0886-A2

Cutting forces for tension wood and normal wood of maple

Iris Vazquez-Cooz ¹ Robert W. Meyer ²

Abstract

This research measured cutting forces for normal and tension wood of hard maple (Acer saccharum Marsh.) and red maple (Acer rubrum L.). Wood blocks were machined to simulate sawing by using a single bandsaw tooth mounted on a three-axis piezoelectric load cell to evaluate principal, lateral, and normal forces when cutting in the 90-90 direction (edge of sawtooth oriented perpendicular to the grain, cutting in the radial direction). Frozen, green and dry wood was cut to produce data related to primary manufacture of frozen or green logs as well as dry wood used in secondary manufacturing.

Three tooth designs were tested. Sawteeth with larger rake angles required less energy to cut green and dry wood. The tooth with the largest rake angle required the least energy to cut dry and frozen wood, and also performed well when cutting green tension wood and normal wood.

Specific cutting force for frozen wood is nearly as great as for dry wood; specific cutting force is least for green wood; specific cutting force is less for tension wood than for normal wood. The results for tension wood are apparently due to the amount and type of lignin present in the tension wood.

Introduction

Wood machining studies using a single sawtooth help to understand the cutting process and eliminate extraneous factors such as vibration of the cutting tool and saw machine components that obscure variables involved in the cutting action. For cutting force analysis, elimination of these extraneous factors is desirable when sawtooth parameters, chip thickness, and wood properties are study variables. We simulated cutting in the 90-90 direction-the cutting edge was perpendicular to the direction of motion of the sawtooth and perpendicular to the wood fibers (McKenzie 1961). A cutting machine using a single sawtooth mounted on a three-axis load cell measured cutting forces precisely in the X, Y, and Z directions, so we could compare cutting forces with wood characteristics.

Sawteeth that minimize cutting forces as the edge passes through the wood are needed, so we considered the following parameters: Lee (1995) states that the cutting process tears cells apart and can be controlled to get a smooth surface. Power requirements increase when feed rate, depth of cut, specific gravity increase and moisture content decreases (Stewart 1980). Side clearance affects cutting power (Koch 1964, McKenzie 1997). Sawteeth sever wood cells across the grain; when cells are not cut cleanly, surface defects are produced. Low rake angles deform wood in compression perpendicular to the grain (Hoadley 1981), which results in severely bent-over fiber ends and splits into the end-grain surface. Fuzzy grain is associated with cutting green tension wood (Panshin and de Zeeuw 1980, Stewart 1980), with greater power consumption and dulling of the cutting edge (Koch 1985). Experiments reported here provide new insight for machining tension wood. Forces were measured in three directions while cutting green and dry tension and normal wood of red and sugar maple at two chip thicknesses. We also extended previous work by measuring cutting forces in frozen normal wood.

Materials and Methods

Three leaning trees containing tension wood and one vertical tree containing normal wood of red maple (Acer rubrum L.) and a like number of sugar maple (Acer saccharum Marsh.) trees were selected for study. All trees grew in central New York.

Specimen preparation. Two matched blocks were cut from each tree, one for testing green and one for testing dry. An additional block for testing frozen wood was cut from each normal wood tree. Matched 25 mm³ blocks were cut to estimate specific gravity (SG) and wood moisture content (MC). SG for green sugar maple ranged from 0.58 to 0.63 and green red maple ranged from 0.49 to 0.59; dry sugar maple was from 0.59 to 0.64, while dry red maple was from 0.52 to 0.60. Cutting blocks were 50 mm (2 in) by 70 mm (2.75 in) by 70 mm (2.75 in) along the grain. A block of ice with the same dimensions was used to measure its cutting force. Blocks to be cut in green condition were stored in a freezer, and blocks to be cut in dry condition were equilibrated in a conditioning room at 65% relative humidity and 70°F. Equilibrium moisture content of sugar maple was 12.2% and red maple was 14.4%. The block carriage (Fig. 1) was cooled with dry ice prior to cutting frozen blocks; temperatures were between -8°C and -19°C.

Dry blocks were wrapped with plastic to keep moisture content stable. Between cuts, plastic was placed on the exposed surface. All cuts were perpendicular to the growth increments to simulate band sawing (Fig. 1). Cuts were made from the bark towards the pith, using two chip thicknesses, 0.50 mm (0.02 in) and 0.76 mm (0.03 in).

Figure 1. Cutting machine used to measure cutting forces simulated band sawing in 90-90 cutting direction.

Individual teeth with stellite tips (Table 1) were cut from bandsaw sections provided by The Experimental University of Guayana, Upata, Venezuela, and mounted on a three-axis piezoelectric load cell to measure principal, lateral, and normal forces. The block of wood was propelled past the sawtooth in a straight line at a speed of 1.34 to 1.50 m/s (53-59 in/s) by a hydraulic cylinder. A precise sawtooth advance system provided desired chip thickness.

Power required for cutting is most directly related to the principal (parallel) cutting force. It acts parallel to the feed and direction of the cutting tool motion, and represents the major effort to sever the chip. Normal force is the component acting perpendicular to both the principal force and the surface generated, it is negative when the tooth engages the wood and positive when the wood repels it. Lateral force measures forces acting on both sides of the sawtooth and indicates if the sawtooth is being pushed to either side.

National Instruments LabVIEW software and a data acquisition board used four input channels, one for each force, one for a speed sensor, and one output channel controlled the cutting machine. LabView allowed us to observe cutting force waveforms immediately and transferred data to a spreadsheet. 2,000 data points were collected for each input channel (2.9 data points per 100 °m over the 70 mm length of cut).

Treatment and experimental design. A factorial treatment investigated cutting forces for normal and tension wood under different conditions. Experimental factors were: wood (three tension and one normal wood), chip thickness (0.50 and 0.76 mm), sawtooth (three designs), and moisture content (green and dry). Frozen green normal wood was also cut.

Cutting experiments followed the sequence green, dry, and frozen wood for each sawtooth. Green and dry wood had 48 treatment combinations and frozen wood had 18. For tension wood three replications (three trees) were used with one normal wood tree used as a control. The number of cuts for each block considered wood variability and cutting conditions. Numbers of cuts (n) differed for each block, using the formula n = [(z*STD)/m]^2; where z = 1.96, m is the desired margin of error for principal force in kg-force, and STD is the standard deviation of the principal force. Four to 7 cuts provided 95% statistical confidence. The data set for sugar maple was 240 observations, for red maple 256, and for frozen wood 70. We used ANOVA to analyze cutting forces; F statistics and P-values were used to test the null hypothesis (no differences in cutting forces of tension and normal wood and forces between dry and frozen normal wood).

Table 1. Geometrical parameters of the sawteeth designed used in the cutting experiments.

Results and Discussion

A unique property of tension wood is the formation of fuzzy (wooly) grain when it is machined, especially in the green condition. The experimental and factorial design helped us interpret cutting force results for tension wood compared to those for normal wood and suitable cutting conditions to obtain better surface quality for tension wood.

Statistical analysis of cutting forces. Specific cutting force (force divided by wood SG) gives an index of resistance to cutting and helps determine whether differences in force are due to factors other than SG. Forces were standardized based on tooth width. Statistical analyses are based on specific cutting forces divided by cutting edge width (cutting force/SG/mm).

Specific principal cutting force. Average specific principal cutting forces are in Table 2. The maximum value for sugar maple was 14.93 kg-force/mm (2.8 % greater than the avg.) when cutting dry normal wood with sawtooth 3 at a chip thickness of 0.76 mm. Maximum value for red maple was 14.25 kg-force/mm (10.7 % greater than the avg.) for cutting dry tension wood (tree 3) with sawtooth 3 at 0.76 mm chip thickness. Dry wood requires more energy to cut. Rubbing on the tooth back occurs with low clearance angle, which increases cutting force. Low or negative rake angles increase cutting forces (Kivimaa 1950, Franz 1958, and Woodson 1979). A corner of sawtooth 1 broke (600 X 400 X 150 μm) cutting the first dry block.

Maximum principal cutting force could be five times as large as the average, and these peak values can significantly increase sawtooth wear (Chardin 1977). For green wood of both species the ratio of maximum to average principal cutting force was 2.1, for dry wood was 1.6, and for frozen wood the maximum ratio was 3.1 for both chip thicknesses. Green wood generally produced an uninterrupted and smooth chip, which indicates that the sawtooth was continuously engaged in the wood. Conversely, dry wood generally produced discontinuous and/or compressed chips with the sawtooth disengaged from the wood some of the time. For dry wood alternate cuts resulted in continuous and discontinuous chips. When discontinuous chips were produced, there was considerable failure at the end-grain surface beneath the sawtooth's edge, the principal force dropped dramatically and the average force was lower (Fig. 2). For green wood of both species, tooth 2, with the largest rake angle (35.1°), produced the best surface and required the least energy. For dry wood, tooth 3, with the smallest rake angle (25°), compressed the surface and crushed fibers, making the surface appear smoother, and required more force.

Table 2. Summary of Specific Principal Cutting Force Data for Green and Dry Wood

The null hypothesis tested cutting force between tension and normal wood for different cutting conditions at α = 0.05 significance. For both species all factors (moisture condition, chip thickness, sawtooth design, and type of wood) were statistically significant. Moisture had the largest effect followed by chip thickness and type of wood. Chip thickness and moisture interacted to influence cutting force more than any other combination of factors, followed by the wood type-moisture interaction. Sawtooth 3 registered the greatest cutting force. The influence of sawtooth design for sugar maple was 6.3 times larger than the wood effect; for red maple, the difference was even larger (16 times).

Figure 2. Relationship between chip quality (top) and cutting forces for dry normal wood of sugar maple (A) and green normal wood of red maple (B). In A the compressed central portion of the chip corresponds to lower principal cutting forces (red data) and normal forces (purple data) that became positive (the tooth was repelled by the wood). In B principal and normal cutting forces were uniform along the cut and the chip is smooth.

It took less energy to cut green tension wood than green normal wood; for dry sugar maple the same effect was found only for thicker chips (0.76 mm). Secondary walls of tension wood are less well lignified (Vazquez-Cooz and Meyer 2002), which may explain why tension wood requires less energy to cut. Dry red maple cutting forces were statistically similar for the thicker chips (0.76 mm), but when tension wood is present in contiguous rings cutting force drops dramatically (Fig. 3). Green tension wood requires less principal cutting force, although it is generally accepted that the power to cut tension wood is high and dulling of the tool may be rapid (Koch 1985). In industry, when tension wood is machined fuzzy grain is produced and its presence causes friction on the body and edge of the cutting tool as it drags against the wood, so more friction and heating is developed (Vazquez-Cooz, 1984). Consequently, energy consumption increases by friction between the workpiece and the cutting tool. The friction forces are added to the whole cutting process, increasing energy consumption, which makes it appear that tension wood requires more energy to cut than normal wood.

Figure 3. Typical cutting forces for dry normal (A) and tension (B) wood of red maple. Lines on the blocks of wood (upper) show cutting locations. Principal force in A is more or less uniform, (15 to 20 kg-force, average 17.46). In B the principal force is uniform (average 16.10 kg-force) but then at the end of the first third of the graph it is dramatically lower (see arrow) at 10.5 to 13.6 kg-force, corresponding with wider growth increments of tension wood (stained blue in the microtome section just above the graph).

Cutting tension or normal wood in the green condition with lower rake and clearance angles requires more energy (Table 3). For dry wood, cutting forces were greatest with the lowest rake angle (statistically significant) and a rough surface was produced. Corners of a sawtooth have a main role in producing side surfaces; therefore, we should be concerned about the effect of the front and lateral angles that form the corners of the sawtooth. The corners of sawtooth 3 were more obtuse, with reduced side clearance (0.50 mm), and cutting force was greater. The reduced cutting force advantage gained when using a small side clearance in dry wood was lost when rake angle was lower: sawtooth 3 was stronger, but cutting force was greater.

Table 3. Reductions in specific principal cutting force for the three saw teeth (tooth 3 required the most energy).

Cutting dry tension wood of sugar maple tree 1 using sawtooth #1 registered a lower specific principal cutting force (6.64 kg-force/mm tooth width). Cutting force was 30% and 38% greater for sugar maple and red maple respectively as chip thickness was increased.

Specific lateral cutting force. No differences were found in lateral force between normal and tension wood. This is interesting for tension wood because there was more fuzzy grain on one side of the kerf, so lateral forces should have been significant on that side. Sawtooth condition significantly affects lateral force. Sawtooth 1 became chipped in its left corner. Cutting red maple with tooth 1 generated lateral forces averaging up to 17% of the principal force, and sawteeth 2 and 3 generated up to 12%. The chipped tooth was no doubt responsible.

Specific normal cutting force. Normal force is sensitive to tooth sharpness, rake and clearance angles, and chip thickness. In our system, when the sawtooth was engaged by the wood, normal force was positive. Tension wood (green and dry) registered lower values than normal wood for both chip thicknesses of both species. As chip thickness increased, normal force became more negative. On average, 98% of normal forces were negative, except for sawtooth 1 which was only 59% negative because of its chipped corner. Sawtooth 2 with larger rake angle generated greater normal force than sawtooth 1 or 3 for both species and moisture contents, which agrees with Woodson (1979); average forces were lower with sawtooth 3 (lower rake angle).

Frozen wood. Cutting force increases with decreasing temperature. Some authors (Kollmann and Côté 1968, Kivimaa 1950) suggest this is caused by ice in the cell cavities. Ice cutting forces (Table 4) for were very low (from 0.35 to 0.55 kg-force/mm). No statistically significant differences in specific principal cutting force between frozen and dry sugar maple occurred, but for frozen red maple they were statistically significantly lower than for dry wood.

Cutting thicker chips of frozen sugar maple increased specific principal force 20%, with dry wood this increment was 35%. Cutting thicker chips of frozen red maple increased the force 23% while cutting it dry almost doubled the force (38%).

Lateral cutting forces for frozen wood were more than for unfrozen wood-frozen red maple with sawtooth 1 generated lateral forces averaging up to 23% of the principal force, and sawteeth2 and 3 generated up to 16%. Frozen red maple forces were statistically significantly greater than dry wood only at 0.50 mm chip thickness.

Table 4. Summary of specific principal cutting force data for frozen normal wood and ice.

Normal forces for frozen green wood of both species were larger than for dry wood; for sugar maple at chip thickness 0.50 mm the difference was statistically significant. For sugar maple, normal forces averaged 44.4% of the principal force, for red maple the increase was 41%.

Conclusions

Sawtooth design is important in reducing energy consumption when processing normal or tension wood.

The sawteeth with larger rake angles cut green red and sugar maple with 20% less force than the sawtooth with lower rake angle. For dry wood, cutting force was about 14% less. Surfaces produced with larger rake angles were smoother, especially when cutting green tension wood. However, for dry tension wood the smaller rake angle cut the wood with crushed cells on the surface, making the surface appear smoother; but below the cutting plane, it split the wood parallel to the grain. The 35.1° tooth registered the lowest specific principal cutting force.

Moisture content produced a larger impact on specific principal cutting force than chip thickness. As expected, green wood registered the lowest specific principal cutting force, followed by frozen wood; dry wood generated the greatest cutting force.

Differences between frozen and dry wood were not statistically significant for sugar maple while for red maple they were significant.

Specific principal cutting force was lower for green tension wood than for green normal wood of both species; the same results were obtained for dry sugar maple. For red maple no significant differences were found between dry tension wood and dry normal wood. When tension wood is machined fuzzy grain is produced and its presence causes friction on the edge and body of the cutting tool. The friction is added to the whole cutting process, increasing energy consumption, making it appear that tension wood requires more energy to cut than normal wood.

No differences in lateral force were found between normal and tension wood of both species, but we found significant differences in lateral force with respect to chip thickness. For red maple the force is greater for thicker chips. For dry sugar maple no differences were found; however, in green condition the thicker the chip, the larger the lateral force. Frozen and dry sugar maple generated the same lateral force results, but frozen red maple generated greater lateral force (statistically significant) than dry red maple only for chip thickness 0.50 mm.

Normal force was lower for tension wood than for normal wood of both species and chip thicknesses used. Frozen normal wood registered greater normal forces than dry wood and we observed that the greatest normal force was generated by sawtooth #2.

Literature Cited

Axelsson, B. O. M. 1994. Lateral cutting force during machining of wood due to momentary disturbances in the wood structure and degree of wear of the cutting tool. Holz als Roh-und Werkstoff. 52(3):198-204.

Chardin, A. 1977. Bandsaw efficiency: developments concerned with the tooth, cutting edge, saw blade, and sawing process. In Proc. 5^th wood machining seminar, October 14 & 15. Richmond, CA: University of California, Forest Products Laboratory.

Franz, N. C. 1958. An Analysis of the Wood Cutting Process. Ph.D. Diss. University of Michigan Press, Ann Arbor. 152 p.

Hoadley, B. R. 1981. Understanding Wood: A Craftsman's Guide to wood Technology. Taunton Press, USA. 256 p.

Kivimaa, E. 1950. Cutting Force in Wood Working. Ph.D. Diss. Finland Institute of Technology. Publ. No. 18. 102 p.

Koch, P. 1964. Wood Machining Processes. Ronald Press Company, New York. 530 p.

Koch, P. 1985. Utilization of Hardwoods Growing on Southern Pine Sites. Handb. No. 605. Vol. II. U.S. Dept. of Agric., Forest Service. Washington, D.C.

Kollmann, F. and W.A. Côté Jr. 1968. Principles of Wood Science and Technology. I Solid Wood. Springer-Verlag, New York. 592 p. Lee, L. 1995. The Complete Guide to Sharpening. The Taunton Press, USA. 245 p.

McKenzie, W. M. 1961. Fundamental Analysis of the Wood Cutting Process. Ph.D. Diss. University of Michigan Press, Ann Arbor. 151 p.

McKenzie, W. M. 1997. Tooth action leading to excessive sawing deviations. In: Proc. 13^th Intern. Wood Machining Seminar, June 17-20. Vancouver, Canada. Pp. 429-443.

Panshin, A. J. and C. de Zeeuw. 1980. Textbook of Wood Technology. 4^th Ed. McGraw-Hill, New York. 722 p.

Stewart, H. A. 1980. Some surfacing defects and problems related to wood moisture content. Wood and Fiber. 12(3):175-182.

Vazquez-Cooz, I. 1984. Personal observations at "El Manteco" sawmill, Guayana, Venezuela.

Vazquez-Cooz, I. and Meyer R. W. 2002. Identification of lignified and unlignified tissues. Biotechnic & Histochemistry. 77(5/6):277-282.

Woodson, G. E. 1979. Tool forces and chip types on orthogonal cutting of southern hardwoods. Res. Pap. SO-146. U.S. Dept. of Agric., Forest Service. 77 p.

¹ State University of New York (SUNY-ESF)
College of Environmental Science and Forestry
One Forestry Drive, Syracuse, NY 13210. USA. Ph.D. Candidate
Fax: (315) 470 6879
E-mail: [email protected]
Website: http://www.esf.edu

²Professor at the State University of New York (SUNY-ESF)
College of Environmental Science and Forestry
One Forestry Drive, Syracuse, NY 13210. USA.
Fax: (315) 470 6879
E-mail: [email protected]
Website: http://www.esf.edu

PhD Candidate and Professor, College of Environmental Science and Forestry, State University of New York, Syracuse, NY, USA. This research was supported with funding from the McIntire-Stennis Program and the New York Center for Forestry Research and Development.