andof WoodThein

CHARLES w. l~cMILLINResarch Engineer, Central Research Laboratory, American Machine and Foundry Co., Stamford, Conn.

Observations of checking frequency, depth of check penetra-tion, veneer thickness, and surface quality were made at 20 machin-ing conditions. An inverse relationship between depth 01: check andfrequency of checking was established. The effect of cutting tempera-ture was demonstrated, and strength in compression pftrpendicularto the grain, tension perpendicular to the grain, and rolling shearwas determined at 4 temperatures. The mechanics of venet!r formationis discussed with respect to the 2 basic veneer types obsE!rved.

geometry, a description of cutting no-menclature is a necessary antecedentto further discussion.

The terminology adopted for use inthis study is defined be1ow, with ge0-metrical relationships shown in Fig. 1.

Line AB-A. line perpendicular to aline tangent to the workand passing through thecutting edge of the knife.

C- Gitting angle, the anglebetween the face of theknife and line A.B.

D- Sharpness angle of theknife.

S- Oearance angle, the an.gle between the back ofthe koife and line A.B.

1- Verticalnosebar~g,the vertical distance be.tween the maximumpoint on the nosebar anda plane described by thecutting edge of the knife.

G- Nosebar bevel, the anglebetween the nosebar andthe horizontal.

H- Horizontal nosebar open.ing, the horizontal dis-tance between the nose.bar and Line A.B.

Gitting velocity-The velocity of thework piece relative to theknife and nosebar.

')(\.

~

Rotary veneer-cutting studies byFleischer ( 1 ) a have revealed that anoptimum knife angle .=xists for a givenspecies and thickness to produce veneerwith a smooth, uniform surface. Fur-ther, Fleischer has shown that, in gen-eral, the cutting anglc~ should increaseas the thickness of tlle veneer cut de-creases, and that a reduction in thehorizontal nosebar opening will resultin veneer that conbUns fewer lathedtecks per inch.

Kivimaa (2), by w;e of a tensile testto determine veneer quality, has dem-onstrated that an o'ptimum nosebaropening exists for a given species.

In work concernin,g veneer smooth.ness, Lutz (3) has shown that smooth-ness of knife-cut veneer is affected bythe orientation of wood fibers, the an-n\Jal rings, and the wood rays withrespect to the plane of the cut.

Behavior of individual wood cellsduring slicing has beoen investigated inCanada (4). Wood a:lls were observedto be compressed by the nosebar andknife edge and by thc~ resistance causedby friction against thc~ nosebar face andthe knife back. In addition, importanttension stresses werc~ observed actingin front of the knife edge in an ap-proximately radial direction.

The investigations summarized aboveprovide plausible foundation to assumethat medtanical properties of wood arean important factoJ: in the veneer.cutting process.

L\-- \'"

T-~--

W OOD VENEEIlS REPRESENT, in ourpresent-day economy, a product

of countless commercial and industrialapplications. Furniture, plywood,molded veneer products, boxes andcrates are, to mention but a few, well-established uses of veneer.

Although veneer is a common andaccepted wO()d product, little basic re-search has been directed toward anunderstanding of the veneer-cuttingprocess. Information accumulated inthe past, although of recognized value,has largely been obtained from experi-ence. Lathe checks, surface irregulari-ties, thickness limitations, and variedproduction techniques are present ob-stacles to continued industrial progress.

It follows that basic research in thefield of veneer-cutting is needed. It isobvious, however, that a comprehen-sive study of the many aspects ofveneer-cutting would be of a magni-tude beyond the scope of an individualresearch effort. The work describedherein, therefore, endeavors to estab-lish what takes place when wood iscut into veneer, and pursues the rela-tionships of mechaniCal properties ofwood and nosebar pressure. The studyis directed primarily to a considerationof the rotary-cutting process.

Review of the Literature

Although numerous publicationsmay be found in veneer-cutting litera-ture ~ning to production methodsand techni~es, only limited materialconcerning the fundamental aspects ofveneer formation has been published.

1 Sccoftd-place wiftfter iJI the 1957 WoodAward competitiOft.. A coftdeo1ltioft of a thesis submitted forthe degree of Master of Wood Technology atthe Uftmnity of Michigaft.

N

//

/'"~

Nomenclature '

Cons!derable conflJSion exi~ts ~n the \veneer mdustry as to the application ofvarious descriptive t:erms Used in theveneer-cuttingrroce;s. Because of the

- . 0 0 0 common use 0 several manufacturingTk AMI'..: Charles McMillm obtamed hIS L_.J. d th Db. ..BS decree from Purdue Unmrsity. He received U1CU~ an e II Igwtles of tool a

his Master of Wood Technology from the Uni. - .venity o~ Michi4&~ in 1957. and is currently oa Numben in parenth.e;es refer to Literature Fig. 1.-Geometrical relationships in orthogo.

encaged In machining research. Cited at the end of thIS paper. nal cutting of ven..r.

Reprinted from the January, 1958, Forest Prod\Jcts Journal (Vol. VIII, No.1), pages 23-32Forest Products Research Society, Po O. Box 2010, University Station, Madison 5, Wisconsin

Fig. 3.-Asa..bled orthogonol ven--cutting fixture showing .ethod of using ref.erenCes foces to polition the nolebar.

fig. 2.-Oisoll...bled orthogonal yen_-cutting ftxture. showing knife. nos. bar. andmounting.

The above species were selected torepresent a harawood and a softwoodboth of commercial importance and ofwidely divergent physical and mccbani-cal properties. Both the sapwood andheartwood of yellow bircft were se-lected for consideration since field 0b-servations indicate that each displaysdistinct veneering dJaracteristics.

Vertical ~-! openings were se-lected to represent a range from zeropressure to pressure resulting in over-compression of dte veneer. Geometri.cal cutting relatiooshi~ were selectedfrom current veneer-cutting practice.

In addition. limited considerationwas given to the determination ofthe coefficient of friction for dteabove-mentioned woods. This wasdeemed necessary. since previous wood.machining studies have indicated dteimportance of frictional forces (5).

Orthogonal Veoeer-Cutting Sys-tem: Because orthogonal cutting ofveneer necessitated the development ofa new approad1 to ven~r research, adetailed aescriptioo of the cutting fix.ture is necessary.

Any cutting system developed shouldmeet the following general ~!Dents: 1) dte systan must allow foracaIrate, easy adjusbnent of the nose.bar in both the horizontal and verticaldirections, 2) deBection of the nosebarand knife as a result of cutting forcesmust be at a minim~~m, 3) the devicemust be of sufficiently heavy constnJC-tion to allow secure positioning of the

demonstrated to bear a linear relation-ship to s~fic gravit), and moisturecontent. Suhberger «(» has shownthat. for a number of 5~es tested at1.5 . per cent moisture content, maxi-

mum crushing strength. modulus ofrupture, and modulus of elasticity de-crease from about 2.5 to 5.0 per centfor ead1 10° F. temperature change.Greenhill's (7) studies of the relationof temperature and moisture contenton woOd properties reveal that, at180° F., the tensile strength and modu-lus of elasticity dkular to thegrain are :d~J~~ to Y2 of theirvalue at room temperature.

Therefore, by alterirlg the tempera-ture of several woods while cutting atvarious vertical nosebar openings, itwas possible to study h,:>w veneer qual-ity was affected by the relationships ofwood mechanical prop'~es and nose-bar pressure.

Experimental Procedure

Design of the Experiment: A totalof 20 cutting conditions with three~lications were used to study theinteraction of mechanical propertiesand nosebar pressure. The followingexperimental design W'8S adopted:

1 ~es-Yellow birdt (&1#/41#1'. Midlx. f.) heartwood andsapwood, and re.iwood (5"'#0;.Jnnp".,;r,nJ D. Don End!.).

2 Nosebar opening-Vertical, 0,3,6. 9, 15 re;r cent less than theveneer thiCkness; Horizontal,0.025 inches.

3 Temperatwe0-800, 120°, 160°,and 200°.

4 Veneer thiCknes o.125 irKh.5 Oearance angle--90 degrees.6 Cutting angle-70 degrees.7 Sharpness angle--20 degrees.8 Nosebar angle--15 degrees.9 Cutting velocit)'-67 feet per

minute.

10 Mechanical tests-Tension per-pendicular to the grain, ampra-sion ~diCUJ~ to the grain,and roll1Dg shea:r.

Purpole and ApproachI'he purpose of this study was to

indicate how mechanical properties ofwood and n<JSebar openings affect thequality of wood veneer.

Exploratory altting investigationssuggested several general areas of re-search. B<Xh horizontal and vernalnosebar openings, as well as cuttingangle, were observed to effect veneer~ty.. Further, it was indicated that!tigher ~n,.. velocities may materiallyIncrease checking.

While it is recognized that all theabove factors inBuence the veneeringprocess, it was necessary that a limita-tion of scope be imposed to consideronly diose variables within the magni-tude of an individual study.

The vertical opening was determinedto be the more critical nosebar adjust-ment, and was selected as a variablewhile horizontal ~ opening washeld constant. In addition, it was feltthat more informative data would beobtained if altbng velocity and altbngangle closely approximated currentpractice.

Further, it was recognized that theuse of production-size equipment pre-sented serious limitations in both a-perimcQta1 design and control of aIt-ting variables. A method was neededto produce veneer on a small, experi-mental scale with precise control ofaltbng conditions. Orthogooal cutting~dicular to the grain, as sug-gested in the wood-mad1ining litera-ture by Franz (5), presented a favora-ble solution to the problem.

In orthogonal cutting of veneer, theknife edge and nosebar are perpen-dicular to the direction of relative m0-tion of the altting fixture and workpiece. This generates a plane surfaceparallel to the original work surface.If a small segment of the outer periph-ery of a large veneer bolt is consideredto approach a straight line, small ve-neer strips produced ~ental1y byorthogooal altting may be consideredanalogous to veneer produced by therotary-cutting process.

During the cutting of veneer, pre-liminary observations indicated that,among <*her stresses, the knife mayinduce stresses in the wood in tensionperpendicular to the grain, compres-sion perpendicular to the grain, androlling shear. Concurrently, the nose-bar may produce stress in compressionperpendicular to the grain and rollingmear. It follows that the strength ofa given species in these properties isan important consideration in analyz-ing the altbng characteristics of thewood.

Most strength properties of wooddecrease as tanperature maeascs. Thedegree of strength change has been

Fig, 4-Orthogonal yeneer.cuHing sYSfe8,showing cutting ftxture, milling machine andyi..,

~

'I\:" -

~~A

-_Sa -,lAm

~ .~~~~::::~::~ ]

~Fig. 5.-Sketch showing method of cuttingveneer test specimens to obtoin a favorablegrowth ring orientation.

knife and nosebar, and 4) the systemmust incorporate a means for obtainingaccurate and uniform relative motionbetween the cutting edge and the work.

The cutting fixture developed is pic-tured in Figs. 2 and 3. Precise p<:>-sitioning of the nosebar and knifemay. be accomplished with two accu-rately machined reference faces. Theknife, attached to the bottom surfaceby two large bolts, was slotted so thatafter sharpening it could be movedforward to the plane of the referenceface from which horizontal nosebaradjustment was determined. The nose-bar was adjusted in both the horizontaland vertical directions by shims placedon the two reference faces. The cut-ting angle was determined by theangle between the bottom face and thehorizontal.

A milling machine provided a sat-isfactory method of traversing thework piece while holding the cuttingapparatus stationary. The s~enswere held in place by a vise affixed tothe milling-machine table. The cuttingfixture was mounted on an extensionof the machine head after the arborhad been removed. By movement ofthe table, a portion of the specimenpassed between the knife and nosebarto form a small veneer strip. Fig. 4pictures the assembled orthogonal cut-ting system.

l~Fig. 7.-Skekh showing dimensions and

grain orientation for rolling shear and cleav-age test specimens.

All mechanical test specimens werestored in water at room temperatureuntil used at the various temperaturelevels. Standard 'procedures were usedto compute the mdividual mechanicalproperties.

Temperature Control: Temperaturecontrol for mechanical properties testswas accomplished by means of con-trolled-temperature baths. Before thes~ens were tested, they wereheated for two hours in water of thespecified temperature. The specimenswere then removed from the water andimmediately tested. Preliminary ex-periments showed that the temperaturedrop during testing was negligible andthat accurate values could be obtainedin this manner.

A more complex means of tempera-ture control was required during ve-neering. Before the veneer was cut,the work unit was heated in water ofspecified temperature for a period oftwo hows. Because of inherent delaysin experimental procedure, the oppor-tunity for tem~rature loss existedduring cutting of the veneer strips.An external source of heat was there-fore required to maintain the testspecimens at a constant temperature.To accomplish this, a heavily Insulatedcontainer with an dectrical heatingunit and thermostatic control was de-veloped (Fig. 8). Water from thecontainer was fed to a spray unitmounted in front of the work piece(Fig. 9), thus permitting 'the heatedwater to flow over the work and main-tain the desired specimen temperature.

Method of Cutting: Prior to cut-ting, the sharpened veneer knife wasadvanced to the lower edge of thereference face, from which the hori-zontal nosebar o~g was deter-mined, and was seCurely bolted to theassembly. Precautions were taken to

Preparation of Test :5pecimens: Inthe selection of material for study, itwas necessary to impose several restric-tions to minimize variation in woodproperties and to facilitate accurateanalysis of the influence of wood prop-erties on veneer quality. The followingrestrictions were placet! on all testmaterial: I) only straigJ:rt-grained, de-fect-free, even-growth material wasused, 2) all veneer aJld mechanicaltest specimens were end-~ tominimize specific gravity variation, and3) all specimens were taken from thesame band of growth rings.

Samples of redwood, heartwood yel-low bird1, and sapwooci yellow birchthat meet the above reqlnrements wereobtained in the green condition andstored in vapor-proof '1frapping untilcut into test specimens. Green woodwas selected so that the test materialwould be reasonably free from internaldefects and stresses resulting from dry-ing. Ead1 board was cut into threeparts, veneer-cutting s}>ecimens beingcut from the central portion, andmechanical test specimens from the endportions.

The veneer test specimens, 4 inchesby 2¥2 inches across th(: grain and 1¥2inches along the grain, were cut fromthe stock with the gro~rth rings orien-tated at an angle of approximatdy 20degrees to the surface lto be machined(Fig. 5). A veneer test specimen fromeach of the three wocod samples wasthen attached to a b31cking strip .forconvenience in handling (Fig. 6),with the growth ring-,i positioned sothat the springwood polction of each in-crement would first be Jpresented to thetool edge. Thus, a uniformly favorablecutting condition was obtained in ac-cordance with the findings of Lutz( 3 ). The composite work unit washeld in water at roc.m temperatureuntil use to make cernun the materialremained above the fiber saturation

point.Two specimen blanks for each of

the three types of mec:hanical tests tobe conducted at the four temperaturelevels were cut from the end portionsof each wood sample.

A cleavage specim(~n was used todetermine strength values in tensionperpendictular to the grain, since itwas reasoned to be a more realistict~t than the standard tension speci-men (5, 8). In both the cleavage andthe shear-test specimc~ns, certain de-partures from standard dimensionswere necessary as a result of the satu-rated moisture condition and the man-ner of grain orienaLtion (Fig. 7).Standard test-specim(n dimensionswere used to determine compressionperpendicular to the Brain (8).

Fig. 6.-Sketch showing work piece unitconsisting of 0 specimen of sapwood andheartwaad yellaw birch and redwood.

Fig. 8.-Genet'Ol view of the experimenloloreG. showing heoting contoiner for moin-toining constont temperuture during cuttingof veneer.

Fig. 9.-Water spray unit for maintenanceof specimen temperoture during cu"ing.

make certain the fixture establishedaccurate orthogonal cutting conditions.

Sufficient shim stock was placed onthe top reference face to raise the ex-treme edge of the nosebar to the cut-ting edge of the knife. By placingadditional shim stock on the two ref-erence faces, precise horizontal andvertical nosebar adjustment was ac-complished. Adjustments of the nose-bar were made during cutting at highertemperatures to com~ for thermalexpansion of the cutting fixture.

The heated work unit of wood speci-mens was securely mounted in thevise, and water of specified tempera-

Table 1.-$UMMAIY OF QUALITY DETEIMINATIONS FOI SAPWOODYELLOW BIICH MACHINED AT VARIOUS NOSEBAR

OPENINGS AND TEMPEIATURES

Fig. 10.-Ven.., $trlPI produced by orthogo_1 cuHing, showIng _rfed edgeand expoled lalhe checks.

ture was allowed to flow over the cutting by employing a two-componentwork unit as previously described. dynamometer. The dynamometer andSeveral light cuts were made to assure experimental method employed is de.a parallel surface with respect to the sciibed in an analysis of the wood.cutting edge of the knife. The milling. cutting process by Franz (5).machine table was thc~n raised 0.125 Interpretive Techniques: To beinch, and the work unit was advanced considered of the highest qUality, ve.past the nosebar and knife. One ve. neer must be 1) of minimum surfaceDeer strip was cut and discarded to roughness, 2) uniform in thickness,assure that the test veneer would have and 3) free f~ lad1e ~ecks. It wascharacteristic surface quality. After recognized that the variables undermeasurements were n1ade for green consideration would produce veneerthickness, the veneer strips were num- that would fail in one or more of thebered consecutively for identifICation above considerations. It was thereforeand placed under wire screens in a felt desirable to have three independ.16 per cent ~uilibrium moisture con. ent measures of veneer quality.tent conditioning roOJD for a period The frequency and depth of latheof two weeks. checking was determined for each ve.

Additional considera.tion was given Deer strip. By lightly staining the looseto the determination of instantaneous side with a fast-drying alcohol staintool forces indu~ on the knife during and beveling a half-inch surface on

1°able 2.--SUMMARY OF QUALITY DETERMINATIONS FOR HEART.WOOD YEllOW BIRCH MACHINED AT VARIOUS

HOseAI OPENINGS AND TEMPERATURES

Depth ofeheek,

m.

0.0780.0680.0820.0.80._1

0.0680.066O.~0.0280.018

o.~0.-o.casO.~0.016

O.02A0.0210.0200.0110

Dry..,; iJo.

0.12.o.~o.~0.1210.116

0.128O.~0.1280.1220.11.

0.1810.1210.1200.1200.11.

0.1290.1260.1280.1210.118

~oleheek,

In.

0.0880.-0.0710.0&10.'"

0.-0.0720.052O.~O0..'

0.0860.~50.-0.-0.012

0..40.0150.0140.0070

~hi.

0.1"0.1240.1240.1220.118

0.11.0.1240.111O.l1A0.11.

0.1100.1%10.1%10.1%1O.l1T

0.1150.1240.1100.110O.Ut

N.-b8r~per _t

0869

15

08II9

15

0I6t

11

0..t

11

N-"ar0Pfttnc. CheekiDCPm' _t f1'equeey

8.'711.0la.115.'722.0

9.010.'714.015.018.'7

9.110.'718.120.150.0

16.028.025.089.0

0

~ . P.

1iO M 1iO 1iO M m m m m ; 180 1 1 1 ~.-

1 1 1::::::::::::::::

100 ~..

0a89

15

0a69

15

0a69

16

0889

16

T~ . r.

10 :::::::::::::::::

10 ; I::::::::=::-:::::

110 110 ;..

110 180 110 110 110 180 100 100 100 =::::::::::~,

4

G~tillea-.

In.

0.1290.1210.1280.1260.121

0.1280.1110.1280.1270.118

0.1S60.1280.126O.IU0.119

0.1880.1110.1260.1260.121

G-~

In.

O.12t0.1800.1800.12'70.128

0.1800.1800.1170.1260.120

0.18'10.118O.U'I0.1170.128

0.1800.1280.1260.126O.IU

Ch8ekiD8frequ8le)'

10.&12.018.817.0!I.O

10..12.01&.018.021.0

10.01..&.8.011.027.&

16.&88.088.0&1.0

0

Tabl. 3.-SUMMARY OF QUAlITY DETERMINATIONS FOI REDWOODMACHINED AT VARIOUS NOSEaAI OPENINGS

AND TEMPEIATURES

NoeebaroP8Dhlc.per _t

036I

15

088I

16

086.

15

008t

15

~ofcbck.

iIi.

0.1280.1150.1070.1000.098

0.0810."0.0760.070O.u..

00000

00000

Cbeekiacfreq_q

6.57.5t.O

10.512.0

6.08.0

10.012.518.0

00000

00000

~.J'.80.80.80.80.BO.

120.120.120.120.120.

180.180.180..180..180.

200.200.200.200.200.-

the stained edge ( 9) , lathe checkswere exposed as dark lines extendingthrough the thickness of the veneer(Fig. 10). The nwnber of checks ex-~ over a 3-ioch length was deter-mined, and the frequency of checkingwas expressed as checks ~r ind1 oflength. The average d~ of checkpenetration was determined by apply-ing the ratio of bevel length and drythickness to the average length of thedIecks exposed on the scarfed surface.

Veneer thickness was determined at5 equi-distant points along the lengthof the veneer strip in the green anddry condition with micrometer calipers.The thickncss of the veneer was con-sidered to be the average of the fivemeasurements. Visual examinationserved to establish surface quality.

Results of Veneet QualityDeterminations

As previously implied, madtiningconditions were sud1 as to produceveneer of a wide range of quality.Data obtained from quality determina-tions are summarized in T abIes 1through 3. Relationships and trendsexttacted from these data are showngraphically in Figs. 11 through 14.

From the data, it will be noticedthat veneer thickness measwed in thegreen condition was greater than thenominal cutting dimension of 0.125iod1. It was observed that the in-creased thickness was accompanied bya corresponding shortening of the ve-neer along the cutting direction. Thesephenomena are similar to those in themachining of metals as described byMerchant (10).

~ Greent.J.1ckn- thlckn_.

in. In.

0.110 0.1820.129 0.1800.118 0.180o.m 0.1280.111 0.124

0.l2t 0.180o.m 0.180o.m 0.1290.128 0.1270.121 0.128

0.119 0.1820.111 0.1810.128 0.180o.m 0.1280.124 0.126

0.128 0.1800.1H 0.1290.116 0.1280.125 0.1270.122 0.124

- -

frequency were relatively shallow. Figs.11 through 14 reveal that lathe check .ing . was materially reduced by in-creasing nosebar pressure or cuttingtemperature.

In general, die surface roughness ofall woods tested decreased with an in-crease in temperature or nosebar pres.$Ute. In the ase of yellow bird1,sUrface quality appeared related to theseverity of dl«king. Shallow checksof high frequency were in all casesindicative of good surface quality.With redwood, however, surface qual-ity did not appear to be so closelycorrelated with dtecking. Although nochecking was observed when machin-ing was done at 1600 P., surfaceroughness was severe and only slightimprovement was noted when veneer-ing at 2000 P.

The reduction in veneer thidcnesswith smaller nosebar openings sug.gests that the attending pressures weresufficient to cause compfl5Sion set ofthe wood fibers. Due to the rigidityof the nosebar, the amount of woodcompression was positive, and defor-mation rather than strength in com-pression perpendicular to the grainappealS to be the limiting factor. Thisis evidenced by measurements of greenand dry thickness dimensions, whidtdisplay relatively uniform differencesat all nosebar pressures and thus reofIect uniform sh~ from the greenthickness during drymg.

An interrelationship appealS to ex-ist between temperature, nosebar pres-sure, checking frequency, and depthof check penetration. 01ecks of rowfreq~ were characteristically severein penetration, while checks of high

fig,

0 3 6 9 12 1&'o.e~1' Open1na - ~

1 .-1..10.. of nosebor preslure 10 depth of check penetrationfer wGod. machined at 80. F.

.100

.090 ..080

SIfi .0'10..

i.

~

. .060

0-

~

!.., .ceo

OS

5~! .040

. 28oW01.u

:I e.~... ~

~

"11.~

.~

,010

200B) 100 120 140 160 160 200 ~ 100 120 1.40 160 1~h_re ture - Op Telllpeft t.ure - Op

Fig. 13.-Relation of temperature to depth af che<k penetration for Fig. 14.-Ielotion of temperature to frequency of Iothe checks forwoods machined with verticol nose bar opening of 9 per cent. woods machined with vertical nosebar opening of 9 per cent.

Results of Mechanical u = the coefficient of friction ments that the cutting process is de-Properties Tests Kn = the normal force component fined by the nature of wood failures

The med1anical properties and co- on the knife ahead o~ the cutting ed~, and thatefficient of friction for redwood, sap-: Kp = the parall~l force component these f~lures are a. function o~ woodwood yellow birch, and heartwoOd on the ~fe mechamcal properties an~ cutting ge-yellow birch were determined at the C = the cuttmg angle ometry ( 5 ). These find!ngs can beprescribed temperature levels. Data It may be note.d that values obtained dem°n.strated to be applicai;>le to theobtained are summarized in Table 4 for the coefficient of friction are very ven.eermg process.. Observations madewith graphical representations shown high, and in some cases greater than during ~e formation of veneer showin Figs. 15 through 18. Only those one. Similarly high values for the coef- tha~ basl.c ven~r trres. are genera~dproperties considered to be pertinent ficient of friction have been reported v.:hlch ?Isplay Identifymg charactens-to the veneer-cutting process are by Ernst and Merchant in metal ma- tiCS. It IS suggested, the~fore, that ve-included. chining studies (12). neer type can be assOCIated to wood

Th .1 . I properties and cutting conditions bye tensl e stress m c eavage was Discussion of Observational Data f anal . f f I ti. nsh.

caluculated b the nation for eccen- means 0 . ys15 0 orce re a 0 .1£!S.tric loading r 11) eq Veneer Type: It has been shown Tw-o basic types of veneer were dis-

F 'Fec in previous wood-machining experi- tinguishable in this study:S = - + - where,A I Tobie 4.-SUMMARY OF MECHANICAL PRO PERTIES AND COEffICI~TS = stress in tension, in pounds per OF fRICTION DETERMINED FOR WOODS TESTED

square inch AT VARIOUS TEMPERATURESF = the applied load, in pounds -

A = the area under stress, in squareinches

e = the eccentricity of the appliedload, in inches

c = the centroidal distance ofarea A, in inches

I = moment of inertia of area A.The coefficient of friction was calcu-

lated from the equation used by Franz(5) in a study of the wood-cuttingprocess.

u = tan (arc tan Kn + C)

Coefticlentof frictionTemp.,. F.

1.181.061.82

80-120.160-200-

Tension Comp~on~dlcuIar ~dleularto the ~in RoniDC .hear to the 2r8in

pal pSi pel at 1'.L.

REDWOOD820 241 815260 202 272192 169 240141 128 161

SAPWOOD YELLOW B :IRCH987 619 684696 572 579571 4S8 889860 840 281

HEARTWOOD YELLOW BIRCH917 518 747792 «5 517596 850 ,.890 269 297

0.990.860.81

BO.120-160.200.

0.870.'710.'78

80.120.160.200.

'K;6

1000 ~ ,~~

~«» ~

~ ow.,~'1tX)

"'i ~~ ~jI~

-~eoo..I.

'100...So..

G400

~11::';

=600~I..

&00

[~COMtassI~~

'.,aoo aoo

('~

~ ~

100100110 100 120 140 180 180 2CX)

\'88P8re tun - Op

Fig. 16.-Ielatian af heartwood yellow birch ..chanlcal properti..ta temperature.

~ 100 18) 1«) 180 1~ BOO

~reture-Op

'-It. 15.-I~lotiOfl of lopwood yellow birch ..echonlcol propert;'1to temperotu,e.

~

300

-~

240..8.

.~..

\j~I. (At P.L.)

~\ \.,.- "K ,"

l~ " \

~110

~1~

1~00 100 1.."0 lfO leG 100 800

T.~l'8ture - Op

fit. . 7.--elatlon of red~ ..chanical properties 10 ".peoatv...

Fig. 20.-Type 8 veneer, showing development of continuous tensionfoilure perpendiculor to the cutting path.

Fig. 19.-Type A veneer, showing split portion ohead af the knifeand foilure as a beam.

resulting from additional movement ofthe knife induces tensile stress perpen-dicular to the cutting path which ulti-mately exceeds the strength of thewood in tension pe~dicu1ar to thegrain. Thus, wood failure more closelyapproximates a continuous peelingaction.

The continuous, uniform process ofwood failure described above resultsin veneer that contains no lathe checksand is of superior surface characteris-tics. Type B veneer, therefore, repre-sents the ideal veneer from the stand-point of quality.

The production of Type B veneerappears to be a balance between forcerelationships and mechanical proper-ties. Factors contributing to formationof Type B veneer include the follow-ing: 1) relatively low strength in ten-sion perpendicular to the grain as com-pared with strength in compressionperpendicular to the grain and rolling

the ultimate check with respect to sub-sequent cycles is clearly de~ndentupon the length of the spltt. Thelength of the splitting failure ap~to be determined by the strength ofthe wood in rolling shear and compres-sion perpendicular to the grain.

The magnitude of tensile stress thata wood may sustain before failurethrough bending determines, in part,the depth of check penetration. Thedepth of check ~netration may, aspreviously discussed, be increased byadditional deformation after bendingfailure has occurred.

Several factors appear conducive toformation of Type A veneer. In thework, low resistance in compressionperpendicular to the grain and rollingshear when accompanied by relativelyhigh tensile strength ~dicular tothe cutting path favor development ofthe splitting failures and ruptures dueto accompanying bending stresses.From the geometry of veneer-cutting,it can be shown that a low coefficientof friction at the interface of the knifeand the work as well as tool force dis-tributions attending the large cuttingangle are also contributing factors.

Type B Veneer Formation: Ve-neering conditions may, within a limoited range, favor the development of acontinuous tension failure that extendsin a plane parallel to the cutting path.In the formation of Type B veneer,relative movement of the knife alongthe cutting path strains the wood aheadof the tool in tension perpendicular tothe grain (Fig. 22). Continued ad-vancement of the knife produces fail-ure in tension at the knife edge andinduces compression and rolling shearstresses parallel to the cutting P8:th.The indUced stresses are not exceededin ultimate strength, however, and thesplitting failure associated with Type Aveneer does not develop. Deformation

Type A veneer is formed when cut-ting conditions are sum that the woodsplits ahead of the knife by shear andcompression until failure of the splitportion occurs in bending as a canti-lever beam, as shown in Fig. 19.

Type B veneer occurs when toolforces cause continuous tension failuresin a direction perpendicular to the cut-ting path, as indicated in Fig. 20.

Formation of Type A Veneer:Type A veneer appears to be producedthrough a cyclic series of events. Atthe start of the cycle, the knife ad-vances to the work edge and produceslocalized areas stressed in tension in adirection perpendicular to the cuttingpath (Fig. 21). Continued movementof the knife relative to the work strainsthe wood ahead of the tool, producesfailure in tension perpendicular to thegrain at the knife edge, and inducesstress in compression perpendicular tothe grain and rolling shear acting in aplane parallel to the cutting path. Asudden splitting failure occurs aheadof the knife edge when the ultimateStrength of the wood in rolling shearhas been exceeded. As the knife ad-vances, the split portion is deflectedup the face of the knife and is stressedin bendIng as a cantilever beam. Asbending progresses, stresses becomecritical and failure occurs as a canti-lever beam at the point of maximummoment (Fig. 21). Continued move-ment of the knife relative to the workmay produce additional deflection andaccentuate the failure until the cuttingedge contacts the point of bending fail-ure and initiates the start of anothercycle. Repetition of the above series ofevents produces veneer that containssevere lathe checks and rough surfacecharacteristics.

Maximum tensile stresses develop atthe point of maximum moment of thecantilever beam. Thus, the location of

R

tJ~ . ,

shear, and 2) high coefficient of fric-tion between the knife face and thework.

The signifiance of force relation-ships within the veneer particle, as in-fluenced by the presence or absence ofnosebar pressure, will be discussed insubsequent paragraphs.

~. +

r,Fig. 23.-Force system suggested for dis-

cussion of Mechonics of ven..,-cutting. No.e-bar pressure obsent. ~

Fig- 24.-Force system suggested for dis-cussion of mechonics of veneer-cutting. Nose.bot pressure present.

The amount of nosebar pressure re-quired to create sufficient tensile stressfor production of Type B veneer ap-~ to be a baJance between forcerelationshiJ':s and ~~cal strength.If the limiting strength of the materialin tension perpendicular to the graindoes not approach or fall below thelimiting strength in rolling shear andcompression, additional nosebar pres-sure is required to produce the desiredtensile stress in the veneer particle. Ifnosebar pressure is insuffiCient or iftensile strength is high relative to thestrength in rolling sheac and compres-sion, Type A veneer will be produced.Thus, with the application of con-trolled nosebar pressure, the occurrenceof lathe checks may be reduced oreliminated.

As previously stated, high frictionalforces appear to favor the productionof Type B veneer. From the geometryof cutting, it can be shown t}lat largefrictional forces at the interface of theknife and the wood have the net effectof reducing tensile stresses resultingfrom bendiny,o

sion perpendicular to the grain is lowwith respect to strength in compressionand rolling shear, conditions are fa-vorable to a continuous tension failurein a plane parallel to the cutting path.The forces applied to the ~cle bythe knife produce failure In tensionbefore the limiting compressive andshear values can be exceeded, and thepeeling action previously descn"bed asproducing Type B veneer results.

Nosebar Pressure Present: Fig. 24approximates the force relationshipsthat exist in the undefonned veneerparticle when the effects of nosebarpressure are taken into consideration.As in the absence of noscbar pressure,the advancing knife exerts resultantforce RJ( on the undeformed particle,and is resolved in horizontal and verti-cal components K. and K.. The nC»e-bar exerts resultant force RJ( to theparticle, and is resolved into horizontaland vertical components Np and N..The force components K. aDd Np areresisted by compressive force FCp andshearing force F.. The com~nent N.is resisted by compressive force F 08'

while force K. is resisted by tensileforce Ft.

Non-colinearity of the horizontaland vertical force components of theknife and nosebar tends to rotatethe particle under consideration, andthereOY creates compressive stresses atthe upper surface of the particle andlarge tensile stresses perpendicular tothe cutting path near the knife edge.In response to these stresses, the parti-cle is initially strained so that bendingstresses resulting from deftection of theveneer by the knife are minimized. Theabove (orce relationships favor theproduction of Type B veneer.

Without nosebar pressure, tensilestrength perpendicular to the grainoften cantiot ~ sufficiently reduced bytemperature adjustment to produce thedesired continuous tension failure andproduction of Type B veneer. Theapplication of noscbar pressure to theparticle, however, creates sufficient ini-tial tensile stress so that the additionalstress resulting from the advancingknife exceeds the ultimate strength ofthe wood in tension perpendicular tothe grain, thus producing the desiredpeeling action.

Application of Visual Observationsand Theoretical Analysis to

Quality and MechanicalProperties Data

Comparison of Tables 1 through 3show that, as machining conditions ap-proach those required for productionof Type B veneer, the severity ofchecking and surface roughness arereduced. It will be noted that, althou9;hseverity of checking is reduced by theapplication of nosebar pressure, thefonnation of Type B veneer requiresthat tensile strength of the material below as compared to compressive andshearing strength.

Table 1 indicates the sapwood ofyellow birch to have slightly superiorveneering characteristics when veneeredbelow 2000 F. Figs. 15 and 16 showthat tensile strength as com palM toshearing and ccxnpressive strength ismore favorable for production of Type

Discussion of Mechanics ofVeneer Formation

An exact theoretical stress analysisof the veneering process is confoundedby complex stress distributions and theanisotropic nature of wood. An ap-proximate solution, however, can bedeveloped that is in reasonable ~ment with observational data of ve-neer types, mechanical properties, andquality determinations.

Nosebar Pressure Absent: Al-though the use of nosebar pressure isaccepted practice, it i$ felt that pres-sure may be completely relieved duringthe veneering process through 1 )changes in the veneer thickness result.ing (rom surface irregularities, or 2)deflection in mechanical elements ofthe lathe .

If the undeformed veneer is consid.ered to be a particle held in mechanical~uilibrium, Fig. 23 approximates theforce relationships existing when nonoseb8r pressure is applied. The ad-vancing knife exerts a resultant forceRK on the undeformed particle. Thisresultant force may be resolved into ahorizontal component ~ and a vertialcomponent K.. ~ is resisted by a com-pressive force Fe acting over area w tof the undeformed particle and a roll-ing shear force F. acting in the planeof the cutting edge. The forces F. andK,. tend to rotate the undeformed par-ticle, and are resisted by internal mo-ment M that originates in the worlc.This resisting moment may be consid-ered to develop through bending ofthe particle as a cantilever beam andfrom tensile stresses distributed alongthe lower surface of the particle in adirection normal to the cutting path.The magnitude of these resisting forcesare determined by the amount of bend-ing present in the particle.

The force relationships describedabove favor the production of Type Aveneer. Woods with relatively highstrength in tension perpendicular tothe grain appear to fail in compressionand shear before the ultimate strengthin tension can be exceeded. Thus, thecharacteristic splitting failure occurs,which ultimately results in a lathecheck.

Strength properties within the unde-formed particle may be such as toresult in Type B veneer, even thoughno noscbar pressure is applied to dieparticle. If the limiting stress in ten-

0

B veneer in the sapwood. In addition,it has been shown that the sapwood istougher than the heartwood ( 12) .. When yellow birch was machined at200° F., the depth of check penetra-tion was observed to be shallower inthe heartwood than in the sapwood.At 200° F., the relative strength prop-erties conducive to formation of TypeA veneer are slightly in favor of thesapwood. However, coefficient of fric.tion data (Fig. 18) suggest that atapproximately 1800 F. frictional forcesexerted on the heartwood becomegreater than those on the sapwood andthus reflect the improved heartwoodveneer quality.

Table 3 shows that redwood con.tained no checks when machined at160° F. and 200° F., regardless of thenosebar opening used. Fig. 17 revealsthat at approximately 1600 F. the ten-sile strength, as compared to the com.pressive and shearing strengths, ap-proaches the conditions favorable forformation of Type B veneer. In addi-tion, Fig. 18 indicates that high fric.tional forces are imposed on the ve-neer particle.

2. Kivimaa, E. 19~. Inves~ rotaryveneer-cutting with the aid of a tensiontest. Forest Products Journal, Vol. 6,No.7, p. 2~1.

3. U. S. Forest Products Laboratory. 19~~.Structure orientation on smoothness ofknif~-cut veneers. Report No. 924.

4. Ottawa Forest Products Laboratory.19'4. Observations made during theveneer-cutting process.

~. Franz, N. C. 19S6. An analysis of thewood-cutting process. University ofMichigan Press, E.R.I., Bull. 39, AnnArbor, Michigan.

6. Sulzberger, P. H. 1943. The eHect oftemperature on the strength of wood,plywood, and glued joints. JournalCoun. Sci. and Ind. Res., Australia,Vol. 16, No.4, pp. 23~23~.

7. Greenhill, W. L. 1936. Strength testsperpendicular to the grain of timber atvarious temperatures and moisture con-ten ts. Jour. Coun. Sci. and Ind. Res.Australia. Vol. 9, No.4, pp. 26S-76.

8. American Society for Testing Materials.1949. Standard methods of testing smallclear specimens of timber. DesignationD14H9. A.S.T.E., Philadelphia.

9. Jayne, B. A. 19S3. Finish checking ofveneered panels. J. Forest Products Re-search Soc., Vol. 3, No.3, p. 7.

10. Merchant. M. E. 194~. Mechanics ofthe metal cutting process I-Orthogonalcutting and a type II chip. Journal ofApplied Physics. Vol. 16, pp. 267-7S.

11. Laweson, P. G., and W. J. Cox. 19S~.Mechanics of Materials. 3M Ed., JohnWiley 3( Sons, Inc., New Yorlc. p. 211.

12. Ernst. H. and M. E. Merchant. 1941.Chip formation, friction and high qual-ity machined surfaces. Surface Treat-ment of Metals. Cleveland.

13. Department of Mines and Resources,Canada. A comparison of the mechani.cal and' hysical properties of the heart.wood 0 sapwood of y~llow birch. For-est Service Circular No. S1.

ConclusionsThe research summarized above sug-

gests a nwnber of conclusions regard-ing the veneer-cutting process.

1. The veneer-cutting process ischaracterized by two extreme types ofveneer formation: 1) veneer formedby a splitting and bending process andcontaining severe lathe checks and

rough surface ~ities, and 2) veneerfonned by a peeling process and char-acterized by the absence of lathe checksand superior surface quaijties.

2. The quality of veneer formedunder a given set of veneering condi-tions is determined by the nature ofwood failure ahead of the knife.

3. The surface quality generated is afunction of wood failure.

4. The type of wood failure, andhence veneer quality, is a function ofwood properties and force relation-ships within the veneer particle.

5. Mechanical properties define thenature of wood failure under a givenforce system.

6. Nosebar pressure influences ve-neer quality by establishing additionalforces withfu the veneer particle.

7. Frictional forces at the knife faceare important because they affect forcerelationships within the veneer particle.

8. Wood mechanical properties thatappear to have the most significant ef-fect on veneer quality are: com('re5sionperpendicular to the grain, tension per-pendicular to the grain, and rollingshear.

9. For the woods tested, tension per-pendi.cuiar to the grain is reduced morethan compression perpendicular to thegrain and rolling shear with an in-crease in temperature.

Literature Cited1. Flei$cber, H. O. 1949. Experiments in

cotary veneer-cutting. Proc. Forest Prod-ucts Research Soc., p. 137.

10