The Technique of FURNITURE MAKING

MANUFACTURE OF WOOD VENEERS

Before the introduction of power-driven circular saws all veneers were sawn by hand, usually 1/8 in (3 mm) and over in thickness, with deep saw kerfing which had to be planed off before laying or the ribbing would ultimately show through the polish. At the turn of the century specially large circular saws up to 18 feet (5.48 m) in diameter with very fine saw teeth and little set were introduced which could cut veneers from 1/32 in (0.8 mm) to 1/16 in (1.5 mm) in thickness, or about 12 sheets to the inch (25 mm). In all cases about half the wood substance was lost in sawdust, and with the exception of certain difficult woods, notably the kingwoods, ebonies and some satinwood, which must still be cut with the saw, all veneers are now sliced with the knife and waste is eliminated. It should be pointed out, however, that saw-cut veneers are always better quality, for the slicing action of the knife across the grain does to some extent tear or distress the wood fibres.

Two methods of knife slicing are currently practised: rotary cutting or peeling, and flat slicing. A third method of semi-rotary slicing is now only used in special cases.

Rotary peeling

Most woods can be cut by this method, but apart from certain special effects (bird's-eye maple, etc.) its use is confined to the production of core veneers for plywood and laminated work, etc., using the more common hardwoods of large dimension, clean cylindrical boles and free cutting grain structure which allow easy peeling. Suitable logs are first cross cut to length, debarked, adzed to shape and, after prolonged immersion in boiling water, mounted on a giant lathe while still hot and peeled with a fixed knife which is fed forward with each revolution. The] lathe is run slowly at first until the bole or bolt is a true cylinder and there is little if any torque, then the speed is greatly increased and the bole unwinds as a continuous ribbon of veneer the full length of the log, after which it is cut into sheets by pre-set guillotines and force dried in mechanical driers. Veneers can be cleanly cut by this method from 1/80 in (0.3 mm) to 3/8 in (9.5 mm) in thickness, but the grain effect follows round the annual rings, giving an exaggerated onion-slice effect; moreover in some species the fibres tend to crack under the knife (all veneers are sliced across the grain), and microscopic examination will disclose innumerable slight tears or cracks. For this reason all veneers should be laid at right angles to the grain direction of the groundwork, other­wise the fibres are not locked in position and unseen cracks will tend to open. The true advantages of rotary slicing are cheapness of production, and the extremely large dimensions possible, but despite these factors flat slicing is always the better method.