Bending Timber

8/13/2019 Bending Timber

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TIMBERS SUITABLE FOR BENDING

W.I. DIEHM

Wood is a plastic material when heated ortreated with certain chemicals and it is thereforecapable of being bent to form permanent

curves. Bending ol timber may be necessaryfor a variety of reasons, functional or aesthetic,

and applications for bent timber are alsovariable, ranging from large curved arches tosmall lumiture components.

The informationcontainedinthis note is aimedat helping the enthusiast to select suitabletimbers for bending and to ensure that thearticles produced perform satisfactorily.

TIMBER SELECTION

Selection ol suitable timbers for bending is

based on a number ol factors including thebending qualityol the timber, density (weight),

strength, and availability. Other importantlaclors which must be kept in mind are grain

orientation and seasoning characteristics.Lyctid susceptibility of the sapwood and timberdurability should also be considered to ensurethat the article being made gives an adequateservice lile.

Bent wood is used in such items as furnitureand cabinet work, boat building, cooperage,coachwork, wheel rims and sporting goods.

Each use has its own special requirementswhich must be considered when choosing atimber to use.

Table 1 lists many Australian timbers suitablefor bending. lt also gives an indication ofnatural occurance and availability and listsproperties such as density, strength, durabilityand colour. The species list is divided into lourcategories (very good, good, fair, poor)indicating suitability lor bending.

Some of the individual timber characteristics toconsider are:

Durabillty. Ability of the timberto resist decayand insect attack is an important consideration

if the bent timber is to be exposed to suchhazards. Adecay hazardwill existirthetimberis to be exposed lo the weather, if the articlewill be wet periodically (e.9. in boats), or if it is

to be used in continually damp situations or inground contact.

ln these situations timbers with low durabilityratings (3 or 4) should be avoided. Sapwood

of all species is non-durable and should beimpregnated with a suitable preservative in

accordance with the requirements ol the Timber

Utilisation and Markeiing Act, or excluded lromapplications subject to decay hazards.

Strength. The strengrth rating of the timberselected can be imponant if the article is subject

to heavy loads or sudden shocks e.g. wheelrims.

Strength to weight ratio is an importantconsideration f orti mbers to be used for sportinggoods such as tennis racquets or in aircraftconstruction.

Graln. Straight-grained limber is essential forsharp and difficult bends. Generally, the timbershould be lree from sloping or'interlockedgrain, and free of any delects such as knots

and gum veins. Small knots may be acceptableif contained on the outside of the bend and if anelficient strapping method is used.

Backsawn material is more suitable thanquartersawn material, particu larly with specieswhich have prominent medullary rays, e.g.

silky oak. These rays can cause wrinkling on

the compression face.

ilote l..ued by: Tirber Rosoarch, Old. Forest Servics. BO Meiers Road, lndooroopilly, 4068 ph. gZ, 97OO



Molsture Content. There is some uncertaintyabout the optimum moisture lor timber bendi ng

but most species seem to bend well at a

moisture content of between 20 and 25 percent, andthis range should belaken as aguidewhen selecting timber. The higherthe rnoistu re

content, the rnore likely crumpling is to occuron the inner lace of the bend. Atternatively, ilthe timber is too dry it is likely that lailure willoccur on the outside face ol the bend wherethe timber is in tension.

METHOD

Preparatlon. The timber should be machined(dressed)belore

bending othenruise saw marksand other machining delects can give rise tolocal failures. Thus any local concentrations of

stresses which can crack the timber during thebending process, causing subsequent failure,are eliminated.

To assist the bending operation, it is desirablelor the timber to be dressed to the minimum

thickness required with allowances made lorshrinkage. Only a minimum amount of surf ace

dressing should be done afterthe timberbendhas been set.

Softenlng the Tlmber. The most commonlyused method of softening or plasticizing woodis steaming, using either atmospheric or lowgauge pressure. When steaming atatrnospheric pressure, one hour for every 25

mm thickness should be allowed.Oversteaming ol the ti mbercan cause problems

such as excessive shrinkage, checking andwarping during re-drying.

To steam at a low pressure, a simple steam-

tight box kilnof sufficient sizeto accommodatethe timberto be bent is allthat is required. Thekiln can be constructed out of concrete, brick,welded boiler plate ortimber. lllimberis used,it should be lined in either sheet steel orcopper.

Wood can also be softened by a variety of

chemicals. Such chemicals behave like waterin that they are absorlced and may causeswelling. The most commonly used isamrnonia. Wood members are either immersed

in liquid amrnonia or treated under pressure

with ammonia gas. Following treatment, theycan be easily moulded or shaped and once theammonia has evaporated, the wood stiffens

and holds its new shape.

Bendlng. There are manytechniques available

for bending timber successfully. The nrost

comrnon involves the use ol a restraining strap(see f igure 1 ). lMth this method, the amount ofstretch on the convex lace'is mechanically

limited by means of a supporting band along

the lace and attached to it in such a manner

that it prevents stretching.

Figure 1. Bendingwith the aid of a metal strap.

The restraining strap usually consists of a thin

strip of metalwith an adjustable block or endstop

at each end. The block is placed at the end of

the timberto resist any stretching ol the timber

during bending. Alevercan be attachedtooneend of the strap to aid in bending the timber

around the mould. The restraining strapshould have the same width as the timber

being bent and should be strong enough toresist any forces encountered in the bendingprocess.

Seasoning. Afterthe timber has been bent and

restrained it must be dried. This will usuallyinvolve reducing lhe moisture content lrom

about 25 percent (steaming increases themoisture content) to the equilibrium moisture

content, which, in coastal areas, is within the

range of 10-15 percent.



Table 1. Propertles of Tlmbers Sultabte tor Bendtng

Trade Name Botanlcel Name Distri- Ilensity Strengthbution (kg/m3)

at 12 M.C.

Dura- Avall. Colour Lyctldbllity ablllty Euscepl-

blllty

VERY GOODbeech, myrde Nothofagus Tas

annninghamiibrigalw Aacia harpophylla C,Smahogany spur Dysorylum N

pettigrewianumoak,silkynorthem Cardweliasublimisoak,Ulip,red Argyroden&on

penlatumpine,celery-top Phyllodadus

aspleniloliuspine,Huon Darydium trankliniipine,radiata Pinus radiata

quandong,silver Haeocarpusangustilolius

GOODash,alpine Eucalyptus

delegatensis

ash,silwrQueensland

blackwood

messmate

siris,red

siris,yellow

POOR

ash,Crow'sbox,yellow

gum,rose

ironbark,red

54,SD5

51,SD1(s3),(sD4)

56,SD753,SD4

S4,SDs

57,SD856,SD6

ss,sD6

54,SD4

54,SD3S4,SD5

54,SD5

54,SD452,SD2s3,sD2S5,SD5

S3,SD3

(ss),(sD6)

(s6),(sD7)

S6,SD6

S1,SD2

32,SD2S5,SD5

S5,SD5

(s7),(sD8)

(s7),(sDa55,SD653,SD3S2,SD2

S2,SD3

S3,SD4

S,SD8

53,SD4S2,SD3

(s6),s06s6,sDs

(s6),(sD7)

54,SDs

S

S

N.S.

N.S.

N.S.

S

1 N.S.

1S1S3 N.S,

1,4 S

1S1 N.S.'t s3 N.S.

3S2 N,S.

1 N.S.

2S1S

Tas

Tas

S,NSW

Vic,SA

N,C,S

NSW,Vic

Tas

700

1025

865

560800

650

520

545

495

675

680

675640

640

1010

910

705

770

7n610

945

1M0400

800

1090

575

560

560

540

4,5 S

2,3 S

2,3

3,5

3,5

2

1

I

1

1,2

D

D

D

cc

D

R

c

c

N,C

N

c

ccc

cccD

c

c

c

4C1D2C4C20

4D

4D3C1C1C

575

980

930

530

895

480

480

690

800

1010

4

2

2

3

4

2

2

1

ash,mountain Eualyptus regnans Tas,Vicash,silwr Flindersia schottiana N,C,S

4-3

3

32

33

3

3

3

1

1

4

3

1

4

444

D

R

cccccD

c

S

N.S.

S

s

sS

N.S.

S

S

1,2

1

1

4,54

33

1

3S

1S

2 N.S.

1S1 N.S.

1S3S

gum,spotted Eualyptus maanlata c,skani Eucalyptus diversialor WAmahogany,rose Dysorylum lraseranum S

Eucalyptus

Flin&rsia N,C

bourjotianaAcaciamelanorylon N,S,Tas

S,NSW,

obliqua Vic,Tas

Albizia toona N,C

Albizia xanthorylon N

Flindersia australis N,C,S

Eucalyptusmelliodora S

Eucalyptus grandis N,C,S

Eualyptus siderorylon S

FAIRalder,rose Caldcluvia austaliensis N,Cash,hickory Flindersia illlaiana N

blackbutt Eucalyptus pilularis Sbollywood Litsea retculata N,C,Sgum,red,river Eualyptus N,C,S

amaldulensispine,kauri, Agathis microstachya N

Queenslandpoplar,pink Euroschinus lalata N,C,S

satinash,grey Eugenia gustavoides N

satinay Syncarpia hillii Stallowwood Eucalyptus microarys S

cheesewood,white Alstonia scholais N,C

maple,Queensland Flindersiabrayleyana N,Cpine,hoop Araucaia cunninghamii N,C,Ssilkwood,bolly Cryptocarya oblata N

walnut,yeflow Beilschmedia bancrofli N

Codes

Dietrlbulion: N = North Queensland; C = Central Queensland; S= SouttrQueensland; N.S.W. = Nen,Soutr Wales; Vic

= Victoria; Tas = Tasmania; SA = Souh Australia; WA = Ulsstern Australia. Strength: S = uns€aSoned value: SD =seasoned value. Values containd in brackets are provisional but can be used witr confidence. The lower tre value the

higherthestengtr. Durablllty:1=highlydurable;2=durable; 3=moderatelydurable;4=nondurable. Availabillty: C=@mmooi D=difficulttoobtain; R=rare. Colour:1 =white/yellow; 2=pinUpinkishbrown; 3=redbrown; 4=brown;5 = motded or streaky. Lyctld Surceptlblllty: S . tusceptible; N.S. = not susceptible.

Revlsed 3/92



Careful seasoning of the timber article isrequired if it is to retain the bend. Seasoningis normally carried out in either a kiln or under

atrnospheric conditions (air drying). ll a kiln is

employed, the drying schedule should be mild.

No steaming treatment o, the bend is requiredafterseasoning. The bends shouldbe protected

lrom rain and sun if air seasoning is employed.The bends should be stacked in such a manneras to promote adequate air circulation aroundeach piece.

LYCTID SUSCEPTIBLE TIMBER

The sapwood of some hardwood timbers is

susceptible to attack by the powder post beetle( lyctid borer). lf any wooden articles containi ng

sapwood of susceptible species are to be sold,the sapwood must be preservative treated inaccordance with the requirements of the TimberUtilisation and Marketing Act. Timber Note 31

provides f urther inf ormation on therequirements of this Act.

ll the bent articles are not lor sale and will not

be exposed to the weather in use, there are a

number ol treatment options available as a

lairly reliable means of preventing lyctid borer

attack. Neither of these would be acceptabletreatments against lyctid attack underthe Act,however

1. Treat with borax by preparing a 3 per

cent borax solution and soaking thetimber in it. One week of soaking is

necessary for each 25 mm of sapwood

thickness, for unseasoned timber, while

double this time rniaybe necessary lorseasoned timber.

2. Treat with permethrin (1 gram per litre)

in a light petroleum solvent (e.9.

kerosene). A good brush application

should be adequate but soaking lor a

lew hours would ensure satisfactory

results.

FURTHER TNFORMATION

Further inlormation can be obtained bycontacting:

Timber Research and Development Advisory

Council(TRADAC)Tlmber House, Dunlop Street, NEWSTEAD

PostalAddress: P.O. Box 2014FORTITUDE VALLEY QLD 4006Ph. (07) 8s21344 Fax. (07) 2522607

REFERENCES AND FURTHER READING

Kelsey, J. ed. (1985) Fine Woodworking On

Bending Timber. Taunton Press 122pp.

Davidson, R.W. and Baungordt, W.G. (1970)

Plasticizing Wood Wth Ammonia. Forest Prod.

J. 20(3) 1e-2s.



Table 1. Propertles ot Tlmbers Suttabte tor Bendtng

Trade Name Botanlcal Name DisrrL llenslty Strength Dura- Ava[- Colour Lyctldbutlon (kg/m3) bltlry ablllry suscepr-

ar t2 M.c. buty

VERY GOODbeecfr, myrde Nothofagus Tas

anninghamiibrigalow Acacia harpopttylla C,Smahogany spur Dyaxylum N

penigrewianumoak,silky northem Cardyelia sublimis N,Coak,trlip,red Argyrodendron N

pralatumpine,celery-op Phy'bcladus Tas

asplenifoliuspine,Huon Daaydium franklinii Taspine,radiata Pinus radiata S,NSW

Vic,SAquandong,silver Elaeocarpus N,C,S

angustifolius

GOODash,alpine Eualyptus NSW,ViC

delegatensis Tasash,mountain Eu@lyptus regnans Tas,Vicash,silwr Hindersia schottiana N,C,Sash,silver Flindersia N,C

Queensland bourjotianablackwood Acaciamelanorylon N,S,Tasgum,spotted Eualypws maaiata C,Skani Eualyptus diversialor WAmahogany,rose Dysorylum fraseranum S

54,SD5 4

2

2

3

4

2

2

4

4

S

S

S

S

S

N.S.

N.S.

N.S.

S

D 2,3

D 4,5o 2,3

c 3,5

c 3,s

D2

R1c1c1

S

N.S.

S

S

S

sN.S.

S

S

S

S

4,54

3

3

1

3

1

c 2 N.S.

DlSc 1 N.S.

clsD3S

D 1 N.S.

messmate

siris,red

siris,yellow

POOR

ash,Crow'sbox,yellow

Eualyptusobliqua

Albiiatana

1025 51,SDl865 (S3),(SD4)

560 S6,SD7800 s3,sD4

650 S4,SD5

520 S7,SD8545 S6,SD6

495 S5,SD6

675 S4,SD4

680 S4,SD3

675 S4,SD5640 S4,SD5

640 S4,SD41010 s2,sD2910 S3,SD2

705 Ss,SDs770 S3,SD3

7n (ss),(sD6)

610 (S6),(SD7)

55,SD6sI,sD252,SD255,SD655,SDs

(s7),(sD8)

(s7),(sDa55,SD653,SD352,SD2

S2,SD3

33,SD4S,SD8

53,SD4S2,SD3

(s6),s0656,SD5

(s6),(sD454,SD5

4C

4C3Csc3C2C3C3D3C

3C3C

1,2

1,2,|

1

S,NSW,

Vic,Tas

N,C

Albiia xanthorylon N

FAIRalder,rose Caldcluvia australiensis N,Cash,hickory Flin&rsia ifflaiana Nbfackbutt Eualyptus pilularis Sbollywood Litsea reticulata N,C,Sgum,red,river Euelyptus N,C,S

amaldulensispine,kauri, Agathis microstachya N

Queenslandpoplar,pink Euroschinus lalata N,C,S

satinash,grey Eugenia gustavoides Nsatinay Syncarpia hillii Stallowwood Eualyptus microorys S

575

980

930

530

895

480

480

690

800

10't0

945

1040

400

800

1090

575

560

560

640

4

1

2

4

2

4

4

3

1

1

DlSc1sc 3 N.S.

c1,4S

Flindersia australis N,C,SEucalyptusmelliodora S

1

,|

4

3

1

4

44

4

D

R

cccccD

c

1S1 N.S.

1S3 N.S.

3S2 N.S.

1 N.S.

2S1S

cheesewood,white Alstonia scholaris N,Cgum,ros€ Eucalyptus grandis N,C,Sironbark,red Euelyptus sideroxylon Smaple,Queensland Flindersia brayleyana N,Cpine,hoop Araucaia cunninghamii N,C,Ssilkwood,bolfy Ctyptocatya oblata Nwalnut,yellow Beilschmedia bancrofli N

Codes

Distrlbutlon: N = Norh Queensland; C = Central eueensland; S= Soufi eueensland; N.S.W. = NewSoutr Wales; Vic= Victoria; Tas = 1356611ia; SA = South Australia; WA = \4lss1e, Australia. Strength: S = unseasonod value; SD =seasoned value. Values contained in brackets are provisional but can be used with confidence. The lower tre value the

higherthesrengh. Durabltity: l =highlydurable; 2=durable; 3=moderatelydurable;4=nondurable. Availablllty: c=@lTllrlohi D=ditficulttoobtain; R=rare. Colour;1 =white/yellow; 2=pinUpinkishbrown;g=red/brown; a=brorn;5 = motded or streaky. Lyctid Surceprlblfity: S r rusceptible; N.S. = not iusceptible.

Revlsed 3/92

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Bending Timber