The individual slats can be 1/4 in (6 mm) wide for small delicate tambours, up to 3/4 in (19 mm) wide for large carcasses, with V2 in (12.5 mm) or 5/8 in (16 mm) about the usual. Thicknesses of the slats can be from3/16 in (5 mm) for small work, up to 5/16 in (8 mm) for average shutters, with a 3/16 in (5 mm) tongue on the latter running in a 1/8 in (3 mm) or 3/16 in (5 mm) deep groove. Total overall length of the slats will be the exact opening plus the depth of the two grooves and minus a bare V16 in (1.5 mm) clearance, and the net length between the shoulders of the tongues pitched to give a clearance of V32 in (0.75 mm) either side or top and bottom, so that the slats ride in the grooves on the ends of the tongues and not on the shoulders. These tolerances may prove too tight in practice, but it is better to start with a tight tambour and ease the shoulders, for loose-running tambours are always noisy. Grooves which are sunk and not formed by attached guides must be accurately worked to a constant depth throughout, either by plough-plane on the straight and chiselling out for the bend, or with hand or portable electric router against a suitable template, with a trial slat prepared and put in place to test the travel. This trial slat can then be used as a pattern for all the other slats, which must be accurately sawn and planed to finished dimension, with a generous allowance for rejects. Figure 250:4 shows a method of working chamfered slats by hand from a solid width, and 250:8 levelling off to thickness by cross-planing on a supporting board. When sufficient slats have been prepared they are assembled on a flat surface, sprung slats rejected, the remainder matched for colour and grain, spreading any pronounced differences over the total width and then numbering them in the exact order in which they are to be assembled. The tongued ends can now be spindled, or the lasts laid out and firmly held in position with covering battens (250:5) exactly on the shoulder-lines and the shoulders cut with
a fine dovetail-saw, and then planed off with a metal shoulder-plane, working to a template pinned alongside to yield a constant thickness of tongue throughout. After checking that the shoulders line up and the net shoulder length is constant throughout, the faces are sanded, the slats turned over and the backs sanded. The slats can also be polished on the face and the edges also sealed, as this will prevent surplus glue from adhering and will make the final finishing much easier. The flexible back can now be glued on, the slats firmly held by rebated/rabbeted battens (250:6) with folding wedges top and bottom to drive them together. This backing can be silk for delicate slats, fine linen canvas, reinforced if necessary with three webbing straps spaced apart and tacked over the glued canvas, or preferably a good quality leathercloth with a closely woven twill backing. It should be long enough to cover the locking rail, and any semi-flexible glue which will not crack at the joints can be used (hide, PVA, etc.), rolling it evenly over the back of the slats and smoothing down the cloth with hand pressure. The flexible backing should be kept back from the edges (250:6A) about 3/8 in (9.5 mm) on either side. Very oily timbers—teak, rosewood, etc.—may have to be degreased for increased glue adhesion, using cellulose thinners or methylated spirits, and not carbon tetrachloride which is often advocated and which can be highly poisonous under certain conditions. When the glue is dry the tambour can be rolled and unrolled a few times to make it flexible, the tongued ends rubbed over with a candle end and fed into the grooves. The locking rail is then sprung in, the flexible backing temporarily attached and the tambour tried. Only when it is sweetly running should the backing be anchored with screws and a few dabs of glue. Locking rails can be shaped to form handle grips by running a length of moulding as in 250:7A, and then cutting and rounding as shown in the sections, and can be made wide enough to accommodate a suitable lock, or the lock can be mortised into a pilaster or apron rail.
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