of the board  10 . 
   The Wood Handbook provides a tabular summary for mechanical properties of commercially important woods. In the utilization of most framing lumber, the strength property of greatest concern is modulus of rupture (MOR) in edgewise static bending. The MOR is defined in psi, i.e. pounds of stress per inch 2 . The formula for determining the stress is: 
           S   =     MC   I           
where S=stress in psi, M=bending moment in inch-pounds, C=mid-depth in inches of the bending member and I=moment of inertia in inches to the 4th power, i.e. (inches) 4 . The moment of inertia I for a rectangular member in bending is determined as follows:
 
           I   =       bd   3     12           
where b=thickness of the member and d=depth of the member. The importance of depth (board width d) to the value of moment of inertia I is apparent from its being raised to the 3rd power. Thus, for a given load in edgewise bending, the larger the moment of inertia I, the lower the stress. To achieve the greatest drying benefit with the minimum loss in moment of inertia I, the stability-kerfs  12  should be positioned as much as possible in the center of the wide faces  14  and away from the narrow faces  16  of the board  10 .
 
   An analysis of moment of inertia can be done for the cross-sectional view of the stability-kerfed, dried S4S board  10   a  depicted in  FIG. 4 . For a standard (unkerfed), nominal 2×4 S4S board, I S =1.5(3.5) 3 /12=5.36 inches 4 . Even if both stability-kerfs  12  on opposite board faces  14  are aligned with each other (and thus stability kerfs  12  on both sides  14  subtract from the moment of inertia I), the stability-kerfed S4S board  10   a  shown in  FIG. 4  still has a moment of inertia I K =4.70 inches 4 . That is, the ratio of I K  to I S , in the preferred stability-kerfed S4S board  10   a  depicted in  FIG. 4  is about 0.88. 
   Stability-kerfing in accordance with the present invention can easily be added to the conventional processing line common to the production of lumber. One preferred kerfing device  18  is illustrated in  FIG. 5 . A long saw arbor  20  is fitted with a plurality of kerf sawblades  22  spaced at the selected interval s. The saw arbor  20  should be sufficiently long to extend over substantially the entire length l of the boards  10  being processed. For example, for stability-kerfing of 100 inch long boards  10 , the saw arbor  20  should extend over about 96 inches. A blade stiffener  24  is provided for each blade  22 , though the blade stiffeners  24  may alternatively be omitted if experience shows they are unnecessary. In the preferred processing line, the kerfing device  18  is added at a station immediately after the headrig. With the board  10  firmly held in straight configuration, the saw assembly  18  moves downward and the blades  22  penetrate the wide face  14  of the board  10  to a desired mid-point depth p of the stability-kerf  12 . The saw assembly then quickly retracts to an upward location while the board  10  is flipped 180° about its longitudinal axis for quick stability-kerfing of the opposite wide face  14 . If the stability-kerfs  12  are to be offset on the two wide faces  14  of the board  10 , then the board  10  when flipped should be moved longitudinally, such as the 3 inch offset. An alternative is to have two saw assemblies  18 , one for each wide face  14 . Simultaneous stability-kerfing of both wide faces  14  can be thereby accomplished without rotation or flipping of the board  10 . 
     FIGS. 6-8  show alternative embodiments of the present invention. In  FIG. 6 , the stability-kerfing is applied in a nominal 2×10 board  30  with a double-arbor arrangement and 5½ inch diameter blades. The two arbors are part of one assembly (not shown) that moves vertically similar to the single arbor arrangement  18  as described earlier with respect to  FIG. 5 . 
     FIG. 7  depicts stability-kerfs  42  in a profile as formed in a nominal 2×4 board  40  from use of circular sawblades of 1¼ inch diameter mounted on  2  parallel arbors incorporated into one assembly (not shown). The four near half-circle stability-kerfs  42  shown create an amount of end grain nearly identical to the stability-kerfs  12  shown in  FIG. 1 . The stability-kerfing of both wide faces  14  can be realized by having one two-arbor assembly (not shown) and flipping the board 180°, or having two assemblies (not shown), one for each wide face  14  of the board  40 . The stability-kerfing could also be formed by using a single arbor assembly  18 , applied four times (two for each wide face  14 ) to the board  40  at desired locations. If a two-arbor assembly is used, it is preferred that the blades on one arbor be located midway to the spacing of the blades on the second arbor on the assembly, so the stability-kerfs  42  on a single nominal 4 inch face  14  of the board  40  alternate between “high” and “low” when the board  40  is oriented as shown in  FIG. 7 . In the most preferred arrangement, only one stability-kerf  42  is positioned at any single longitudinal location on the board  40 , and thus  FIG. 7  depicts three of the stability-kerfs  42  hidden in dashed lines at the particular cross-section shown. 
   One alternative to circular sawblades  22  used to create the stability-kerfs  12 ,  32 ,  42  depicted in  FIGS. 1-7  is the use of saber sawing to create stability-kerfs  52  such as shown in  FIG. 8 . Saber sawing permits the formation of right angle corners  54  to the stability-kerfs  52 . A sequence of saber-type blades can be mounted in an assembly (not shown) whereby a single arbor actuates the sequence of blades in unison. The assembly is then powered to move perpendicular to board length l for the desired length k and depth p of the individual cuts  52 . An alternative to movement of the saw assembly is to move the board horizontally for the desired distance. If a right-angle  54  at each end of the kerf  52  is not desired, the extension of the saber saws can alternatively be controlled to produce a curvilinear penetration during both ingress and regress of the saber-type sawblades. 
     FIG. 8  particularly depicts a cross-sectional view of a stability-kerfed nominal 2×10 inch piece  50  of framing lumber, kerfed by saber-sawing, in its dried, S4S condition. The actual dimensions are 1.5 inches in thickness b by 9.25 inches in depth (width d). In the green, unseasoned condition the actual dimensions in thickness b g  and depth d g  were close to 1.65 inches and 9.75 inches respectively. After being dried to about 10% MC, the preferred stability-kerf profile produces stability-kerfs  52  with a length k of 5.45 inches long and a depth p of 0.4 inches, centered in alternating locations on opposing wide faces  14  of the board  50 . The moment of inertia I value for the solid cross section of the nominal 2×10 is 
             I   S     =           (     1.5   ″     )     ⁢       (     9.25   ″     )     3       12     =     98.9   ⁢           ⁢       inches   4     .               
The moment of inertia I value for the stability-kerfed cross section is obtained by subtracting from the 98.9 inches 4  the moment of inertia contribution or I value lost in the parts of the cross section penetrated by kerfing. The lost value is approximated as follows: The I value
 
           lost   =           (     0.4   ″     )     ⁢       (     5.45   ″     )     3       12     =     5.4   ⁢           ⁢       inches   4     .               
Thus, if the stability-kerfs  52  on opposing sides  14  of the board  50  are spaced sufficiently relative to the load that a rupture location only includes one stability-kerf  52 , the kerfed moment of inertia I K  value is 98.9 inches 4 −5.4 inches 4 =93.5 inches 4 . If the stability-kerfs  52  on opposing sides of the board  50  are close enough together that the rupture location includes both stability-kerfs  52 , then a smaller moment of inertia I is appropriate. The worst case scenario is to model the stability-kerfs  52  on opposing sides  14  of the board  50  as being aligned at the same longitudinal location, so the board strength matches that of a milled, wooden I beam. In this case, the kerfed moment of inertia I K  value is: I K =98.9 inches 4 −10.8 inches 4 =88.1 inches 4 . The worst-case ratio of I K  to
 
             I   S     =         88.1   ⁢           ⁢     inches   4         98.9   ⁢           ⁢     inches   4         =     0.89   .             
Thus the stability-kerfed 2×10, if for example used as a floor joist, should have 89 percent the bending strength of what it would have unkerfed. However, the strength values for wood increase with decreasing MC, which can cause the stability-kerfed 2×10 to have a higher bending strength than that calculated by merely comparing moments of inertia I.
 
   The present invention can be equally applied to other dimensions of boards. For a nominal 2×12 member the actual dry S4S dimensions are 1.5 inches thick (b) by 11.25 inches wide (d). If the 2×12 were routed on each wide face  14  in rectangular manner, leaving flanges 1.5 inches wide by 2.5 inches deep and a web 0.5 inches thick, the numerical I value for the cross section is 178−20.2˜158 inches 4 . This is nearly 90% of that for the solid 2×12 and the engineered I-joist. With a rectangular shaped kerf (preferably produced by saber-sawing, though it could also be obtained by routing), and at a kerf depth p of 0.4 inches and a kerf length l of 6.75 inches in the S4S board, the ratio of I K  to I S  for the nominal 2×12 is 0.90. Thus, to attain an I K  to I S  ratio in the dried lumber of about 0.90, the preferred depth p g  of each kerf should approximate 25 to 30% of the green thickness b g  with the preferred length k g  equal to 60 to 65% of green board width d g . Using roughly these percentages, and making the comparison at equal MC&#39;s, will result in a framing member with essentially 90% of the edgewise bending strength it would have as a solid cross section framing member. Wood is anisotropic and comes in different species, and the most-preferred kerf dimensions should be selected as appropriate for particular samples and species of boards. 
   While the 90% I K  to I S  ratio is appropriate for analyzing boards in edgewise bending, the manner of use of the kerfed board is not limited to edgewise bending. Many 2×4&#39;s are used in framing lumber either in vertical arrangements (typically supporting a compressive load like a column), or in horizontal arrangements wherein the wide face is oriented horizontally. The preferred 2×4 of  FIGS. 1-4  is equally appropriate for such uses. Due to the increased straightness and dryness of the boards, kerfed 2×4s may be less likely to fail than unkerfed 2×4s even in such vertical and horizontal loading arrangements. If it is known that a board will be loaded in facewise bending, stability kerfs may be placed upon the narrow faces of the board rather than on the wide faces of the board. Another example is with lumber such as nominal 4×4s and 6×6s, which can be very difficult to dry without inducing warpage. For such square boards, the kerfs can be placed upon two opposing faces, or can be placed in all of the four faces of the boards. 
   As an alternative to either circular or saber sawing, the stability-kerfs of the present invention can be formed by a roller incisor  60  as depicted in  FIG. 9 . Two steel rollers  62  have three high strength tapered blades  64  mounted parallel to the roller length. The rim speed of the rollers  62  is synchronized with the in-line speed of the advancing board  10 , so the incisor blades  64  experience primarily resistance to board penetration and not a severe bending moment. The blades  64  make incisions at the selected interval s perpendicular to the grain on the respective wide faces  14  of the board  10 . For instance, for nominal 2×4 boards the blades can be 2 inches in length (k) and ½ inch in depth (p). The blades  64  make incisions centered on the wide faces  14  of the 2×4 board  10 , leaving a non-incised band on the narrow edges of the board  10  which is 0.85 inches wide. This kerfing profile again provides an I K  to I S  of approximately 0.90. 
   An alternative to a roller incisor is a pressure incisor (not shown) similar in design to that for saw kerfing of  FIG. 5 . The saw arbor is replaced by a non-deformable strip of steel having incisor blades of the desired length k, depth p and spacing s, such as 2 inches in length ½ inch in depth and at 3 inch spacing. With the freshly sawn board held in place in a straight configuration, the incising “ram” or press thrusts downward to cut the stability kerfs. If a single ram is employed, the board is flipped to receive stability-kerf incisions on the opposite wide face  14 . More preferably, the board is pressed between opposing rams to incise both wide faces simultaneously, which facilitates removal of the board from the press. Both the roller incisor and the pressure incisor can be properly modified to accommodate boards of any standard length l or width d. 
   Table 1 is copied from the Wood Handbook: Wood as an engineering material, Agric. Handbook. 72. USDA 1987. 
                                                         TABLE 1                   Approximate middle trend effects of moisture content on       mechanical properties of clear wood at about 20° C.                Relative change in property from           12 percent moisture content                At 6 percent   At 20 percent       Property   moisture content   moisture content                    Modulus of elasticity   +9   −13       parallel to the grain       Modulus of elasticity   +20   −23       perpendicular to the grain       Shear modulus   +20   −20       Bending strength   +30   −25       Tensile strength parallel   +8   −15       to the grain       Compressive strength   +35   −35       parallel to the grain       Shear strength parallel   +18   −18       to the grain       Tensile strength   +12   −20       perpendicular to the grain       Compressive strength   +30   −30       perpendicular to the grain at       the proportional limit                    
Table 1 gives the approximate effects of MC on the mechanical properties of clear wood at a temperature of 20° C. Strength values are normally obtained at a wood MC of 12% and a wood temperature of 20° C. The Wood Handbook table gives the relative change for each property in going from 12% MC down to 6% (strength increase) and for a change from 12% to 20% MC (strength decrease). Of immediate interest are the relative changes for bending strength. The approximate increase in strength for each percent decrease in MC is 5 percent. The approximate decrease in strength for each percent increase in MC is more than three percent.
 
   The Southern Yellow Pine (SYP) species as a group are a large contributor to the production of framing lumber. The Wood Handbook gives the modulus of rupture (“MOR”) at 12% MC for Longleaf Pine as 14,500 psi. In contemporary processing, SYP species are commonly kiln dried to an average MC of 15%. Thus its average MOR entering the market chain at 15% MC is 14,500 psi minus the strength loss due to having a MC of 15% rather than 12%. The loss calculates to 1359 psi. The 14,500 psi, minus 1359 psi, results in a MOR value of 13,141 psi. For those pieces at the upper end of the MC distribution, a MC of 19% or even greater, the loss in strength due to the additional MC is truly significant. At 19% MC the bending strength is reduced to 11,328 psi. On the other hand, if the drying were to a 10% average MC, the bending strength is 14,500 psi plus 906 psi which equals 15,406 psi. The ability to efficiently dry to lower and more uniform MC&#39;s with stability-kerfing more than compensates for the approximate ten percent loss in bending strength resulting from decrease in moment of inertia. 
   EXAMPLE 1 
   Forty red pine boards, 20 controls and 20 stability-kerfed as depicted in  FIGS. 1-4 , were dried as one charge in a steam heated experimental lumber dry kiln. Sixteen of the full length boards, 8 stability-kerfed and 8 controls, (all boards≅100 inches long) served as sample boards to be weighed periodically during the kiln run. The dry bulb temperature was maintained at 192° F. throughout the kiln run while the wet bulb temperature tracked at about 173° F. 
     FIG. 10  compares drying rates for stability-kerfed and controls. Accelerated drying due to stability-kerfing is readily apparent. Stability-kerfed boards, even though higher in initial average MC, reached 10% MC in about 23 hours while for the controls this required over 41 hours. This stability-kerfing design created a 45% reduction in the time required for reaching a highly desired level of final MC. The 10% average MC is in good agreement with the equilibrium moisture content (EMC) the lumber will seek during subsequent storage, transportation, marketing and final end-use structural applications. At 10% average MC the range in MC for the 8 stability-kerfed boards was 7.6% to 11.8% while for controls at their 10% average it was 7.9% to 11.5%. The similarity in range shows that the 45% faster drying did not unfavorably increase the range in MC. 
   Table 2 below summarizes warp data for the 40 boards, comparing warp values of boards stability-kerfed in accordance with the preferred stability-kerfing profile of  FIGS. 1-4  relative to standard 2×4 control boards. Each warp form was measured to the nearest 1/32 inch. 
   
     
       
             
           
             
             
             
           
             
           
             
             
             
           
             
           
             
             
             
           
         
             
               TABLE 2 
             
           
           
             
                 
             
             
               Warp Comparisons - Controls vs. Kerfed - No Restraint 
             
           
        
         
             
                 
               Controls - Avg. MC 8.8% 
               Kerfed - Avg. MC 7.9% 
             
             
                 
                 
             
           
        
         
             
               Number Of Boards Meeting Stud Grade 
             
           
        
         
             
               Crook 
               10 (50%)  
               17 (85%)  
             
             
               Bow 
               20 (100%) 
               20 (100%) 
             
             
               Twist 
               4 (20%) 
               3 (15%) 
             
           
        
         
             
               Average Amount of Warp 
             
           
        
         
             
               Crook 
               0.27 in. 
               0.11 in. 
             
             
               Bow 
               0.15 in. 
               0.06 in. 
             
             
               Twist 
               0.59 in. 
               0.65 in. 
             
             
                 
             
           
        
       
     
   
   The average absolute amounts of crook and bow for the stability-kerfed boards were less than half of those for the controls, even though the stability-kerfed had a lower average MC of 7.9% compared to 8.8% for controls. With respect to meeting stud grade, using crook as the criterion, only 10 of the 20 controls made stud grade while for the stability-kerfed  17  made grade. With bow as the criterion, all 20 of each met grade. Due to the high allowance of the grading rule for bow, all controls made grade in spite of having over twice the average amount of bow as that for stability-kerfed. For twist, the absolute amount for both stability-kerfed and controls was very high and the grade recovery for each was very low. In a small kiln charge of only 40 boards there is a negligible dead weight of lumber to restrain warp. In this experimental drying with the near absence of restraint, stability-kerfing produced more than a two-fold reduction in absolute crook and bow but had no benefits for twist. In a commercial kiln charge twist would be greatly reduced for both stability-kerfed and controls due to dead-weight loading. 
   Table 3 summarizes the strength-testing data obtained for the 20 stability-kerfed and 20 unkerfed red pine boards. 
                                                                             TABLE 3               Strength And Moisture Data Obtained In The Determination Of Bending Strength In Edgewise Centerpoint       Loading Of Nominal 2 × 4 Kerfed And Unkerfed Boards At A Clear Span Of 82 Inches                   Strength Data in Edgewise Bending                No. of   Average Peak   Range of Peak Loads   Avg. Extension at   Average           Studs   Load lb. of Force   lb. of Force   Peak Load inches   MOE psi               Kerfed   20   709   1295-143   1.612   949,170       Controls   20   745   1228-409   2.161   823,277                    Moisture Content* at Time of Strength Testing                No. of   Average MC of   Range of Average %   Range of % MC Values   Range of % MC Values           Studs   Boards - values in %   MC values   Obtained for Shells   Obtained for Cores               Kerfed   20   9.7   9.2-10.9   8.2-9.5    9.2-10.6       Controls   20   10.2   9.6-10.9   9.0-11.6   9.5-11.7               *Calculated as a percentage of the constant weight obtained at a drying temperature of 220° F.            
The average breaking force for edgewise bending in pounds of force was 709 for the stability-kerfed boards and 745 for the controls. The ratio of stability-kerfed to controls is 0.95, considerably higher than the 0.88 “worst-case scenario” value estimated earlier. The elevated value likely arises for two reasons. The first is that in making the estimate the kerfed regions were treated as rectangles while in reality the actual kerfs left wood that contributed to the moment of inertia I value. Secondly, as Table 3 shows, the average MC for the stability-kerfed at time of strength testing was lower than that for the controls and this also contributed to higher strength. The lower and more uniform MC for kerfed also translated into a 15% higher modulus of elasticity for kerfed than for controls. The greater stiffness is well evidenced by the average extension at peak load for kerfed being only 75% of that for controls.
 
   The present invention thus attains the following results:
         1. The use of end grain creation via stability-kerfing in green dimension lumber to greatly accelerate its drying to the desired low and uniform moisture content while simultaneously reducing the warp that commonly accompanies the drying.   2. The created end grain diminishes just slightly the moment of inertia and thus the lumber retains its ability for use as structural lumber with no inhibition to nail, screw or adhesive use.   3. The slight reduction in strength due to the stability-kerfing is more than recaptured due to the ease in achieving a lower and more uniform final moisture content than that attained in contemporary commercial practice.   4. The unique use of stability-kerfing for end grain creation will greatly enhance the treatability of lumber with preservatives and the post-treatment removal of the vehicle employed.   5. Recognition of a variety of stability-kerfing designs that can reduce the drying time for green lumber to the final desired moisture condition to one-half of that required for comparable unkerfed lumber.   6. Innovative design of sawing equipment for quick and efficient stability-kerfing of lumber.   7. The use of end grain creation in green dimension lumber to reduce drying time, energy requirements and warp for large batches of lumber such as in a kiln.   8. The creation of a technique which when incorporated into the drying process for green lumber produces a dimensionally stable product free of significant distortion during subsequent storage, marketing and structural applications.       

   The stability-kerfing technique of the present invention thus increases the contribution of end-grain drying and greatly reduces drying time and also improves uniformity of final MC within and between pieces, and thereby improves the overall recovery and grade of dried lumber from a given input of logs. 
   Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.