As the sawn boards fall off the log, they land on "live," that is, revolving rollers, which carry them along at the rate of 200 to 250 feet a minute. Stops are provided farther along to stop the boards wherever wanted, as at the edger, Fig. 43, or the slasher. From the live rollers the boards are transferred automatically, Fig. 44, by chains running at right angles to the rollers and brought within reach of the edger man. About one-third of the boards of a log have rough edges, and are called "waney." These must go thru the edger to make their edges parallel. The edger man works with great speed. He sees at once what can be made out of a board, places it in position and runs it thru. From the edger the boards are carried to the trimmer, which cuts the length. The lumberman"s rule is to "cut so that you can cut again." The so-called 16" logs are really 16" 6". The trimmer, Fig.
45, now trims these boards to 16" 1", so that if desired they can still be cut again. The trimmer may be set to cut at any desired length according to the specifications.
[Ill.u.s.tration: Fig. 45. Automatic Gang Lumber-Trimmer. It may be set to cut automatically to any desired length.]
[Ill.u.s.tration: Fig. 46. Lumber Sorting Shed. Virginia, Minnesota.]
[Ill.u.s.tration: Fig. 47.]
The boards are now graded as to quality into No. 1, No. 2, etc., Fig.
46, and run out of the mill, to be stacked up in piles, Fig. 47. Big timbers go directly from the saw on the rolls to the back end of the mill, where the first end is trimmed by a b.u.t.ting-saw or cut-off-saw which swings, Fig. 48. The timber is then shoved along on dead rolls and the last end trimmed by the b.u.t.ting-saw to a definite length as specified, and shoved out.
One of the most remarkable features of the modern mill is its speed.
From the time the log appears till the last piece of it goes racing out of the mill, hardly more than a minute may have elapsed.
[Ill.u.s.tration: Fig. 48. Cut-off-Saw. This saw trims the ends of timbers.]
A large part of the problem of sawmilling is the disposal of the waste. The first of these is the sawdust. In all first cla.s.s mills, this together with shavings (if a planing-mill is combined) is burned for fuel. It is sucked up from the machines and carried in large tubes to the boiler-room and there is mechanically supplied to the fires.
The slabs, once considered as waste, contain much material that is now utilized. From the live rolls, on which all the material falls from the main band-saw, the slabs are carried off by transfer chains, and by another set of five rollers to the "slasher," Fig. 50, which consists of a line of circular-saws placed 4" 1" apart. This slasher cuts up the slabs into lengths suitable for lath or fence-pickets.
Fig. 49. Or they can be resawn into 16" lengths for shingles or fire-wood.
[Ill.u.s.tration: Fig. 49. Ten Saw Gang Lath Bolter. This machine cuts up material lengthwise into laths.]
[Ill.u.s.tration: Fig. 50. Slab-Slasher. This machine cuts up the slabs into lengths suitable for lath or fence-pickets.]
From the "slasher" the 4" 1" lengths are carried on by traveling platforms, chains, etc., to the lath-machines, Fig. 51, where they are sawn up, counted as sawn, bound in bundles of 100, trimmed to exactly 4" in length and sent off to be stored. The shingle bolts are picked off the moving platforms by men or boys, and sent to the shingle-machine, Fig. 52, where they are sawn into shingles and dropped down-stairs to be packed. Shingle-bolts are also made from crooked or otherwise imperfect logs.
Of what is left, a good part goes into the grinder or "hog," Fig. 53, which chews up all sorts of refuse into small chips suitable for fuel to supplement the sawdust if necessary. Band-saws make so little dust and such fine dust that this is often necessary.
[Ill.u.s.tration: Fig. 51. Combination Lath-Binder and Trimmer. With this machine the operator can trim the bundles of lath simply by tilting the packing frame over from him causing the bundles to pa.s.s between the saws, thereby tr.i.m.m.i.n.g both ends at one movement.]
[Ill.u.s.tration: Fig. 52. Hand Shingle-Machine. This machine is used in Sawmills in which it is desired to utilize slabs and tr.i.m.m.i.n.gs by sawing shingles therefrom, or to saw shingles from prepared bolts.]
If there is any refuse that cannot be used at all it goes to the sc.r.a.p-pile, Fig. 54, or to the "consumer," the tall stack shown in Fig. 37, see p. 33.
Boards ordinarily sawn from logs are "slash-sawn," i. e., they are tangential or b.a.s.t.a.r.d, each cut parallel to the previous one. By this process, only the central boards would be radial or "rift" boards.
[Ill.u.s.tration: Fig. 53. Edging grinder or Hog. It cuts any kind of wood into coa.r.s.e or fine chips suitable to be handled by chain conveyor or blower.]
[Ill.u.s.tration: Fig. 54. Sc.r.a.p-Pile. Oscilla. Georgia.]
But, for a number of reasons, radial boards are better. They warp less because the annual rings cross the board more evenly. Yellow pine flooring that is rift-sawn is more valuable than slash-sawn, because the edge of the annual rings makes a more even grain, Fig. 55. Where slash-grained flooring is used, the boards should be laid so that the outside of each board will be up in order that the inner rings may not "sh.e.l.l out."
[Ill.u.s.tration: Fig. 55. Slash-Grain and Comb-Grain Flooring.]
In sawing oak for valuable furniture or trim, the log is first "quartered" and then the quarters sawn up as nearly radially as is desired. There are various methods of cutting quartered logs, as ill.u.s.trated in Fig. 56.
[Ill.u.s.tration: Fig. 56. Methods of Sawing Quartered Logs.]
In making staves for water-tight barrels, it is essential that they be cut radially in the log, in order that the staves be as non-permeable to water as possible.
[Ill.u.s.tration: Fig. 57. Lumber-Kiln.]
SAWMILLING.
REFERENCES:[*]
Trout, Ca.s.sier 11: 83, 184.
Woodcraft 5: 56, May "06.
[Footnote *: For general bibliography see p. 4.]
CHAPTER III.
SEASONING.
The seasoning of wood is important for several reasons. It reduces weight, it increases strength, it prevents changes in volume after it is worked into shape, and it prevents checking and decay. Decay can also be prevented by submergence and burying, if by so doing logs are kept from fungal attacks. The piles of the Swiss Lake dwellings, which are in a state of good preservation, are of prehistoric age. Wood under water lasts longer than steel or iron under water. But for almost all purposes wood has to be dried in order to be preserved. The wood is cut up, when green, to as thin pieces as will be convenient for its use later, for the rate of drying depends largely upon the shape and size of the piece, an inch board drying more than four times as fast as a four inch plank, and more than twenty times as fast as a ten inch timber.
There are various methods of seasoning:
(1) Natural or air-seasoning is the most common, and in some respects the best. In this method, the wood is carefully and regularly piled in the seasoning-yard, so as to be protected as far as possible from sun and rain, but with air circulating freely on all sides of the boards, Fig. 47, see p. 38. To accomplish this, "sticking" is employed, i. e., strips of wood are placed crosswise close to the ends and at intervals between the boards. In this way the weight of the superposed boards tends to keep those under them from warping. The pile is skidded a foot or two off the ground and is protected above by a roof made of boards so laid that the rain will drain off.
Fire-wood is best dried rapidly so that it will check, making air s.p.a.ces which facilitate ignition, but lumber needs to be slowly dried in cool air so that the fibers may accommodate themselves to the change of form and the wood check as little as possible. Good air-drying consumes from two to six years, the longer the better.
(2) Kiln-drying or hot-air-seasoning is a much more rapid process than air-seasoning and is now in common use, Fig. 57. The drying is also more complete, for while air-dried wood retains from 10% to 20% of moisture, kiln-dried wood may have no more than 5% as it comes from the kiln. It will, however, reabsorb some moisture from the air, when exposed to it.
The wood of conifers, with its very regular structure, dries and shrinks more evenly and much more rapidly than the wood of broad-leaved trees, and hence is often put into the kiln without previous air-drying, and dried in a week or even less time.
Oak is the most difficult wood to dry properly. When it and other hardwoods are rapidly dried without sufficient surrounding moisture, the wood "case-hardens," that is, the outer part dries and shrinks before the interior has had a chance to do the same, and this forms a sh.e.l.l or case of shrunken, and often checked wood around the interior which also checks later. This interior checking is called honeycombing. Hardwood lumber is commonly air-dried from two to six months, before being kiln-dried. For the sake of economy in time, the tendency is to eliminate yard-drying, and subst.i.tute kiln-drying.
Kiln-drying of one inch oak, takes one or two weeks, quarter-sawn boards taking one and a half times as long as plain-sawn.
The best method of drying is that which gradually raises the temperature of both the wood and of the water which it contains to the point at which the drying is to take place. Care is therefore taken not to let the surface become entirely dry before the internal moisture is heated. This is done by retaining the moisture first vaporized about the wood, by means of wet steam. When the surface is made permeable to moisture, drying may take place rapidly. Curtains of canvas are hung all around the lumber on the same principle that windows in newly plastered buildings are hung with muslin.
The moisture is absorbed on the inner surface of the curtain and evaporates from the outer surface. Improvements in kiln-drying are along the line of moist air operation. In common practice, however, the moist air principle is often neglected.
There are two methods in operation, the progressive method and the charge method. In the progressive, the process is continuous, the loads going in at one end of the kiln, and out at the other, the temperature and the moisture being so distributed in the kiln, that in pa.s.sing from the green to the dry end, a load of lumber is first moistened, then heated, and finally dried out. In the charge system, the process is intermittent, one charge being removed before a new one is admitted. This gives the best results with high grade lumber for special uses.
A modification of hot-air-seasoning is that which subjects the wood to a moderate heat in a moist atmosphere charged with the products of the combustion of fuel.
(3) Small pieces of wood may be effectively seasoned by being boiled in water and then dried. The process seems to consist of dissolving out alb.u.minous substances and thus allowing freer evaporation. Its effect is probably weakening.
(4) Soaking in water is sometimes used as a good preparation for air-seasoning. Previous soaking hastens seasoning. River men insist that timber is improved by rafting. It is a common practice to let cypress logs soak in the swamps where they grow for several months before they are "mined out." They are eagerly sought after by joiners and carpenters, because their tendency to warp is lessened. Ebony is water-soaked in the island of Mauritius as soon as cut. Salt water renders wood harder, heavier, and more durable and is sometimes applied to ship timbers, but cannot be used with timbers intended for ordinary purposes, as the presence of salt tends to absorb atmospheric moisture.
(5) Boiling in oil is resorted to for special purposes, both for preservation and to give strength. For example, the best handscrews are so treated. The oil also prevents glue from sticking, the most frequent cause of injury to handscrews.