Some machining qualities of selected lesser-used timber species in Ghana. Addae-Mensah & Ayarkwa

SOME MACHINING QUALITIES OF SELECTED LESSER-USEDTIMBER SPECIES OF GHANA

A.G. Addae-Mensah and J. Ayarkwa

Forestry Research Institute of Ghana, P. O. Box UP63, UST Kumasi, Ghana

ABSTRACT

In this paper, boring, shaping and turning characteristics of eleven lesser-used species (LUS) are presented.The results showed that, generally, the high-density species have better boring, turning and shaping qualitiesthan the low-density species. That is, whereas Afina (Strombosia glaucescem), Albizia (Albizia ferrugenia),Essia (petersianthus macrocarpus) and Esa (Celtis mildbraedii) have superior machining qualities, ONe(Pycnanthus angolensis), Ceiba (Ceiba pentandra), Bediwonua (Canarium schweinfurthii) and Bombax(Bombax brevicuspe) do not have vet:y high grade boring, turning and shaping characteristics. Furthermore, theresults of the boring test do not seem to indicate any clear relationship between density of wood and the ojJ-sizesin the bored holes.

Keyword~'.· Lesser-used species, machining properties, ecologicaL zones.

INTRODUCTION

Among the important classes of properties thataffect the general utility of any wood are itsmachining properties. Machining properties ofwood are associated with standard wood

processing operations such as planing,shaping, turning, boring etc. Unlike metals,wood in general is easy to cut or shape. Forvarious usages such as in furniture andfixtures, the smoothness and ease with whichwoods can be worked may be the mostimportant of all properties. Unless a woodmachines fairly well and with moderate ease, itis not economically suitable for such usesregardless of its other virtues.

According to Dinwoodie (1980), machining isa stress-failure process and therefore it isconvenient to analyse as an action of a cuttingtool on a piece of wood. The direction ofmotion and configuration of the tool determinethe' way the stress develops and the woodresists it. He described machining as involvingcutting, shaping and surfacing actions.According to Herbert et. al. (1984), the lowerthe density of the wood, the easier the wood iscut with a tool. Dinwoodie (1980) confirmsthat as density increases, the blunting time ofcutting tools decreases. Kollman and Cote(1968) state that surface quality depends onthe wood species and becomes better fordenser, harder and drier wood. According toDinwoodie (1980), the quality of cut depends

GhanaJ. ojForestry Vol. 61998

on the grain direction and mechanics of chopbreakage. Cutting along the grain is said to bemore efficient and results in better finish

quality than cutting against the grain.According to Davis (1938), the rate of feedingsamples during machining is quite independentof species and is chosen in relation to the typeof knife and the rotation speed of thecutterblock, so that the smoothness of the cutis suitable for the end use of the piece.Extremely low feed speeds may actually resultin tearing instead of cutting, producing poorfinish. Generally low feed speeds are reportedto produce better finish during machining thanhigher speeds (Davis, 1938). Some tropicalhardwoods such as Milicia excelsa (Odum)and Distemonanthus benthamianus (Ayan) areoften found to have some hard deposits likecalcium and silica. These deposits are said tohave pronounced dulling effect on cuttingtools in all directions (Farmer, 1972;Dinwoodie, 1980). The presence of resins andgums are also said to stick to tools and resultin overheating and changing the normalworking temperature. Dinwoodie (1980)further stated that timber of high moisturecontent does not machine as well as that oflow moisture content due tendency for the thinwalled cells to be deformed rather than cut dueto their increased elasticity under wetcondition. Deformed fibres easily pick up aftercutting resulting in poor surfaces.

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Some machining qualities of selected lesser-used timber species in Ghana. Addae-Mensah & Ayarkwa

Boring, for example, is commonly donewhenever dowels, spindles and screws areapplied in the manufacture of chairs, tablesand other hardwood products. Even thoughsome carpenters of today still bore using braceand bits, in industrial woodworking electricpower has replaced manpower. The simpleststationary-boring machine is the singlespindle, hand-feed type and at the otherextreme are automatic multiple-spindlemachines that bore several holes ofpredetermined depth at the same time.Although, not one of the most importantwoodworking operations, the quality of boringeither adds to or detracts from the generalutility of any species. A smooth cut, accuratelysized hole is necessary for the best glue joint.The extent of off size in bored holes also helpsto explain why some species split considerablymore than others when doweled and also the

difference between dry fit and a loose fit withaccurately sized dowels.

A wide variety of turned products such as tooland implement handles, spools and bobbling,and sporting goods, chair, furniture and toyparts, can be produced using the lathemachine. Lathes vary from simple handoperated to specialise automatic machinescapable of making several turnings per hour.Although turnings are not very commonpractice in woodwork, a number of highquality products are often produced fromturnings.

Shaping is chiefly done in the furnitureindustry. The commonest is the cutting ofpatterns on some curved edge like that of around table top. There are power-feedautomatic shapers, but the most common typeis the spindle.

Unlike the physical and mechanical propertiesof the lesser-used species (LUS), machiningproperties have had little or no systematicstudy at all, and there are hardly anypublications on the subject. Due to lack ofadequate information on the machiningproperties of the LUS, the wood workingindustry very oven shuns their use.

Some of the everyday working qualities andmachining characteristics of some GhanaianLUS are presently being studied at the

Ghana J of Forestry Vol. 61998

Forestry Research Institute of Ghana (FORIG),under an Intt;rnational Tropical TimberOrganisation (ITTO) Project. The objective ofthe study is to assess the merchantability of theselected LUS so that the industry could beproperly advised, and confidence in machiningthem improved. This paper presents in part theresults of the studies made for the benefit ofcabinetmakers, furniture manufacturers, andother wood users who can undertake withassurance the use of these new marketablespeCIes.

EXPERIMENTAL PROCEDURE

Test Materials

Wood test samples for the investigation werecut from lumber samples from logs of thefollowing eleven species. Three tree samplesof each species were collected from each ofthree forest ecological zones in Ghana for theexperiment namely Wet Evergreen (DrawRiver Forest Reserve), Moist Evergreen (BuraForest Reserve) and Dry Semi-DeciduousForest Zones (Opra Forest Reserve):

1. Yaya (Amphimas pterocarpoides)2. Ayan (Distemonanthus benthamianus)3. Bediwunua (Canarium schweinfUrthii)4. Denya (Cylicodiscus gabunensis)5. Bombax (Bombax brevicuspe)6. Afina (Strombosia glaucescens)7. Albizia (Albiziaferrugenia)8. Ceiba (Ceiba pentandra)9. Ohaa (Sterculia oblonga)10. Esa (CeltiS mildbraedii)11. Essia (petersianthus macrocarpus)

The logs were sawn and the lumber kiln-driedto about 10% moisture content at FABITimbers in Kumasi and later conditioned to12% moisture content for the studies. The

selection of test samples for the study was inaccordance with ASTM, D 143-152 (1979).

Testing Methods

Boring, shaping and turning qualities of driedsamples of the above species were investjgatedusing equipment at the timber engineeringworkshop of the Forestry Research Institute ofGhana (FORIG). Commercial sized machinesin good condition were used for the study.

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Some machining qualities o{selected lesser-used timber species in Ghana. Addae-Mensah & Ayar"wa

Sharpness of the cutting tools or knives werekept uniform in all the tests through re­sharpening. The quality of the machined piecewas used as the basis for evaluation in

accordance with ASTMD 1666-64 (1978).Under the study, the quality of machinedpieces was assessed by both visual and tactileinspection. All the test specimens wereconditioned to 12% moisture content for thetests. The tests were conducted as follows:-

Boring quality test

In this study, the equipment used was ageneral-purpose stationary type borer with asingle spindle employing hand feeding. Newsingle twisted, solid centre bit of size of 28mmwas used for boring samples of each species.The selected bit size was large enough forholes to be well inspected. Two separate bitspeeds of 600 rpm and 1400 rpm wereinvestigated in the study. A constant bit feedspeed of 0.5mm/min was adopted for all tests.Twenty-five test specimens of 50mm x300mm x 27mm thickness were cut from

lumber samples randomly selected from thepile from each ecological zone. For eachspecies a total of 75 specimens were testedfrom the three ecological zones. Four separateholes were bored in each block sample whilstholding the block firmly in a vice. In all 300holes were bored for each test condition foreach species.

After boring, each hole was carefullyexamined and graded for smoothness of cut ona scale of 1 to 3 (Fig. 1). Holes with clean,smooth cut with a minimum of fibre tear-out

or crushing on the cut surface were graded I orexcellent, whilst fibrous, rough holes withmuch fibre tear-out were graded 3 or poor. Thepercentage of holes in each grade wasdetermined. The exact size of each bored hole

was measured with a caliper both in a directionparallel and perpendicular to the grainimmediately after boring. The differencebetween the average measurement in the twodirections and the size of the bit for eachspecies was determined as amount of off sizein the bored hole. The overall off size for each

species was calculated.

Ghana 1. of Forestry Vol. 61998

Turning quality test

In this investigation the equipment used wasthe lathe machine of the type that is commonlyfound in small wood workshops. The machinewas used to produce 20 identical turnings fromlumber samples randomly selected from thepile from each species from each ecologicalzone. In all 60 specimens of each, of 37mm x37mm x 187mm dimensions were investigatedfrom the three ecological zones. Theinvestigations were carried out using a singleturning speed of 7900 rpm, and the turningpatterns were chosen such that cuts were madealong the grain, across the grain and diagonalto the grain.

After turning, each sample was carefullyexamined and graded taking into accountsharpness of cut and smoothness of surface ona numerical scale of 1 to 3 (Excellent, goodand poor). The poorest point in a turning wasconsidered the controlling factor in the gradingsince that point governs the anlount of sandingrequired to make it commercially acceptable.Excellent turning with sharp details andsmooth surface were graded 1 whilst poor oneswith splits, rough surfaces etc. were graded 3(Fig. 2). Turnings with quality intermediatebetween the two were graded 2. Thepercentage of excellent and good sanlples wasthen calculated.

Shaping quality test

This investigation was conducted usmg aspindle moulder and a bandsaw. The testsamples were cut from lumber samplesrandomly selected from the stockpile of eachspecies from each ecological zone. Samples,which were of 25mm x 75mm x 325mmdimensions, were bandsawed into the shapeshown in the attached photograph (Fig. 3) suchthat it included straight and curved sawing.

Thirty test samples of each species from eachecological zone were cut into the above shapeand carefully shaped by an experiencedoperator using a spindle moulder running at aconstant speed of 600 rpm. In all, 90specimens of each species from all the threeecological zones were investigated. Aftershaping, each sample was graded on the basis

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Some machining qualities of selected lesser-used timber species in Ghana. Addae-Mensah & Ayarkwa

Fig. 1 Boring

Fig. 2 Turning

. .. ,. .

. ~ "~'i:'~ -"~,I.!

Fig. 3 Shaping

GhanaJ. ofl<orestry Vol. 61998

of defects such as raised grain, fuzzy grain andtom grain (see definitions in the Appendix).The worst defect in a shaped sample was usedto determine the grade of each sample. Threegrades were adopted - Excellent, Good andPoor. Excellent samples were those free fromany of the defects mentioned above whilstpoor ones had several of the defects.Intermediate between the two extreme gradeswas graded as good (Fig. 3).

RESUL TS AND DISCUSSION

The results of the boring quality test have beenpresented in Table 1 showing the relativesmoothness of cut of the individual species at600 rpm and 1400 rpm respectively and inTables 2 and 3 showing variations from size ofbored holes at the above speeds. The relativeturning and shaping qualities of the differentspecies have also been presented in Tables 4and 5. From the results of the tests, all thespecies investigated were grouped into qualitygrades I, II and III as shown in the tables.

Boring Test

Results of the boring studies showed that allthe species investigated did not differ sowidely in boring qualities for both 600 rpmand 1400 rpm bit (spindle) speeds. Tables Iindicate that Afina has the best boringcharacteristics whilst Ceiba has the poorest.The results further indicated that the heavier

species produced greater percentage of goodand excellent holes compared with the lighterweight species. Thus the heavier speciesseemed to have superior boring properties.This is in agreement with Davis (1938),Kollman and Cote (1968) and Farmer (1972) .

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Some machining qualities of selected lesser-used timber species in Ghana. Addae-Mensah & Ayarkwa

Table 1: Boring: Relative Smoothness of Cut at600rpm and 1400 rpm

Species % Good and ExcellentGrader--- 600 rpm

1400 rpm

Afina

100100Albizia

9694Essia

9292IEsa

9091

Ayan8886

Denya8585

Ohaa

8479Bediwonua

8075Bombax

7872IIOtie

7668Ceiba

7465

Results based on 300 holes bored with 28mm bit at

feed speed ofO.5mm/min.I = High Quality Boring Species (85% - 100%).

II = Medium Quality Boring Species (55% - 85%).III = Low Quality Boring Speed (Below 55%).

Table 2: Boring: Variation from size of Bored Holesat 600 rpm

Species Amonnt Off-size (mm)

Esa

0.35Afma

0.35Essia

0.36Albizia

0.37Bombax

0.38Ceiba

0.40Otie

0.41Bediwonua

0.45Ayan

0.49Ohaa

0.57Denya

0.69

Av. for both across and parallel to the grainmeasurements feed speed of O.5mm/min and bit size of28mm.

Ghana}. of Forestry Vol. 61998

From Table 1, Afina, Albizia, Celtis, Ayan,Denya and Ohaa (producing over 85% goodand excellent holes) were grouped underGrade I (ie. High Quality Boring species). Thesuperior boring qualities of the denser speciesis in agreement with Irvine (1961) and Farmer(1972).

However, in the case of Ohaa, no fibrousfinish was observed on end grains. No gumbuild up was also observed in Ayan, causingcharring and consequent blunting of thedrilling bit as asserted by Farmer (1972).Bediwonua, Bombax, Otie and Ceiba(producing 55% to 85% good and excellentholes) were grouped under Grade II (ie.Medium Quality Boring species).

From the species studied there were no poor orlow quality boring species (ie. below 55%).The results in Table 1 for a bit speed of 1400rpm produced almost the same grading resultsexcept for Ohaa, which was, graded underGrade II species. The results in Table] furthergave an indication that quality of bored holesslightly decreased with increased bit speed.This is in agreement with Davis (1938) andKollman and Cote (1968). The resultspresented in Tables 2 and 3 showed that boredholes differed from the actual size of the bit byamounts ranging from between 0.35mm and0.69mm for bit (spindle) speed of600 rpm and0.31mm to 0.68mm for bit (spindle) speed of1400 rpm.

The results seemed to indicate that whereas for

Celtis, Afina and Albizzia the average off­sizes were smaller, that for Denya and Ohaawere bigger than the lightweight species ie.Bediwonua, Otie, Ceiba and Bombax. Thusthere seemed to be no clear relationshipbetween density of wood and the off-sizes inthe bored holes. The results in Tables 2 and 3showed that there also seemed to be no clear

relationship between bit speed and the amountof off size in the species. There were also nofibres that had been flattened, bent orcompressed during boring.

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Some machining qualities of selected lesser-used timber species in Ghana. Addae-Mensah & Ayarkwa

Table 3: Boring: Variation from size of Bored Holesat 1400 rpm.

Species Amount Off-size (nun)

Esa

0.31Afina

0.32Essia

0.32Albizia

0.34Bombax

0.38Ceiba

0.39Otie

0.48Bediwonua

0.50Ayan

0.61Ohaa

0.64

Denya

0.68

Av. for both across and parallel to the grainmeasurements Feed speed of 0.5mm/min and bit size of28mm.

Table 4: Turning: Relative Turning Qualities at7900rpm

Species % Good and ExcellentGrade

Afma

100A1bizia

90ICeltis85

Essia

80Ohaa

80IIDenya

80Ayan

65

Bombax

45Bediwonua

45IIICeiba0

Otie0

I = High Quality Turning Species (85% - 100%)II =. Medium Quality Species (55% - 85%)

1lI O~ Low Quality Species (Below 55%)

Table 5: Turning: Relative Shaping Qualities at 7900Rpm

Species % Good and ExcellentGrade

Afina

97Albizia

90Essia

87IDenya

87

Celtis

83Bombax

83Ayan

80Ohaa

80Bediwonua

73IICeiba

70Otie

63

I = High Quality Turning Species (85% -100%)II = Medium Quality Species (55% - 85%)

11l = Low Quality Species (Below 55%)

GhanaJ. of Forestry Vol. 61998

Turning Test

The results of the study presented in Table 4indicated that, whereas all samples of Afinatested produced good and excellent turnings,all samples of Ceiba and Otie produced pooror inferior turnings and the variation in qualityfrom excellent to poor was so wide.

The results again seem to indicate that, ingeneral, the medium and heavy speciesproduced better turnings than the lightweightspecies. This is in agreement with Davis(1938), Kollman and Cote (1968) and Farmer(1972).

The trend of the results may be due to thelower densities of Otie, Ceiba, Bediwonua andBombax, which had relatively low percentagesof good and excellent turnings and the higherdensities of species ranging from Afina toAyan (Table 4), which have comparativelygreater proportion of good and excellentturnings. For the purpose of selection in thefurniture and woodworking industry, Afina,Albizia and Celtis (with 85% - 100% good andexcellent turnings) have been grouped underGrade I or High Quality species category,whilst, Essia, Ohaa, Denya and Ayan (with65% - 85% good and excellent turnings) havealso been grouped under Grade 2 or MediumQuality species category. Bombax,Bediwonua, Ceiba and Otie (with 0% - 45%good and excellent turnings) have also beengrouped under Grade 3 or Low Quality speciescategory.

The investigation also revealed that althoughAfina turns better than all other species, itshigh splitting characteristics renders itunattractive for users. Otie and Ceiba werealso too fibrous or woody and turning wasgenerally not useful.

Shaping Test

The results of the study showed that generallyall the species tested had good shapingqualities, which ranged from 63 to 97 percentof samples tested.

The trend of results again gave someindication that the heavier species hadsomewhat better shaping qualities than the

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Some machining qualities of selected lesser-used timber species in Ghana. Addae-Mensah & Ayarkwa

lightweight species. This is in agreement withDavis (1938), Kollman and Cote (1968) andFanner (1972). From the results, Afina,Albizzia, Essia and Denya were grouped underGrade 1 (ie. High Shaping Quality Specieswith 85% to 100% good to excellent samples)and all the remaining species grouped underGrade 2 (ie. Medium Shaping Quality specieswith 55% to 85% good to excellent samples).

It was also observed in the study that raisedand tom grain defects were more common inlight weight species whilst surface roughnessand fuzzy grain defects were also prevalent inheavier species. These results are in agreementwith Davis (1938) and Kollman and Cote(1968). It was observed that shaping parallel tothe grain and diagonal to the grain gave betterresults than across the grain in almost allspecies tested, in agreement with Fanner(1972) who reported that Ayan and Albiziahave the tendency to tear and break away atarises. There were also instances of split inshaping in Afina which rendered such samplesnot useful.

CONCLUSION

The results showed that, generally, the high­density species have better boring, turning andshaping qualities than the low-density species.That is, whereas Afma (Strombosiaglaucescens) Albizia (Albizia ferrugenia),Essia (petersianthus macrocarpus) and Esa(Celtis mildbraedii) have superior machiningqualities, Otie (Pycnanthus angolensis), Ceiba(Ceiba pentandra), Bediwonua (Canariumschweinfurthii) and Bombax (Bombaxbrevicuspe) do not have very high gradeboring, turning and shaping characteristics.Fmthermore, no clear relationship betweendensity of wood and the off-sizes in the boredholes was established under the study.

The results of the study have indicated thatmost of the lesser-used species investigatedhave promising boring, turning and shapingcharacteristics to justify their utilisation in thefurniture and wood working industries.

GhanaJ. of Forestry Va/: 61998

ACKNOWLEDGEMENT

We wish 0 acknowledge the financial supportfrom the International Tropical TimberOrganisation (ITIO) for this study.

REFERENCES

American Society for Testing and Materials(1978). Annual Book of ASTM Standards,ASTMD 1666-64 (1978). Philadelphia. pp 486- 512.

American Society for Testing and Materials(1979). Annual Book of ASTM Standards,ASTMD 143-52 (1979). Philadelphia. pp 116.

Davis, EM (1938). Experiments on planing ofhardwoods. Transaction American Society ofMechanical Engineers, TASMA 60(1). pp 3-5.

Dinwoodie, J.M. (1980). Timber: Its Natureand Behaviour. Van Nostrand Reinhold Ltd.,Berkshire. pp. 190.

Farmer, R. H. (1972). Handbook ofHardwoods 2nd ed. Dept. of Environment,

. Building Research Establishment, PrincessRisborough Laboratory HMSO. London. pp 3­52.

Herbert, C. F., Erwin, H. B, Jeanne, P. D.Fahley, D.F. Koeppen, R.C. MacGovern,J.H. and Zerbe, J.I (1984). Forest ProductsUtilization. Forestry Handbook Section II.2Ed. John WiIey and Sons Inc., New York. pp565-635.

Irvine, F. R. (1961). Woody Plants of Ghana.Oxford University Press, London. pp. 30-35.

Kollman, F. P. & Cote, W. A. (1968).Principles of Wood Science and Technology 1.Solid Wood. SpringIer-verlag. Berlin,Heidelberg, New York. pp. 292.

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