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
316 Solid wood bending 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:
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