Manufacture Of Wood Veneers

Before the introduction of power-driven circular saws all veneers were sawn by hand, usually V8 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 V32 in (0.8 mm) to Vi6 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 780 in (0.3 mm) to /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.

Flat cutting or slicing

Originally carefully selected flitches of wood suitable for veneers were dogged down to a horizontal bed within the machine, and a heavy knife mounted on a rigid carriage slid backwards and forwards over the table to give a shearing cut. In more modern machines, however, the bed is vertical, moving upwards and downwards, again with a diagonal shearing action, against a fixed knife usually V2 in (12.5 mm) thick, 7 in (178 mm) wide and capable of cutting a 17 ft (5.18 m) length with every stroke. The knife is fed forwards by the thickness of the veneer being cut for each downward stroke of the bed, and is capable of very fine adjustment and accurate cutting of veneers from 1/250 in (0.1 mm) to 1/4 in (6 mm) or more in thickness.

Flat slicing of veneers gives a smoother, finer cut than rotary peeling, but even so there is always a tendency for the fibres to be plucked or torn, as in hand-planing across the grain. The best and most decorative results are obtained from true quartered flitches, where the direction of the cut is at right angles to the annual rings; whereas if the cut transverses the full width of the log it will go with the rings for half the width and against them for the other half. The effect can often be seen in sliced gaboon construction veneers, where one half of the width of the sheet is cleanly cut and the other half woolly and uneven.

The cutting of all veneers is highly specialized, for the logs must be steamed correctly, at exactly the right temperature for cutting (wet heat softens wood), and dogged down at the right angle, with the thickness and speed of cut determined by long experience only, and an instinctive appreciation of how the wood is likely to behave. Face veneers, as distinct from construction veneers, are usually cut either 0.88 mm, 0.83 mm or 0.71 mm thick (28. 30 or 35 to the inch), the last familiarly known as "seven mil' (0.71 mm), with the thicker veneers a better compromise against excessive glue penetration and torn grain. Continental veneer-cutters often cut 40, 50 or even 60 sheets to the inch (25 mm) with little regard for plucked grain and tissue-thin patches. Such veneers are much cheaper, but should not be used in good work.

Reference has already been made to "Semi-rotary" cutting, in which suitably shaped flitches are mounted off centre in the lathe so that the cut is half-slice, half-rotary peeling action. This cutting method is sometimes employed for special grain effects, notably in Rio rosewood, etc.. but has been largely abandoned owing to the enormous torque created by the off-centre flitch, and consequent heavy wear on the lathe.

Veneer thicknesses

Veneer

thickness

Approx.

thickness

uge no.

(in)

no. to inch

(mm)

(25 mm)

8

0.160

61/4

4.064

11

0.116

8V2

2.946

12

0.104

93/4

2.642

14

0.080

12V2

2.032

17

0.056

18

1.422

19

0.040

25

1.016

20

0.036

28

0.914

21

0.032

311/4

0.813

22

0.028

3 5%

0.711

25

0.020

50

0.508

27

0.016

621/2

0.406

35

0.008

125

0.203

TYPES OF VENEER Page 47

While rotary peeling is usually reserved for plywood manufacture, constructional veneers for laminated work are normally sliced in 1, 1.5, 2.5, 3 and 4 mm thicknesses from beech, obeche, gaboon, limba and similar woods. Backing quality veneers used as balancers on the underside of groundworks (see Veneering, marquetry and inlay, Chapter 32) are usually peeled or sliced to V32 in (0.7 mm) or thinner out of sapele, straight-grain mahogany, makore (cherry mahogany), etc. Both constructional and backing veneers are normally sold in bulk quantities, but can be obtained in small amounts from retail dealers if specially ordered. They are considerably cheaper than the usual run of face veneers, and one suspects that backing veneers are often used for face-work in cheap furniture. There can be no objection to this for the veneers are cleanly cut from prime logs and are usually lacking only in decorative effect, although occasional bundles will be the equivalent of a good African mahogany or sapelewood face veneer. Saw-cut veneers are now virtually unobtainable except as odd lots, but kingwood is still saw-cut. Face or decorative veneers are usually sold by the cutters in part or complete logs, composed of several bundles according to the number of flitches the log has yielded. This 'buying as they rise' and taking the good with the bad—for practically all trees have some defect or other—is obviously much cheaper than choosing selected bundles from any one stock, and most veneer-cutters in a large way of business will not permit 'breaking the log'. For the small user there are many retail dealers who buy in bulk and sell a few leaves at a time, although the price asked must necessarily be several times that of a bulk purchase.

In choosing veneers it is always a wise precaution to examine each bundle very thoroughly, for beautiful veneers are international currency. They may pass through several hands and are easily damaged. Reputable firms do not hide defective or damaged leaves in the middle of the bundle as a matter of policy, but transposition can occur. Curls, burrs/burls and feathers, etc. are numbered and should be in strict order as cut, for although there may be no apparent difference between adjacent sheets, the small variations build up, and the difference between the first and last sheet of, say, a 30-leaf bundle can be very considerable. Such bundles are rarely, if ever, split, and must be bought as a whole. Highly decorative veneers should never be ordered from one sample leaf only; inspection of the whole stock is essential. To cite just one example—a single Rio rosewood log may yield bundles ranging from a very plain light tan to rich inky black figuring.

Veneer varieties

It would be impossible to list all the varieties available at any one time, for every wood species is capable of yielding veneers, and every tree in the species is an individual with different grain structure, coloration and abnormalities. Add to this different methods of cutting, rotary peeling, semi-rotary peeling, quartered slicing, crown cut (tangential cut), etc., and the permutations are endless. There are, for example, over 300 different species of oak commercially available; the mahoganies form a vast family, while walnuts include English, French, Italian, Ancona (Italian), Circassian, Caucasian (Persian), Australian and American. Names also change according to the locality or at the whim of the dealer, with common agba elevated to tola branca, white tola, etc., to give only one example. Add to this abnormal grain effects— roe, bird's eye, blister, dapple, mottle, quilted, stripy, swirled, burr/burl, crotch, stump, pollard, fiddleback, curl, fan, feather and flame—and the endless variety ensures that wood as a means of decoration is irreplaceable so long as people remain individuals and hanker after individual things. A full description of the matching, jointing, preparation and laying of all types of veneer is given in Chapter 32.

Woodworking Tools and Installation Tips

Woodworking Tools and Installation Tips

There are a lot of things that either needs to be repaired, or put together when youre a homeowner. If youre a new homeowner, and have just gotten out of apartment style living, you might want to take this list with you to the hardware store. From remolding jobs to putting together furniture you can use these 5 power tools to get your stuff together. Dont forget too that youll need a few extra tools for other jobs around the house.

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