Felling And Sawmilling

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Exceptionally valuable timbers standing alone or in isolated groups are usually bargained for separately; each tree is measured and the price per foot cube of usable timber in each butt negotiated.

It is customary to fell standing timber in autumn and winter, although there is no real objection to summer felling, for a growing tree can transpire up to 50 gal (227 1) of water a day through its leaves in hot weather, thus depleting the moisture content of the trunk. However, summer felling does increase the risk of too rapid drying and consequent severe checking and splitting of the logs, and therefore winter schedules are usually adopted. Tropical timbers are felled at any time dependent on the prevailing conditions, for they have no fixed period of growth (and therefore no pronounced annual rings), while certain heavy timbers, notably teak, greenheart, etc., are girdled or ring-barked at the base of the trunk about two years before felling, thus cutting off the supply of food and sap. As the tree continues to transpire through its leaves the overall weight is drastically reduced, enabling it to be hauled or floated.

If a tree is a large one and the timber above the first main break is usable, then the whole trunk is felled and taken to the saw-mill; alternatively the second length above the break (branch or fork) is split for cordwood, and only the butt or main trunk is used. Except in tropical rain forest trees, where lack of light and overcrowded conditions encourage very straight, clear cylindrical poles often up to 180 ft (55 m) high in emergent trees, second length timber is usually inferior in quality, with large knots, twisted grain and a much smaller percentage of heartwood. Such timber is, therefore, only suitable for building construction work, although an occasional second length will yield good-quality narrow planks. Branches are not used, for not only is the proportion of heart-wood very small but the pith is off centre, with the annual rings in tension at the top and in compression underneath, therefore causing considerable shrinkage. Occasional prime walnut, mahogany and ash trees, etc. yield at the convergence of the two sets of annual rings the exceptionally beautiful crotch, curl and feather figuring, while good burrs or burls, when they occur, are cut off and sold to the veneer-merchants. The stumps or root bases of some walnuts, notably the American black walnut, can also yield the very bold stump figuring, and these trees are uprooted bodily and not felled. It is a little sad for the craftsman that all the best of timber—prime oaks of 40 in (1 m) quarter girth and up, fine walnuts, clean sycamores, and in fact all exceptional trees—are immediately bought for veneer cutting at prices far higher than they would reach for conversion into solid board and plank.

Home-grown hardwood logs are not debarked at the saw-mill, for, although the bark does encourage insect pests, it. helps to protect the trunk from over-rapid drying. Imported timbers are, however, always debarked, and sometimes also desapped which helps to cut down the weight, for the logs are sold with the customary allowance for bark and wane, and there is little point in incurring heavy transport costs for useless tonnage. Where logging costs are high, with long distances to be covered, and where there are ample forest reserves to cover the capital costs, local saw-mills are usually erected to convert the logs into square edge timber or lumber. Teak and other valuable hardwoods are invariably exported square edge, while most of the common woods can be obtained in round logs trimmed of sap or in square edge timber, the latter usually only 'shipping dry', with a moisture content below the danger level for fungus attack (25 per cent), but otherwise unseasoned.

Methods of saw-milling

Plain sawn or flat sawn

Produces full width waney/wane edge and squared up boards (Figure 3.5) with the annual rings in a series of contour markings. Boards cut this way are known as tangential cut.

Quarter sawn

Figure 3.4 shows a true quarter sawn or radial cut log. However, due to cost, this is rarely done, and the majority of quarter sawn timber is produced as in 3.3. Here, many boards do not r. '

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3 Wood conversion or milling have the annual rings running at 90 degrees to the surface as on the true radial cut board.

Billet sawn (Figure 3.2)

This is frequently done to produce more stable timber than results from the plain sawn method, and it is often passed off as quarter sawn timber, whereas in fact only a few centre boards (3.6) are produced on the true quarter. The differences in behaviour between plain sawn and quarter sawn timber are described later under Movement and shrinkage (p. 15).

4 Waney/wane-edge board showing square edge wood (A) and short and long narrow boards (B and C)

Saw-milling

Prior to the conversion of the logs into timber or lumber they are cross-cut to length, odd kinks adzed or sawn off, and debarked if the bark is loose or gritty or if hidden nails are suspected (a constant hazard in hedgerow timber).

Treatment of the butt, the side or angle from which it is to be cut, whether it is to be plain sawn to provide waney/wane edge boards, or cut 'billet' fashion for quartered, is at the discretion of the head sawyer, for inexperienced saw-milling can destroy much of the value of a prime butt. The actual 'breaking down' of the log is done either by large circular saws, or more usually by vertical band mills in which only 3/32 in (2.4 mm) is lost in sawdust for each cut, as against 1/4 in (6.3 mm) for the heavy circular ripsaws. Logs are also cut with the horizontal band mill, which is slower but gives a more even cut, as the weight of the plank is downwards and not against the side of the saw.

The primitive method of pit-sawing by hand, in which the log is placed over an open pit with one man on top of the trunk and another underneath to pull the long pit-saw downwards, it still used in native districts where labour is cheap and power facilities not available. The expression "top-sawyer' originated from this method of sawing, for the man on top guided the saw, while his helper below did most of the hard work in cramped and difficult conditions.

Where the cut includes the natural shape of the log without preliminary squaring or trimming then the plank or board is known as waney (or wane) edge, and square edged is obtained by trimming off on either side. If the plank is wide enough the offcuts/cut offs are used to provide narrow boards {short narrows and long narrows), both of which are sound, but correspondingly cheaper. It is obvious that square edge timber/lumber will be more expensive than waney edge, owing to the extra sawing, waste, etc., but the economy in the latter is largely illusory. Imported sawn hardwoods are usually square edge to save shipping space, but some producers export in the form of 'boules' or complete logs totally flat cut, while a fair proportion of African timbers are exported in the round. Very valuable timbers—ebonies, kingwood, blackwood, lignum vitae, etc.— which are of small dimension are exported in log form and sold by weight.

Portable chain mills

These are, in essence, large chainsaws that operate horizontally on a felled log to produce sawn planks to any thickness required. Their great advantage is that they can be taken into the woodlands and the log can be converted on the site, which enables the individual boards to be easily transported by normal van or trailer.

This machine does, therefore, enable the furniture-maker to recover logs that might otherwise simply rot away due to their not being economically viable in terms of normal commercial considerations. Furthermore, in some parts of the world it is the only way to obtain complete logs of sawn timber as opposed to the squared up and graded boards.

5 Sperber portable chain mill
6 First cut with ladder support
7 Subsequent cuts do not require the ladder support. (By courtesy of Philip Cole)

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Wood Working for Amateur Craftsman

Wood Working for Amateur Craftsman

THIS book is one of the series of Handbooks on industrial subjects being published by the Popular Mechanics Company. Like Popular Mechanics Magazine, and like the other books in this series, it is written so you can understand it. The purpose of Popular Mechanics Handbooks is to supply a growing demand for high-class, up-to-date and accurate text-books, suitable for home study as well as for class use, on all mechanical subjects. The textand illustrations, in each instance, have been prepared expressly for this series by well known experts, and revised by the editor of Popular Mechanics.

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