The Technique of FURNITURE MAKING

Curved work

Permanent curves in wood can be achieved by the following methods:

1 Cutting to shape from the solid timber

2 Steam bending

3 Coopering

4 Saw kerfing

5 Laminating thin sections together

CURVES FROM SOLID WOOD

The process of finding wood with grain that naturally follows the curve required goes back centuries. It was this tradition that led to the oak forests of Britain being scoured for timber which would naturally supply the structural curves which were necessary for roof stresses and boat building. Sadly, because of industry's demand for straight-grained, easily worked wood, it is more difficult today to find these natural curves. It is this shortage of naturally curved timber that has led to an increase in the use of the other methods, and laminating is now the main technique used to achieve curved shapes, especially where any quantity is required.

STEAM BENDING

Any piece of wood bent or bowed within the limits of its normal stress range, i. e. where the neutral axis between the stretched convex face and the compressed concave face is roughly equal (316:1) will revert to its normal shape immediately the strain is released, owing to the natural elasticity of its fibres. If the limiting stress according to the wood species is exceeded (316:2) some permanent deformation will occur, but there is always a danger that the fibres will be disrupted (316:3). If, however, the wood is rendered semi-plastic either by heating it with live steam or immersing it in boiling water or heated wet sand, its compressibility ratio will be greatly increased, and provided it is locked in position until the fibres have cooled and dried it will set rigid to the desired curve. There will be some recovery as the created internal stresses equalize themselves, but while this slight tendency to straighten out cannot be precisely calculated, a test-piece in the same species and of the same dimension will usually show the extent of the movement.

In practice prolonged immersion in boiling water or heated wet sand is never as efficient as steam bending, and the latter is to be preferred. The technique is straightforward and all that is required is a sufficient head of steam at atmospheric pressure to heat the wood thoroughly to 212° F (100° C), and to maintain that temperature for at least 45 minutes for every inch of thickness, i. e. a 11/4 in (32 mm) section will require approximately one hour's steaming. A simple apparatus using a large kettle or oil-drum, a source of heat (Primus stove, coke fire, etc.), a length of rubber hosepipe and a wooden box or metal or earthenware pipe sealed at each end with wooden bungs is shown in 316:4, and is quite sufficient for the occasional bend. Production in quantity would, of course, call for more sophisticated methods if consistent quality with low waste factors are to be achieved.

Bending methods

Immediately the wood is soft enough, i. e. uniformly heated throughout, it is taken from the steam chest and bent by hand, or by mechanical means if of heavy section, round a suitable former or form. A rough and ready method is to use sturdy pegs driven into a board, but this is inclined to bruise the wood, the bend may not flow easily between the pegs and the ends are inclined to split as they dry. A more sophisticated method is shown in 316:5 in which an 18 swg spring steel strap, slightly wider than the wood section, greatly minimizes the actual disruption of the fibres in the stretched convex face, permitting bends of much smaller radii. The strap is firmly anchored to heavy wood or metal back plates which prevent the ends

swivelling, and if oak or other acid timber is to be bent then the strap should have an interleaving of thin aluminium sheet or other protective material. End stops fixed to the back plate are also necessary to keep the strap tight against the bend; they should be spaced to allow a certain fractional creep in the bend length (not exceeding 2 per cent) with the slackness taken up with wooden wedges.

Detachable handles can be clipped or bolted to the back plates, or the plates can be extended to form handles for easy manipulation, and once the bend is completed it can be anchored by a tie-rod and lifted from the former. It should then be placed in a dry heated atmosphere to set for about 12 hours, after which it can be freed from restraint and allowed to settle naturally for about 2 weeks. With acute bends it is always better to leave the metal strap and tie-rod in position during the actual setting time (316:6), but simple bends may need only a batten nailed across (316:7), thus releasing the strap for further use.

Bends of fairly large radii usually tend to straighten out a little after they have dried. The recovery movement is not calculable, and it is usual to bend to slightly smaller radii to allow for the movement, but bends of small radii may tend to turn inwards, although they should not move once they have set. Very green timber will obviously bend more readily than dry seasoned wood, although hydraulic pressures induced in the moisture-choked cells may cause extensive rupturing, while old dry wood is usually too stubborn or too brittle to bend well; therefore the ideal is probably partially seasoned wood with a moisture content of around 25 per cent which the steam heating and subsequent drying will further season to within acceptable limits. Shaped, moulded or round section timber can be steam bent, but some deformation of profiles and slight flattening of rounded faces against the strap must be expected, therefore it is better to bend first and profile after whenever possible. Woods vary greatly in their bending properties, with elm outstanding and home­grown ash, beech and oak fairly equal. Comparative minimum radii of curvature in inches to be expected from (a) supported and (b) unsupported sections 1 in (25 mm) thick are as follows:

No precise data are available for English walnut but the writer has always found mild, straight-grained walnut roughly comparable to oak or beech, ebony fairly malleable and even oily rosewood capable of simple curvatures if taken in several stages. The quoted figures given in the table are taken from the Forest Products Research Taboratory data.

COOPERING

This traditional method of building curves or cylinders is best illustrated by the traditional beer barrel. The basic method involves bevelling sections of wood to form curves, and it has been undergoing quite a revival in small workshop production. Many of James Krenov's designs incorporate coopered doors.