101;;1

Moisture Content of Southern as Related

Charles W. McMillin George E. Woodson

Abstract Direction of borioB;=-tangential, radial, longitudinal. Nominal moisturecontent: 0, 3, 6, 10, 1', 30, Holes 3-1/2 inches deep were bored with a I-inch '0, and 80 peIceDt. spur machine bit in southern pine having specific gravity of 0.53 (ovendry weight and volume at 10.4 percent Held roobnt were: moisture). The bit was rotated at 2,400 rpm and re- type: spur mad1inebit. moved chips 0.020 inch thick. For wood moisture con- Drill diameter: 1 ind1. tents ranging from ovendry to saturation, thrust was Spindle speed:2,400 rpm lower when boring along the grain (average 98 pounds) Olip thickness: 0.020 inch (plunge speed of 1.6 than across the grain (average 138 pounds), while torque indICSper S«ODd). was higher when boring along the grain (average 42 Wood specific gravity: 0.'3 (ovendry weight and inch-pounds) than across the grain (average 33 inch- volume at 10.4 percentmoisture rontent). pounds). For both boring directions, torque and thrust The dlip thicknessand spindle speedare representa- increased with increasing moisture content to a maximum tive of commercialpractice. The spur madtine bit was at about 5 to 10 percent, then decreased to a constant value at about the fiber-saturation point. For the type of selected becauseit produces holes of good quality in bit tested, net power at the spindle required to cut 0.020- southern pine; geometrical specifications are given in inch-thick chips at speeds of 2,400 rpm or less should Figure 1. not exceed 2 hp., regardless of boring direction or mois- New bits were used for ead1 replication and ead1 ture content; thrust should not exceed 200 pounds. Chip drilling direction. Except d1at ~ minor imperfections types resembled those obtained in orthogonal . were correctedby hand honing, bits were put into test as they ame from the ~ of acturer. Severalhundred bO81'dfeet of rough-sawnPJtberD WOOD MOISTURE CONTENT affects tool forces and thus pine 4 by 4'5 were kiln-dried to 12 percentmoisture con. the energy consumedin madtining. Moisture also tent and arolratdy surfacedon four sidesto 3-1/2 by 3- interacts with other factors to influence the processof 1/2 indIes. They were then crosscutto foml 3-1/2-ind1 chip fo~tion and, hence,affects the shapeand size of cubes; only dear, defect-free wood was accepted. The the separatedparticle. While these subjectshave been cubes were placed on stickers (widt the end grain ex- explored for such operations as periphetal and posed) in a room maintained at 60 pettmt relative hu. sawing.few specific data are availablefor macltineboring midity and 73°F. Fans assuredadequate air circulation (5, pp. 347-358). dtroughout the stacksuntil the samplesread1ed constant In the researchtq)()rted here, an analysiswas made weight. of the rdationship of wood moisturecontent to the thrust force and torque required to madtine-boresouthern pine (Pm.s spp.) in the tangential, radial, and loogihldinal directions. Limited considerationwas also given to cer- tain aspectsof chip formation. The authors are Principal Wood Scientist and Ass0- ciate Wood Scientist, Southern Forest Experiment Station, ProcedUR USDA Forest Service, Pineville, La. They acknowledge A factorial experimentwith three replicationswas the assistance of Mr. Arthur Flack, H~t, Inc., MIl- waukee, Wis., in desi~ the boriDI machine. This paper designedwith variablesas follows: was recevied for publication in January 1972.

FOREST PRODUCTS JOURNAL Vol. 2a. No.ll 55 FI",re 1. - Geometrical.pedflcatlons of spur machinebit. A = m_n rake an91_20. P = meansharpne.. angle--6O. p. = sharp_s angle of spur.-35. 'Y = clearanceangle of 11,_10. . = angle of lead (.pur to lip, measured at drcumferencej-180. h. = heightof brad-O.20 inch hs = heightof spur-O.1 0 Inch L = le"gthof spurat root-O.53 InCh D = bit dlameter-1.00 Inch ft = bit radlu-o.50 inch ft = radiusof brad at root-O.09 Inch

~ .)..-,

From a 2-percentsample of the total population, the Specimenswere clamped in a attachedto a strain- averagemoisture content was 10.4 percent; the standard gage dynamometerdesigned to isolate torque from the deviation was 0.55. The averagevolwne per blodc was thrust force exertedon the workpieceand to measure~ 693.80 cc. with e. standarddeviation of 6.71. separately. The output of the dynamometerwas charted Blocks weighing 410:f:3 g. (i.e., those with 0.53 on a two-channd oscillographic recorder. The photo- specific gravity) were selectedfrom the population and sensitive relay system momentarily actuated an auxiliary separatedinto samplessuitable for radial, tangential, or longitudinal drilling by visual inspection of the annual ring orientationon the end grain (Fig. 2). Samplesfor each of the three directions were ran- RADIAL domly assigned to the eight nominal moisture-content classes. Blocks to be bored at zero percentmoisture con- tent were ovendried at 212°F.; hwnidity d1amberswere used to condition blocks to moisturecontents of 3, 6, and 10 percent. Moisture contents of 15, 30, 50, and 80 GRAI percent were obtainedby enclosing-el()c:ksin plastic bass with sufficient water to raise the moisture content to the / desired level. / Holes were made with a boring ma~i!ine especially / designedfor research. A 5-hp., synd1rooous-speed,3,600- rpm, alternating-currentmotor with timing belt drive as- ~ ~ sured ronstant spindle speed (2,400 rpm) while Wlder TANGENTIAL load. The spindle rotatedin a hydraulicallyoperated quill ~ / assembly;the desired feed rate of the quill was main- / tained by a temperature-pressurecompensated flow-control 1/ /' valve on the out-flow port. An electronictimer, acblated by a photo-sensitiverelay systemat the beginning and end LONGITUDINAL of the stroke,was usedto set and monitor the feed rate. ~re 2. - PrI-ry !lOrInI'directions.

56 NOVEMBER 1972

~ Table 1 RESULTS OF THRUST, TORQUE, AND MOISTURE CONTENT DETERMINATIONS.

0.0 146 32 0.0 109 39 4.4 186 35 4.5 135 42 6.8 175 36 6.6 120 48 10.7 150 34 10.6 107 52 19.0 120 32 18.7 81 39 32.7 117 33 32.4 78 39 56.3 100 30 56.7 84 40 79.9 108 32 78.2 72 38

pen on the oscillographwhen the tip (brad) of the bit WQSat depths of 1, 2, and 3 inmes. Torque and thrust were calculatedby applying a calibrationfactor to the pen deflection at eachdepth and averagingthe results. After holes were bored, the moisturecontent of ead1 block was determinedby ovendrying. Results Thrust and Torque When averagedover all levels of moisture content, the 'Valuesfor thrust and torquewere:

Drilling direction Thrust Torque - Figule3. - Effed of moisturecontent on thrust. (Lbs.) (In.-Ibs.) Tangential 139 33 Radial 136 32 Longitudinal 98 42 By variance analysis (0.01 levd) , thrust was lower and torque was higher in the longitudinal direction than in ei~ the tangential or radial direction. Thrust and torque in the tangential and radial directions were not significantly different by Duncan's multiple range test (0.01 level); this result agreeswith that reportedby Good- child (2). Accordingly, data for these two directions were averaged. Table 1 lists thrust and torque when blocks at the severalmoisture contents were drilled across -.. and along the grain.

FOREST PRODUCTS 10URNAL Vol 22, No. 11 57 where it was about equal to that required for ovendry wood. At all moisture contents, torque was less across the grain than along the grain. For engineering purposes, torque along the grain averaged about '2 inch-pounds for air-dry wood and 39 indt-pounds for green wood. Values aaoss the grain were aboot 35 and 32 indt-pouods. These torques may be used to compute net horsepower at the spindle at var- ioos spindle speeds (when 01tting 0.020-indt-thick dlips), as shown in the following tabulation. Tests with this bit type indicated that torque was unrelated to spindle speed when chip thickness was held. coostaot. . Spindle Plunge AIoas the IraJD A~ the IlaJD speed speeCI ~ ~ ~ ~ (Rpm) (In./sec.) - - - - - (Hp.)- - - - - 1,200 0.8 0.99 0.74 0.66 0.61 2,400 ~ 1.6 1.98 1.48 1.33 1.22 3,600 2.4 2.97 2.23 2.00 1.83 Power inaeases linearly with inaeasing spindle speed. since this parameter appears in the numerator of the power equation. In boring aloog the grain. aboot 25 percent less power was required for green than for air-dry wood. For cross-grain boring, moishlre content had little effect on power requirement. Olip Formation When drilling is in the longitudinal direction, the actioo of the lips (the cutting edges generating dlips) ap- proximates orthogonal cutting Iaoss the grain. Two gen- eralchip t)Ipes have been described by McKenzie (7) and observed by Woodson and Koch (12) for this direction. McKenzie Type I dtips form when splits 0CQ1tparallel to the grain below the cutting . The splits may be small. or they may be frequent and deep. The dlip gen- erated above the cutting plane is essentially equal to the depth of cut and consists of numerous subchips formed by FlgUN5. - TypicalchIps fonned when boring in two cllNc- lions 01 Ihree molsluN conlenls. The scale In A Is applicable 10 shear parallel to the grain. Type I dtips form more 8 ancl C as _II; lhe Icale In D II applicable 10 E ancl F. readily in wet than in dry wood. Type II chips form when wood failure occurs per- pendicular to the gram and at a variable distance below In ~-grain boring. trends in dJip formation were the cutting plene. The failure may be intermittent (Type more difficult to identify. becausethe cutting action of lIa) or continuoos (Type lib). The dtip above the cut- the lips continuouslyalternates between the veneercutting ting plane may be sheared into numerous subd1ips of ir- direction and the planing direction. regular shape and thickness or may be relatively continuous. Severalbasic modes of veneer formation have been Type II chips are most frequent in wood of low moisture recognizedby McMillin (9). Leney (6). and Woodson content. and Kod1 (12). Sample chips frmn this study were collected and ex- 1) Continuous- An unbroken sheet of veneer in amined with a low-power microscope in an attempt to whid1 the original wood structure is essentially detect general trends in formation with ci)aDgesin wood und1anged. unsture content. Typical dlips are illustrated in Figure 5. 2) Cantilever beam - Veneer formed when wood Otips produced at zero and 10 ~t moisture con- splits aheadof the knife and fails as a cantilever tent resembled those of McKenz.ie Type II (Fig. 5A, B). beam. Maximwn tensile stress in bending may aups formed above the cutting plane (upper portions of occur at variable distancefrom the cutting edge. the figures) were sheared into numeroos small subchips 3) Compressiontearing - Vmeer formed when the at zero percent moisture; they were longer and more con- cutting edge deflects the wood into a slight bulge tinuous at 10 percent. Otips formed below the cutting ahead of the knife. The compactedcells then plane (lower portions of the figures) were somewhat fail in tensi911either' above or below the cutting smaller for holes bored at zero percent moisture than for plane. boles bored 8t 10 percent. At 80 percent moisture. typ- Three basic dJip types are observedwhen madlining ical Type I chips WeIe formed (Fig. 5C). .Although in the planing direction (I. 3. 4. 12). shear failwes were present, most particles remained rela- Franz Type I dJips are formed when the wood splits tively intact. aheadof the cutting edge and the split portion fails as a

58 NOVEMBER 1972 cantilever beam. Low moisture contents favor formation dry wood (8). Temperatures near the cutting edge may of this chip type. exCeed300.C. (10), and low conductivity of ovendry Type II chips are characterizedby continuous diagonal wood may prevent the heat £ rom dissipating- with con. shear failures that extend from the rotting edge to the sequentreduction in strength and hencein thrust. Lower chip swface. An unbroken, smooth, spiral dtip is formed. thrusts may also have resulted from strength lossesthat Intermediate to high moisture contents favor formation of occurred when the sample blocks were ovendried. Type II chips. Torque is primarily related to forces exerted by the Type III dtips form when the wood ahead of the spurs and brad and the parallel tool force component of tool ruptures in shear and compression parallel to the the lips. Analysis of the oscillographs indicated that a grain and the deformed wood is then compacted against rdativdy constant torque o£ about 12 indl-pounds was the tool face. When the acromulation of compressed imposedby the spurs and brad for all moisture contents material becomes critical, buckling ocrors and the chip and drilling directions. By contrast, torque associated with the paralld tool force exerted by the cutting lips escapesupward. Examination of the particles produced at zero and 10 varied with both drilling direction and moisture content. percent moisture content indicated that Franz Type I chips In drilling along the gRin, the lips sever tradteids were generally formed when the lips were rotting in the perpendicularto their long . In cross-graindrilling, planing direction (lower portion of Fig. ,D, E). The fibers are cut in a plane paralld to their axes. Since chips generated at zero percent moisture were considerably tradleids ere strongerwhen in the perpendiculardiredion, shorter and less curled than those produced at 10 per- grClter torque would be expected when boring along than cent At 80 percent moisture, chips similar to Franz Type acrossthe grain. II were most frequently formed (Fig. 'F). For certain combinetioos of rake angle, depth of cut, Failures of the cantilever beam type were generally and cutting direction, the paralld tool force has been observed when cutting in the veneer direction at zero and shown to exhibit a maximw:n value with increasingmois- 10 percent moisture content (upper portion of Fig. 'D, E). ture content. For example, in orthogonal madtining of Failure occurred closer to the rotting edge for wood at loblolly pine earlywood in the planing direction with a zero percent than e.t 10 percent moisture. At 80 percent knife having a 25. rake angle, Woodson 8.DdKodl (12) moisture, chips appeared to form by failure in compres- reported average paralld £orces of 1.8, 5.7, and 2.9 sion tearing (Fig. 'F). pounds per 0.1 indl of kni£e at 7, 15.5, and 30 percent moisturecontent, respectively. It is possiblethat the paral- Discussion Id tool force exerted on the cutting lips of the bit ex- It is generally held that the thrust force in machine hibited a similar trend with increasing moisture. boring is relatedto the force requiredto advancethe spurs and brad into the work and to the normal force component exerted by the cutting lips (2). The relative influence Literature Cited of the normal force could be estimatedfrom the oscil- lographs by comparing the maximum dynamic thrust re- 1. FRANz. N. C. 1958. An analysis of the wood-cutting quired to advancethe spurs and brad into the surfaceof process. Ph.D. thesis. Univ. Mich. Press, Ann Arbor, Mich. 152 pp. the sampleblock to the averagethrust while boring the 2. GOODCHILD,R. 1955. The machine boring of wood. G. hole. Analysis indicated that no more than 5 percent of B. Dep. Sci. Ind. Res., Forest Prod. Res. Bull. 35, the total thrust was associatedwith the normal tool force London. 19 pp. on the cutting lips. It was concludedthat, for the type 3. KOCH, P. 1955. An analysis of the planing process: Part I. Forest Prod. J. 5:255-264. of bit used here, effects associatedwith the normal tool 4. . 1956. An analysis of the lumber planing force are small and can be neglected,i.e., 95 percentof the process: Part ll. Forest Prod. J. 6:393-402. trust force observedwas attributable to spurs and brad. 5. . 1964. Wood Processes. Ronald In boring acrossthe grain, the brad exerts forces Press Co., New York, N.Y. 530 pp. perpendicularto the long axis of fibers while the spurs 6. LENEY, L. 1960. Mechanism of veneer formation at the cellular level. Mo. Agric. Expt. Sta. Res. Bull. 744. cut in a direction which continuouslyalternates between 111 pp. the parallel and perpendicular axes. Since fibers are 7. McKENZIE, W. M. 1961. Fundamental aspects of the strongerwhen in a direction perpendicularto their axes, wood cutting process. Ph.D. thesis. Univ. Mich. Press, greaterthrust forces would be expec.tedacross than .aloog Ann Arbor, Mich. 161 pp. 8. MAcLEAN, J. D. 1941. Thermal conductivity of wood. the grain. Heat., Piping and Air Condo 13:380-391. Most strength properties of wood decreasewith in- 9. McMILUN, C. W. 1958. The relation of mechanical creasing moisture from ovendry to the fiber-saturation properties and nosebar pressure in the production of point. Above the saturationpoint, strength remainscon- veneer. Forest Prod. J. 8: 23-32. stant. It is probablethat the force requiredto advancethe 10. OKUSHJMA, S., H. SUGIHARA. and M. UMEMOTO. 1969. Temperature of cutter-cusp in wood cutting. J. Jap. spurs and brad is proportional to workpiecestrength, and Wood Res. Soc. 15(5):197-202. it was expectedthat thrust forces would follow the same 11. U. S. DEPARTMENT OF AGRICULTURE, FOREST PRODuCTS generaltrend with variation in moisture. However, oven- LABoRATORY. 1955. Wood handbook. USDA Agric. dry wood required less thrust than did wood at 5 percent Handb. 72. 528 pp. 12. WOODSON, G. E., and P. KOCH. 1970. Tool forces and moisture. chip formation in orthogonal cutting of loblolly pine. wood weakenswith increasingtemperature (11, p. USDA Forest Servo Res. Pap. SO-52, South. Forest 89), but wet wood has greater thermal conductivity than Expt. Sta., New Orleans, La. 25 pp.

FOREST PRODUCTS JOURNAL Vol. JZ. No. II 59