ANALYSIS OF THE SAWMILLING PRACTICES IN THE STATE OF DURANGO, MEXICO. DAVID ZAVALA ZAVALA B.Sc. Escuela Nacional de Agricultura Chapingo, Mexico, 1972

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1 ANALYSIS OF THE SAWMILLING PRACTICES IN THE STATE OF DURANGO, MEXICO by DAVID ZAVALA ZAVALA B.Sc. Escuela Nacional de Agricultura Chapingo, Mexico, 1972 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in THE FACULTY OF GRADUATE STUDIES Department of Forestry We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA April 1981 David Zavala Zavala, 1981

2 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of PAXtTs T 2y The University of British Columbia 2075 Wesbrook Place Vancouver, Canada V6T 1W5 Date April /?f/ DE-6 (2/79)

3 i ABSTRACT Control of sawmilling operations, including log bucking practices and sawing processes, is one reasonable solution to the Mexican lumber shortage problem. This is particularly so if available techniques to improve sawmill efficiency can be included in normal manufacturing processes. Sawmilling analyses were carried out to assess the relationship of log volume input to lumber volume recovery, evaluated in actual and nominal dimensions. Six sawmills were selected, based on the most frequent type of band headrig in the State of Durango. A statistically representative sample size of sawlogs was used in each sawmill, amounting to a total of 870 logs. The proportion of log volume breakdown into lumber and byproduct volumes was analyzed. It was found that the proportion of chippable residue accounted for 26 per cent of the total log volume throughput. This suggests the possibility of allocating a large amount of this volume to pulp mills, rather than continuing present practice of burning as waste with no economic return. Sawlog types and lumber recovery characteristics under normal manufacturing processes were included in the study. A significant difference of per cent was found between the two expressions of lumber recovery percentages based on actual and on nominal dimensions. Major emphasis, however, was given to the analysis of log bucking practices, lumber dimensions, and sawing variation, in respect to their

4 i i effect on both potential lumber recovery percentage and potential revenue to the sawmill industry. It was found that excessive log trim allowance resulted in a 4.34 per cent wastage of the total log volume input at the trim saw. Over allowance in lumber thickness dimension resulted in a 3.55 per cent loss of the total lumber volume recovered, and sawing variation accounted for a 2.76 per cent loss. It was concluded that closer control of sawmilling operations to minimize poor bucking practices and sawing variation has significant potential for lumber recovery increment. It was also concluded from this study, that future sawmill analysis would require inclusion of log length and sawing variation in an assessment of sawmill performance.

5 i i i TABLE OF CONTENTS Page ABSTRACT TABLE OF CONTENTS LIST OF TABLES LIST OF FIGURES ACKNOWLEDGEMENTS i i i i vi viii ix 1. INTRODUCTION 1 2. LITERATURE SURVEY Lumber Recovery Definition Log Characteristics Sweepy Logs Taper Logs Infested Logs Logs from Dead Trees Cull Logs Log Diameter Type of Sawmill Cutting Patterns Log Scaling Log Sample Size Log-Lumber and Byproduct Proportion Volume and Weight Method Saw Kerf and Cutting Patterns Method 21

6 iv Page 2.3 Quality Control Log Length Allowance Lumber Dimension Allowance MATERIALS AND METHODS Lumber Recovery Sawmill Selection Log Sample Size Log Scaling Log and Lumber Grading Sawmilling Procedure Lumber Yield Evaluation Log-Lumber and Byproduct Proportion Quality Control Log-Length Allowance Lumber Dimension Allowance Board Thickness Selection Sample Size and Board Measurements Board Sawing. Variation Target Size Determination RESULTS AND DISCUSSIONS Raw Material Characteristics Raw Material Log Sample Size 43

7 V Page 4.2 Characteristics of Lumber Recovery Lumber Thickness Lumber Grades Log Volume and Lumber Volume Relationship Lumber Recovery Percentage Lumber Volume Allowance Log-Lumber and Byproduct Proportion Quality Control Log Length Sawing Variation Potential Lumber Recovery Increment by Cutting to Target Thickness Potential Lumber Recovery Increment by Reducing Sawing Variation Economic Analysis CONCLUSIONS AND RECOMMENDATIONS Conclusions Recommendations 57 BIBLIOGRAPHY 58 TABLES 67 FIGURES 87

8 vi LIST OF TABLES Table Page 1 Proportion of lumber, chippable residue and sawdust by log diameter range and sawmill type (18, 29, 35, 37, 39, 41) 67 2 Summary of references in log-lumber and byproduct relationship 68 3 Type and number of sawmills in the State of Durango Log grades 70 5 Sawlog distribution by length and diameter categories 71 6 Number of logs and volumes by grading classes 72 7 Number of logs sampled to give 95 per cent confidence interval about the population mean 73 8 Volume and percentage of pieces of lumber recovered by thickness classes 74 9 Lumber volume recovery by grade categories (based on log volume) Distribution of lumber yield by nominal and actual dimension of thickness, width, length and volume Allowance and over-allowance in volume for different board thickness Nominal, actual, and allowance volumes related to lumber recovery based on actual and nominal dimension Log-lumber and byproducts proportions Over-length allowance volume by log length categories Log volume lost by over length with 4 and 6 inches nominal allowance Thickness variation distribution for nominal 3/4-inch boards Mean thickness - sawing variation and target thickness for nominal 3/4 inch boards 84

9 vii Table Page 18 Potential lumber volume recovery in 3/4 inch nominal lumber by cutting to target thickness and by reducing sawing variation Potential lumber recovery in different nominal thickness by cutting to target thickness and by reducing sawing variation 86

10 viii LIST OF FIGURES Page 1 Distribution of logs by diameter classes 87 2 Distribution of logs by length classes 88 3 Distribution of logs by grades and volumes 89 4 Proportion of different lumber thicknesses produced under nominal manufacturing conditions 90 5 Thickness variation in 3/4 inch nominal lumber 91

11 ix ACKNOWLEDGEMENT I am greatly in debt to the Consejo Nacional de Ciencia y Tecnologia (CONACYT) for the scholarship granted during the study period at the University of British Columbia. Gratefully acknowledged is the support of the Instituto Nacional de Investigaciones Forestales (INIF), which allowed working time to collect and analyze the data used in this thesis. Special thanks are due to Dr. N.C. Franz for directing the write-up, for his constant assistance and encouragement during the most difficult part of my task. Thanks are also due to Dr. R.W. Kennedy for his valuable comments and to Dr. L. Paszner for his advice and comments. Appreciated also is the help of Mr. J. Rocha in collecting and analyzing the data. Special thanks are due to Tom and Dobriela, for their constant encouragement.

12 1 1. INTRODUCTION The sawmill industry in the State of Durango, with 124 mills, represents approximately 12 per cent of the total number of sawmills in Mexico (16, 103). Some of the most outstanding characteristics of the sawmill industry are: (1) the large number of small sized mills with nearly 70 per cent of them processing less than 20,000 board feet of mixed pine species per shift; (2) high manpower requirements, with almost 2 men per 1000 board feet of lumber produced, or with an average of 28 workers per mill; (3) low level of automation, which is reflected in the high number of employees and in the equipment characteristics; (4) low capital investment with most of the mills having just the required equipment to breakdown the logs in the simplest way, i.e. a single headrig, an edger with three circular saws and a single trim saw (114, 115). These characteristics have a direct effect on lumber volume produced per unit of time and on sawmill efficiency. Sawmill utilization efficiency is measured as the percentage of sawn lumber volume produced from the log volume throughput. Many factors affecting sawmill efficiency are not taken into account in most sawmill assessments, although they have a definite influence on lumber volume recovered from log volume input, and thus, on the net revenue to the enterprises.

13 2 Some of these factors are: (1) the characteristics and working conditions of the equipment, particularly the headrig; (2) characteristics of raw material, such as log grade and log size; (3) characteristics of the output defined by lumber grades and lumber dimensions; (4) log length allowance practices and log volume lost by bucking errors; (5) lumber dimension allowance and excessive oversizing to avoid skip during dressing due to sawing variation (2, 4, 12, 14, 29, 30, 41, 43, 95, 101). Log length variation and over-thickness allowance analyses generally are not included in evaluating sawmill efficiency. It has been pointed out by some researchers that these two factors are of great importance due to their effect on lumber volume recovery, and thus, on the revenue to the enterprises. However, lumber recovery analyses have shown the benefits from adaption of quality control programs to monitor log length and sawing variation (2, 4, 11, 13, 30, 56, 68, 76, 102, 106, 107, 115). The main purpose of this study was to determine the dimensions and volume of lumber that is produced under current industrial manufacturing practices from the various grades and sizes of logs available to sawmills in the State of Durango. A major emphasis was given to evaluate log bucking practices and their effect on lumber wasted at the trim saw. To cope with the lack of information on lumber thickness variation and lumber over-dimension effects on lumber recovery variation, analysis of

14 3 these factors was also Included as the main objective. With the results of the analysis it is expected to identify the main sources of errors under normal sawmilling practices and suggest the correcting actions to increase lumber yield.

15 4 2. LITERATURE SURVEY 2.1 Lumber Recovery Definition The evaluation of a sawmill operation, considering equipment, raw material characteristics and lumber yield, in terms of lumber-log relationship, usually has been expressed in three different ways: Lumber Recovery Factor (LRF), Overrun, and Lumber Recovery Ratio (LRR). Lumber Recovery Factor is defined as the nominal board feet of lumber recovered per cubic foot of log input to a sawmill (22, 105). Thus, LRF is the ratio of nominal lumber recovery (N) to the cubic scale of a log (V). Anything that effects N or V will influence LRF (43). Overrun is defined as the difference between board foot volume, estimated by various log scaling rules, and the actual lumber recovery (51, 55, 59). Overrun is usually expressed as a percentage determined from the equation (61, 110): Overrun % = Lumber tally-net scale Net scale x 100 [1] Lumber Recovery Ratio (LRR) is defined as the ratio of cubic volume of lumber recovered to the cubic volume of logs sawn. It may, for convenience, be based on log and lumber weights which provide an indirect indication of volume subject to errors due to density variations (51). It is sometimes expressed as Per Cent Recovery (84). Irrespective of the way expressed, the lumber recovery or the lumber-log relationship values obtained will change according to the

16 5 following factors: log diameter, log length, log taper, type and number of defects, sawmill types, sawmill recovery practices or product mix, dimension of lumber produced, saw kerf, sawing variation, condition and maintenace of mill equipment, and the ability, conscientiousness and fatigue level of the sawyer or other mill personnel (43, 101). Many studies have been carried out to determine lumber recovery associated with some of the variables listed. The selection of the variables mostly depend on the study objectives. Type and number of defects in logs, combined with log diameters, log length, and taper have been used in lumber recovery studies to formulate log grading rules. Analysis of sawmilling processes associated with equipment, cutting patterns and log characterisics, has become increasingly important in the last 15 years. Small logs in particular have received much attention recently in lumber recovery evaluations Log Characteristics The size and volume of lumber that is produced under current manufacturing practices has a direct relationship with the various grades and sizes of logs (75). The relationship between log surface characteristics and the lumber yield produced, usually has been used to develop log grading and tree evaluation systems (113). In some instances, if log grading rules have already been developed, the lumber grade yields and recovery ratios obtained from various log types are used to update grade and volume recovery when utilization standards, manufacturing practices, quality or marketing requirements have changed (89, 93). Lane et al. (75) determined the relationship between the size and volume of lumber that would be produced from various grades and sizes of Sitka spruce with

17 6 the purpose of developing a tree grading system. The sample size of long merchantable logs was not intended to be representative of a typical log mix; rather, the objective was to obtain adequate saw log recovery information for the full range of size and quality of timber available. Woodfin ej: al. (113) used lumber yield values and log surface characteristics to develop a log grading and tree evaluation system for western hemlock. The sample of 1165 sawn logs yielded a cubic lumber recovery of 48 per cent of the gross cubic volume. Kerbes and Mcintosh (70) developed a log and tree classification system as a guideline in predicting optimum end-use values from spruce, based on exterior log characteristics and end-product values. The most important characteristics in the samples examined were top diameter, the number of clear sides and the percentage of defect due to rot and sweep. Bailey and Dobie (6) concluded a lumber quality study of 1086 trembling aspen logs and 428 balsam poplar logs, and developed a four grade quality system for both species according to log top diameter, percentage of decay and log sweep. The LRF increased with diameter at breast height (dbh), and the net effect was increasing value per cubic foot of tree as tree size increased. To evaluate a log grading system already established, Dickinson and Prestemon (22) estimated the yield of hardwood factory lumber grades which were expected from tanoak logs. Dickinson et^ al. (23), and Prestemon et al. (94) with similar objectives as the former analysis, predicted the yields of factory lumber grades from Pacific madrone and chinkapin logs. The three studies concluded that, because hidden characteristics may influence the relative yield of the several grades of lumber from a log, log grades should be applied only when a

18 7 large number of logs are being classified; otherwise, widely varying results may be obtained. The effect of log grades on lumber quality yield has been evaluated for different tree species and log characteristics. Some of these studies have considered a very large sample of trees and logs to covering the full range of size and quality of sawn timber available (73, 90, 93). It has been found that in general the trend of lumber quality decreases with log quality (62, 90, 93) and that conversion returns diminish as log defects increase (33) Sweepy Logs The effect of a specific log defect on lumber recovery has also been evaluated through some lumber recovery studies. Brown and Miller (12), Dobie (24), and Dobie and Middleton (38) analyzed the effect of sweepy logs on lumber grade and volume recovered. In general, they found that sweepy logs yielded less lumber than straight logs of the same top diameter and length. The dimensions of lumber from sweepy logs were shorter and narrower. Sawing time was almost 40 per cent longer at the headsaw. As a general rule of thumb, Dobie and Middleton (38) established that each 0.1 increase in sweep/diameter ratio led to a reduction in lumber yield of about 7 per cent as compared to straight logs Taper Logs Yield of lumber per cubic foot of log, as stated by Dobie (25), decreased as log taper increased. In addition, sawing time at the headsaw per thousand board feet of lumber increased with log taper. Large-taper logs require 12 per cent more time and yield 5.7 per cent

19 8 less lumber than more cylindrical logs. Hallock (55) and Hallock et al. (59) established that, within reasonable limits, when logs are scaled by board foot rules, the greater the taper the more lumber the sawmill operator will get from his log investment. On the other hand, if cubic scale is used, his lumber yield decreases per dollar of investment as taper increases Infested Logs The lumber yield from beetle-infested logs was analyzed by Dobie and Wright (45). In their study, the selection of trees to be evaluated was based on tree appearance, which in turn was a reflection of time since beetle attack. Grade 1 trees had a green top, grade 2 trees had a red top, grade 3 had grey tight bark and grade 4 had grey loose bark. Trees with green and red foliage yielded similar values per 100 cubic feet of logs, with positive conversion returns. Those with no foliage but tight bark also yielded positive returns but at a lower level. For trees sloughing bark the returns were negative. Similar results were found by Sinclair et^ al. (97), who reported lower lumber yield for beetle-infested timber, as well as a decrease in the grade of lumber recovered. An analysis of lumber recovery from mistletoe-infested trees is reported by Dobie and Britneff (32), but the results showed a different trend from the former studies. On the average, no important differences between sound logs and infested logs were found, nor was any particular difference associated with tree size. Differences in lumber grade yields and recovery factors between lodgepole pine with severe crown infections of dwarf mistletoe and non-infested lodgepole pine trees were not evident.

20 Logs from Dead Trees The effect on lumber yield from dead trees is quite evident according to Snellgrove (98). His analysis showed that the longer a dead tree stands the greater the wood loss; the total loss ranged from 28 per cent for trees dead from 0-2 years, to 71 per cent for older dead material. This statement is also confirmed by Woodfin (111) who found that approximately 28 per cent of the tree value was lost in the first two years after mortality. According to Dobie and Wright (46), only the largest diameter classes of the better log quality group is likely to yield positive conversion returns. Thus, unless there is a real scarcity of fiber, this potential source of additional supply would not appear, on the average, to be economically recoverable. Plank (88) also reported a very high percentage of low quality lumber from dead trees as compared to that obtained from live trees Cull Logs With the objective to evaluate the possibility of processing cull logs into sawn lumber, Snellgrove and Darr (99), and Woodfin and Plank (112) studied the effect of cull logs on lumber yield. Although 47 per cent of the gross cubic volume of cull logs could be manufactured into lumber, the lumber produced would be of low grades. Both analyses indicate that chips and lumber values recovered from cull logs would not exceed an estimate of the average cost of logging and processing merchantable logs. The economic feasibility of using cull logs for lumber manufacture is marginal except in times of extremely high lumber prices, according to these two studies.

21 Log Diameter Log diameter has a definite relationship with quality and quantity of lumber. Thus, it appears in most grading specifications (72). As log diameter increases defects generally decrease with a consequent improvement in grade recovery of lumber (5, 62, 82). However, in some cases (65) the reverse is true, since for any given grade of log, the larger logs may be more defective and lumber quality recovered is diminished. It has been found that the volumetric lumber yield commonly increases with log diameter, ranging from 40 to 43 per cent in 10 to 12-inch logs, and 58 to 65 per cent from 24 to 28-inch logs (19, 21, 85, 86, 87). A general concept that would work for both cases would be that, for logs of the same grade, there is an increase in per cent recovery with an increase in diameter and for logs of the same diameter there is a decrease in recovery with a decrease in log grade (69, 92) Type of Sawmill In sawing large-diameter logs with traditional headsaws, little change has occurred with respect to recovery and product distribution as lumber, sawdust or chippable residue (105). On the other hand, due to the tendency of diminishing supplies of high quality logs, a great deal of innovation has taken place in processing small-diameter logs, resulting in effects on both recovery and product distribution. The most important single factor in successful small-log milling is high speed processing (35). Logs are processed as quickly as possible with emphasis on speed rather than on recovery, resulting in low lumber recovery for small logs (80). When combining the lumber yield and log throughput estimates in the calculation of volume productivity,

22 11 the throughput has far greater impact than lumber yield (4). Changes in small-log sawing methods which have taken place in the last decade have not resulted in increased lumber yields from small logs. Rather, they have resulted in increased productivity and increased yields of pulp chips (29). The percentage of lumber yield increases with the log top diameter class, at least for chipper headrigs and scrag mills (31, 41). The log quad-band system achieved the greatest lumber yield in 9 to 12-inch top diameter range, whereas the chipper canter was most effective in the 6 to 8-inch range (4). Chip yields vary with sawmill type, log size and mill cutting practices (39). Comparing band-headsaws, circular round-log gang, scrag mill and chipper headrig, the chipper headrig yielded a much lower percentage of sawdust and a correspondingly higher percentage of pulp chips (29, 35). The average volume of sawdust obtained headrig was seven per cent of the log volume, or about with a chipper half of the normal sawdust volume from a band mill, and less than one third of that from a scrag stud mill. The data reported in Table No. 1 shows the relationship between sawmill type and the proportion of lumber, sawdust and chips, from small logs ranging from 4 to 14 inches top diameter (29, 35, 41). The big differences in yields are of course in sawdust and pulp chips, with chipper headrigs yielding a much lower percentage of sawdust and higher percentage of pulp chips. The effect of log length on lumber recovery with different types of mills was analyzed by Dobie and Parry (39). They established that the ratio of board feet of lumber recovered per cubic foot of log does not

23 12 change appreciably in logs of 12, 16 and 20 feet. Thus, the same percentage of lumber, sawdust and solid residue could be expected from these length classes. On the other hand, it was pointed out by Dobie and McBride (35), that the LRF decreases as log length increases, whereas the percentage of chip volume increases with log length. A common explanation is that the ratio of board feet to cubic feet of log decreases with log length because taper in logs under 40 feet is not considered in board foot scale rules. The potential lumber volume, which is normally cut out of long tapered logs in conventional mills, is recovered instead, as chips by the chip-and-saw mill. Thus, the ratio of board feet of lumber per cubic foot of large logs would be higher in conventional mills. Apparently no significant difference in the percentage of log volume made into sawdust from large logs was found among various types of sawmills (81). Without taking into account the LRF, the potential advantage of a double-cutting over a single-cutting band headsaw is an increase in productivity, which results in increased gross revenue per shift and reduced cost per unit of output (90). Sawing in single pass units like chipper headrigs, scrags and log-gangs suggests that throughput should increase with log size. It was pointed out by Dobie (27) and by Dobie et al. (41) that in chipper headrigs, sweep or heavily flared butts had a tendency to become jammed and in scrag mills feed rate decreased as top diameter increased. In chipper headrigs and scrag mills, however, the cubic feet of log processed per unit of time increased as log size increased. In log-gang mills, productive capacity increased and processing cost per unit of time diminished with increasing diameter to a certain optimum, whereafter, the trend in both reversed.

24 13 In general, for any type of sawmill, considerable processing time can be saved at the headsaw by processing the longest logs. However, lumber lengths are controlled by market demand (37) Cutting Patterns A large number of factors affect the volume of lumber obtained from any given log by the sawing process. These factors are of two types. The first, such as kerf width, lumber roughness, target size, smallest lumber saved, and slabbing and edging practices are commonly recognized. The second include log breakdown procedures (56, 60). It is apparent that no large difference in LRF exists between grade sawing and live sawing (78). For similar quality logs, LRF from live sawing slightly exceeds that from grade sawing (1). The same relation is true for long logs; live sawing results in a slightly higher per cent recovery and value recovery (4). It has also been established that live sawing on the headsaw takes 18 to 32 per cent less time on the average than grade sawing, and yields considerably higher productivity (9, 52). According to Hallock et al. (60), basically eight log breakdown systems may be used for converting small softwood logs from 5 to 20-inch diameter to dimension lumber. Short logs, less than 16 feet, with a taper of 3 inches or less per 16 feet are best cant sawn, using fulltaper on the log and full-taper on the cant. Logs longer than 16 feet, with taper over 3 inches, are best cant sawn using split-taper on the log and full-taper on the cant. The margin of these advantages can vary from 0.5 per cent to 6.6 per cent, depending on the log mix. In attempting to optimize the conversion of each log, the sawyer faces the difficult task of choosing the best of many different sawing

25 14 patterns in a limited length of time. Since LRF is inherently lower for smaller logs, the choice of the right pattern is even more difficult and critical for small logs (77). Because of pressure for high production the sawyer may run varying sizes of logs through the unit without changing to the appropriate patterns. Recovery could be cut in half by selecting the wrong pattern, as frequently happens for 7-inch diameter logs (3). The variable opening-face live-sawing method is shown to be the best, increasing the volume recovered by approximately 10 to 14 per cent over that of the poorest opening-face (57, 63). To select the best alternative cutting patterns, the use of computer in the sawing process has been useful to obtain high production and the best recovery with small logs. This trend will continue in the future due to the increasing scarcity of large quality logs and the continually rising cost of roundwood. The investment to cover the cost of purchasing computer controlled equipment may be recovered in a relatively short time (63) Log Scaling Log volume is expressed in two common ways: by board foot rules, and by cubic scaling procedures. Log rules are one of the oldest means of estimating lumber recovery from logs. They have been used by the lumber industry in log transactions by both buyers and sellers to epxress the volume of a log in terms of the board feet of lumber it is expected to yield. There are more than 100 log rules and for most of them allowance must be made for shrinkage and waste in estimating the volume of boards that could be recovered (15, 65). Most of the board foot scale rules estimate volume somewhat lower than the actual tally of lumber

26 resulting from sawing the log, even in the most inefficient mills (59). The difference is commonly referred to as overrun. A method expected to replace the board foot rules, as a mean of scaling logs, is that based on log volume, usually expressed in cubic feet or cubic meters. The most widely used cubic scaling method for calculating the volume of logs is the Smalian formula (55, 59). This formula assumes the cubic volume of a log can be closely estimated by multiplying the average area of the two ends of the log, by the length of the log in the same units. There is a fundamental difference between the two systems of volume measurement. Basically, any differences in taper of logs are ignored when board foot rules are used, as the small end diameter of the log and its length are the only variables considered. When logs are cubically scaled, both end diameters are considered and the taper is accounted for in calculating the volume of logs. When logs are scaled by log rules, the lumber yield increases as the taper increases. If cubic scale is used the lumber yield decreases as taper increases. It is also common to express the log volume by both systems in studies where the precision of estimators is desired. Hanks and Brisbin (62) used the International 1/4-inch and the Scribner log rules, as well as cubic scaling by the Smalian formula. Henley and Plank (65), Lane ejt al. (73), and Snellgrove et al. (100) used the Scribner Decimal C log rule, and for gross cubic volume the equation: 2 2 V = L (D + D X D 2 + D 2> [2] Where D^ is the log scaling diameter, small end; D 2 is the diameter,

27 16 large end, and L is the log length. Woodfin (110), and Fahey and Martin (48), used the Scribner log rule and the cubic volume equation listed (65, 73, 100). Dobie and Wright (43) compared two cubic scales used in British Columbia: the lumber cubic scale, and the firmwood cubic scale, using the Smalian formula and the lumber recovery factor. Inadequate deductions for butt-flare, difficulties in assessing the degree of defect, and inaccuracy in measurement were the main limitations found. For defective logs estimates of volume will vary with the scale used, and consequently the LRF will vary between scales. Over-generous allowances for sweep, flare, and defects will tend to decrease volume and increase LRF, whereas conservative deductions will have the opposite effect. In general it was concluded by Dobie and Wright (43), that there could be as many different estimates of volume for a given log as there are scalers, so that the LRF for the same log could vary accordingly Log Sample Size The sample size required for analysis of LRF depends on the precision desired, cost limitations, and the type of information needed to satisfy the requirements of the study objectives. Commonly, to evaluate the LRF in a sawing process, a sample of 100 logs selected at random from the log yard has been used. These logs are processed in batches in a normal manner (17, 105). When more precise information is needed and more variables are analyzed for the required data the sample size usually is increased. Plank and Henley (89), in their study to relate timber characteristics to end product yield values, used a sample size of 1009 merchantable logs to cover the full range of log size available even when samples were not representative of a typical log mix.

28 17 With similar purpose Henley and Plank (65) selected a sample size of 428 logs that were representative of the available tree characteristics. Pong and Fahey (93) used a sample size of 1126 logs which was representative of the full range of size and quality of the logs. Bailey (5), with a sample size of 609 logs, ranging from 7 to 17 inches in diameter, obtained a representative sample size of straight trembling aspen logs, while Mueller and Bager (82), for merchantable logs ranging in diameter from 7 to 24 inches, needed 675 logs. Dobie et al. (34), with a sample size of 2585, representative of the major commercial species, compared the difference between the lumber cubic scale and the firmwood cubic scale. Lane et: al. (73), to determine the lumber yield and recovery ratios for old-growth Douglas-fir, using two log scaling and grading practices, processed 2980 woods-length logs and 4974 sawn-length logs. With different objectives Lane and Woodfin (74) selected a sample size of 4009 commercial sawlogs to evaluate lumber yield by grade. To satisfy statistical requirements, the sample size should be selected according to the variation of the quantity estimated (26, 54). The required number of logs is determined by the formula: 2 2 N - [3] E o Where N is the sample size, s is the population variance, t is the appropriate value of Student's t, and E is the allowable error. If the volume variation of the log population is unknown, then a preliminary

29 18 sample of 60 logs should be taken at random to determine the variance by the formula v 2. lexl 2 Where Zx z is the sum of squared values of all individual measure - ments, ( x) is the square of the sum of all measurements. Using this procedure Dobie (26) found that in B.C. the sample size for small interior log mills ranges from 249 to 343 logs and for coastal sawmills ranges from 210 to 779. Dobie and Warren (42) reported that the number of logs in two-inch diameter classes diminished as diameter increased. For 95 per cent confidence limits, with half-width of 5 per cent of the mean LRF, about 100 logs of the 4-inch class and 60 of the 10-inch class should be sampled. 2.2 Log-Lumber and Byproduct Proportion Sawmill residues such as slabs, edgings, trims, sawdust and bark, that were once wasted, are now being sold for pulp chips, fiber and particle wood products, mulches, soil amenders and ground covers. Woodchips are sold to pulp mills, fiber or particle board plants for processing into paper or boards; bark residue is screened, washed and marketed as soil amenders, mulch and ground covers, sawdust is sold for mulching and livestock bedding or burned at the mill as fuel (19, 20, 21, 85, 86, 87). Because of the increasing value of sawmill residues, increased attention has been given to estimating the amount of each byproduct. Factors affecting the lumber yield of log input also affect the yield of

30 19 sawmill residues: log diameter, log quality, scaling practices, sawmill type, saw kerf, dimension of lumber produced and sawmill recovery practices, among others Volume and Weight Method Using a band headsaw to process shortleaf pine logs, ranging in diameter from 9 to 20.4-inch, Phillips and Schroeder (87) determined the yield of lumber and byproduct from the sawlogs. Each log was weighed with and without bark, and scaled prior to sawing in a 3/16 inch kerf band headsaw into lumber of unusual dimension, 1 by 5 inches and 1 by 3 inches, for export. The proportion of the different log components was: 54 per cent lumber, 26 per cent chippable residue, 10 per cent bark residue and 10 per cent sawdust. Lumber yield increased from 43 per cent in small trees to 58 per cent in large diameter trees. The percentages were determined by weighing the chippable residue from each log. Sawdust weight was determined by substracting weight of chippable residue and lumber from debarked log weight. Phillips (85, 86) studied black-oak logs varying in diameter from 11.9 to 25.6 inches. He processed them on a band headsaw into grade yield of 4/4 and 5/4 inch lumber with a minimum of wane, and found a relation of 55 per cent lumber, 20 per cent chippable residue, 15 per cent bark residue and 10 per cent sawdust. As scaling diameter increased lumber yield increased from 46.3 to 61 per cent; 25 per cent of small diameter logs went into chippable residue compared to only 16.4 per cent for large diameter logs. The percentage of bark residue and sawdust remained relatively constant over the range of tree diameters. Clark (19) and Clark et al. (21), sampled 230 yellow poplar sawlogs ranging from 11.7 to 28.4 inches in diameter. The logs

31 20 were weighed with and without bark and scaled by the Smalian formula prior to sawing them into 4/4 inch lumber, with a 3/16 inch kerf band headsaw. A proportion of 54 per cent lumber, 15 per cent bark, 18 per cent chippable residue and 13 per cent sawdust was found. They also found that chippable residue decreased from 29 per cent in small logs to 16 per cent in large logs, while bark residue decreased from 17 per cent in small logs to 12 per cent in large logs. Lumber yield increased as tree size increased, ranging from 42 per cent in 12-inch diameter trees to 59 per cent in 28-inch diameter trees. Using a conventional 5/16 inch kerf circular saw headrig, Clark and Taras (20) processed slash pine sawlogs ranging in diameter from 9.6 to 21 inches into 4/4 and 8/4 inch lumber and determined a relation of 51 per cent lumber, 22 per cent chippable residue, 10 per cent bark residue and 17 per cent sawdust. They also found the LRF increased with tree size up to 18 inches and then decreased slightly. The average LRF ranged from 5.3 in 12-inch d.b.h. trees to 6.5 board feet per cubic foot in 18-inch d.b.h. trees, and the average was 6.1. Taras et al. (104), with merchantable stems of loblolly pine ranging in diameter from 9.8 to 19.4-inch d.b.h., and processed on a circular headsaw into dimension and board lumber, reported a proportion of 50.3 per cent lumber, 28.5 per cent chippable residue, 7.6 per cent bark and 13.6 per cent sawdust from the sawlogs; 88 per cent of the lumber cut was 8/4 inch dimension and 12 per cent was 4/4 inch boards. Lumber yield increased as tree size increased, ranging from about 37 per cent in 10-inch trees to about 55 per cent in 20-inch trees. Chippable residue on the other hand, decreased, ranging from a high of 39 per cent in 10-inch trees to 25 per

32 21 cent in 20-inch trees. Bark yield decreased as tree size increased ranging from 9 to 7 per cent. Sawdust weight decreased slightly with increasing tree size ranging from 14.4 per cent in small trees to 12.9 per cent in large trees. Fahey and Hunt (50) studied grand fir thinning logs, ranging in diameter from 4 to 14 inches processed at a band mill and from 4 to 12 inches at a chipper headrig, producing standard and better grade lumber. They found that the proportion of lumber, sawdust, and chippable residue was , 35.1 per cent, and 49.5, 5.4, 44.1 per cent, respectively, for the two machine centers. An empty chip bin was used to collect the chips and solid residue volume was determined from the oven-dry weights. Schroeder et^ al. (96), according to the statement that weight- scaling is the accepted method of buying and selling pine logs in the southern United States, formulated tables based on weights to estimate the proportion of lumber and other primary products of pine saw timber trees. Lumber was produced on a circular headsaw which had a 5/16 inch kerf, into 4/4 and 8/4 inch thickness and the proportion was 54 per cent lumber, 26 per cent chippable residues, 16 per cent sawdust and 9 per cent bark. LRF increased with diameter from 5.80 for 10-inch trees to 6.84 for 20-inch trees. In all these studies a similar procedure to determine the proportion of lumber and byproducts from sawlogs was followed. Regression equations were formulated to predict either the weight or the volume, or both, from merchantable stems and the primary products Saw Kerf and Cutting Patterns Method A different approach to determine lumber volume and byproduct proportion from sawlogs was used in the following studies. Sawdust

33 22 volume was determined by using the average saw kerf and the computed surface area of the rough green lumber from each log. Chippable product volume was determined by subtracting the lumber and sawdust volume from the gross cubic log volume. Using band headsaws to process old-growth coastal Douglas-fir ranging in diameter from 5 to 67 inches, Lane j2t al. (73) employed an average saw kerf thickness for each mill which was producing optimum values of board, dimension, select and shop lumber. They found that about 63 to 64 per cent of the cubic content of the log was manufactured into rough green lumber of which approximately 25% was lost as planer shavings and shrinkage. Henley and Plank (65) used a band headsaw with an assumed average saw kerf of 7/32 inch and computed cross sectional area of the lumber in each 6 to 34-inch log of Engelmann spruce. When producing nominal 2 by 4, 2 by 6 and 1 inch boards, they found that an average trim allowance of 6 inches would increase the gross cubic volume by 3.3 per cent and there would be a corresponding increase in the volume of residues. Pong and Fahey (93), applying a saw kerf of 8/32 inch in a band headsaw processed red and white fir ranging in diameter from 7 to 50 inches. They manufactured select, shop, common and dimension lumber, in a similar way to the former studies, and tabulated the varying proportions of lumber, sawdust and chippable residue from different log diameter classes. Snellgrove et al. (100) with a band headsaw and an average saw kerf of 0.25 inch, processed low grade coastal Douglas-fir logs into board, dimension, and select items. He found that about 62.5 per cent of the cubic content of the sawn-length logs was manufactured into rough green lumber and about 8 per cent of the volume was sawdust.

34 23 The remaining per cent cubic content of the logs was considered mill residue. They also found that the residue volume will increase with a 6-inch trim allowance, which increases the gross cubic volume of the average log by 24 per cent. Lane et^ _al. (75), used a single cut bandsaw with an average saw kerf of 0.25 inch, to process Sitka spruce logs ranging in diameter from 6 to 56-inch into rough green cants for export. They determined the proportion of lumber,. sawdust and chippable residue by log diameter class. Fahey and Martin (48) studied 292 second growth Douglas-fir logs ranging in diameter from 7 to 44 inches, using a double cut bandsaw produced lumber of which' 49 per cent was 4 inches or thicker. They reported that the proportion of lumber, sawdust and chippable residue was 60 per cent, 9 and 31 per cent, respectively. The portion of the log converted to sawdust, 9 per cent, was low due to the high per cent of 4-inch dimension lumber produced in this study. Normally approximately 11 to 12 per cent of the log volume becomes sawdust. Processing small diameter logs, Kerbes and Mcintosh (69) using 3/8 inch kerf for the headsaw and edger with logs ranging in diameter from 4 to 14 inches. They determined the proportion of sawlog volume converted in lumber, sawdust and solid residue for each diameter log class. The overall recovery was 61, 19 and 20 per cent of lumber, sawdust and chippable residue, respectively.* Since cubic scaling is becoming more common in sawmill studies Hanks (61), in his analysis to predict the lumber and chippable residues from 10 species of hardwood trees, calculated sawmill residue volume in *The previous information including section has been summarized in Table 2.

35 24 cubic feet for each tree by subtracting the cubic-foot volume of lumber and sawdust from the gross cubic-foot volume of saw log material. The logs ranging in diameter from 10 to 30-inch, were sawn in circular and band mills with kerfs of 7/16 inch and 10/16 inch respectively, processing lumber with thickness of 4/4, 5/4, 6/4 and 8/4 inch. He presented prediction equations and tables to estimate the gross cubic-foot yields of lumber, sawdust and solid wood residues. Bennett and Lloyd (8), in contrast to the former study, used the International and Scribner log rules to estimate the portion of a log that goes into slabs and edgings. In their analysis it was found that the volume of byproduct varies according to the length of the log. This variation results because the International Rule makes no allowance for such increase; thus, the proportion of log volume converted to lumber increases with log length by the International Rule, but decreases by the Scribner Rule. Consequently, the percentage of the log volume going into slabs and edgings decreases slightly with log length by the International Rule but increases by the Scribner Rule. The proportion of lumber and byproducts from small-diameter logs, defined as being from 4 to 15 inches top diameter, changes according to the type of sawmill and processing characteristics. These relations are clearly shown in Table 1, (18, 2 9, 37, 41). Saw kerfs used to calculate the volume of sawdust produced were 11/32 inch for circular headsaws, 3/16 inch for band saws and gang saws, 5/16 inch for edgers, 7/32 inch for trim saws, 1/8 for twin band saws and 1/4 inch for scrag saws. It has been generalized (105) that, for circular sawmills, the appropriate cubic contents that develop from green, debarked logs are: lumber 37 to 47 per cent; kerf 16 to 21 per cent; chips 17 to 35 per

36 25 cent; and planer shavings 12 to 18 per cent. This includes oversizing, sawing variation and planning allowance. The figures for bandmills with the same variables are: lumber 44 to 53 per cent, kerf 12 to 15 per cent; chips 20 to 29 per cent, and planer shavings 11 to 15 per cent. A general procedure is described by Dobie (26) to determine sawdust volume according to log breakdown patterns and the various pieces of equipment in a mill. The saw cuts are recorded and the thickness of pieces removed at each machine center either measured or estimated. Over a period of time sufficient data can be gathered to give sawdust yield for each diameter class at each machine center for each processing pattern used. This procedure increases in complexity as the number of processing stages and cutting patterns grow. A different approach is described by Steele and Hallock (101), analyzing the methods used to calculate the volume of byproduct in the sawing process. They concluded that most of the methods have one weakness in common; relatively few of the variables that can affect residue volumes are considered. An accurate prediction of sawmill residue production requires including in the analysis all the important variables that can affect this production. A geometric model is suggested to calculate volume of green lumber, dry lumber, green chips, green sawdust and dry planer shavings. 2.3 Quality Control Log Length Allowance The most common lengths for logs processed into sawn lumber are 8, 10, 12, 14, 16, 18 and 20 feet in nominal dimension, with an additional

37 over-length ranging from 4 to 12 inches (95). Log length variation has high impact on log volume lost and on value return on investment of raw material. When the actual dimension exceeds significantly the nominal dimension, the lumber wasted at the trim saw is greatly increased. Dobie (30), in his analysis of 13 coastal sawmills in B.C., found that 3.1 per cent of the total log volume processed was lost due to an over allowance of log length considering 6-inch trim allowance for long logs. Aune and Lefebvre (4), in their analysis of 72 small log sawmills in the interior of British Columbia, reported a log volume lost of 1.8 per cent due to over length of the logs. Zavala (115) in his report of the characterization of the sawmilling industry in the State of Durango, found that 3.2 per cent of the log volume processed was lost due to an over length. However, he considered, as a common log bucking practice, an allowance of 4 inches of the nominal log dimension for any given log length category, while Dobie (30) and Aune and Lefebvre (4) considered 6 inches as allowable length for long logs Lumber Dimension Allowance The most common dimensions for cutting and selling sawn lumber range from 4 to 12 inches in width and from 4 to 20 feet in length, in increments of 2 inches and 2 feet, respectively. Thickness ranges from 1/2 inch to 2 inches in nominal dimension. Rough dry lumber allowance for finish machining ranges from 1/4 to 1/8 inch in thickness, at least 1/2 inch in width, and 3 inches in length (95).

38 27 Among the few studies in lumber dimension analysis Rodriquez (95), processing 755 logs in 7 sawmills, reported that the actual volume lost, as a result of over-allowance, ranged from 20 to 34 per cent of the nominal volume. Cardenas (17) established the minimal thickness dimension and volume allowance for sawn lumber most commonly processed. For 1/2, 3/4, 4/4, 6/4 and 8/4 inch, the corresponding lumber thickness should be 16, 22, 28, 41 and 54 millimeters, yielding a volumetric allowance for finishing of 40, 30, 22, 15 and 8 per cent, respectively. The allowance in dimension is a common practice for production and commercialization of sawn lumber, due to the volume lost by sawing variation, by planing, and by shrinkage from green to dry dimension. Minimum size requirements for rough green lumber are determined by taking the required final dry-dressed lumber dimension and adding allowances for planing and shrinkage. The target lumber size is determined by additional allowance which represents sawing variation. Over allowance in sawn lumber to compensate for thickness sawing variation, increases lumber volume lost which results in lower lumber recovery. Sawing variation is a measure of the mechanical precision in manufacturing lumber thickness and widths (102). Skill and training of machine operators and maintenance of machining conditions have a direct influence on sawing variation and on planing variation of rough lumber. It is, therefore, possible to control and reduce these two types of variations (2, 11, 13, 14, 106, 107). Sawing variation can be quite significant as indicated by Kerbes and Mcintosh (69) who reported in their study that, in the actual conversion of trees to finished lumber, oversize amounted to 10 per cent of the