The Technique of FURNITURE MAKING

VENEERING PARTICLE BOARD

Chipboard (particle board) should be veneered with synthetic resin glue, for hide glues may or may not take over the resin impregnated surface and water-based glues tend to swell up the chips. If resin glues are not available the outer skin of the board should be broken through with coarse abrasive paper and a test for adhesion made, or the manufacturer of the board consulted. Synthetic resin glues adhere perfectly and give no trouble. If the boards are supplied already veneered with a common backing quality veneer, usually agba (tola) or African mahogany, the decorative face veneer should be crossed as usual, as this locks the minute cracks always present in knife-cut veneers. Furniture quality particle boards with fine chips in the surface layers should not require counter veneering with backing quality veneers, but if the coarse particle boards (building boards) are used then they should be double veneered (see Counter veneering, p. 307).

JOINTING VENEERS

The trimming and jointing of narrow veneers to form larger panels is done by special saws or power-operated guillotines in production - work, but knife cutting by hand is rarely satisfactory and the veneers should be worked as in jointing narrow boards. Single leaves can be planed up on the shooting-board (293:1) if adequately supported as shown, or a pack of veneers can be cramped/clamped between two wood straight-edges and either held in the vice

(288) or passed over the jointer. If the veneer edges tend to splinter out or crumble they can be supported with strips of adhesive tape while they are being planed. After planing, the edges

are taped together with adhesive tape, with a final strip along the entire length of the joint

(289) , and while gummed paper tape is normally used, clear cellulose tape has very definite advantages, even though it is more expensive, for the joints can be examined for tightness against the light and the tape is more easily peeled off after pressing. Tape-splicers in production-work apply mechanical methods to the same principle, and the two leaves of veneer are pushed together by angled rollers while a continuous strip of gummed paper tape is wetted and pressed down by a front roller. With tapeless splicers the veneer edges are first coated with a mixture of hide glue and formaldehyde and allowed to dry. A heated roller in the splicer then liquefies and sets the glue as the two edges are fed together, with sufficient strength of bond which enables the veneers to be handled as a single sheet.

Wedge jointing (finger jointing) veneers

Short veneers which are merely butt jointed together end to end to increase the overall length show the joint as a hard line saturated with glue no matter how skilfully they are laid. While this is acceptable in matched curls (297:4) it can be objectionable in long runs of straight - grained woods. Finger or wedge jointing will, however, provide an almost invisible match
with the grain of the separate veneers merging into each other.

Laying out finger jointing

The veneers chosen should be reasonably straight grained and not too pronounced in figure. The butt ends of the matched veneers taken in strict order from the bundle should be overlapped by the amount required for the fingering, always remembering that the longer the fingers the more they will approximate to the direction of the long grain and the less evident they will be when laid. A small allowance, say 3/8 in (9.5 mm) over and above the length of the fingers, should be made at each end to support the cuts, and two lines should be squared across the top veneeer 3/8 in (9.5 mm) from the end of the top veneer and 3/8 in (9.5 mm) from the position of the end of the under veneer. Within this box parallel lines should be drawn and the position of the fingers marked in with a hard pencil; the fingers are then cut in with a sharp knife, the waste lifted and the two veneers pushed tightly together and taped in position. Figure 292 illustrates the various stages, and the completed finger joint, which was not 'faked' in any way, shows the joint as a slight deviation only in the grain direction on the right-hand side of the completed panel. The method can also be used in random lengths of veneers, providing the grain is reasonably uniform throughout the several sheets.

Finger jointing is a traditional hand process, but wedge joint veneer shaping machines working on the principle of a small fly-press are now available with up to 12 right-hand and 12 left-hand knives to cut joints up to 14 in (355 mm) in width. The principle is also employed in the scarf jointing of heavy construction timbers, but precision machines are required as the fingering cannot be sawn by hand accurately enough to give the required tightness at the tips.