Standard lengths of logs range from 10 feet to 24 feet, depending on the type of wood. (They are always cut in multiples of two feet.) It is possible to obtain lumber of greater length, but the log from which such lumber is to be cut must be delivered to the sawmill on special order.
Logging is a year-round operation and the time of cutting has very little effect on the durability or other properties of the lumber, provided the logs are properly cared for after they have been cut. Logs cut in the spring and summer must be handled in a manner different from those cut in the f all and winter.
Springcut and summer-cut logs are more likely to be attacked by insects and fungi. Opportunity for decay by attack of fungi can be reduced to a minimum if the log is stripped of its bark when it is cut, then piled openly on skids. Under no condition should a log be allowed to lie in direct contact with the ground. Another point which makes the handling of logs cut in the early part of the year different from those cut in the colder weather is seasoning, which we shall discuss in detail later.
The manner in which a log is cut into boards and planks has an important bearing on the quality of the lumber. There are two methods used: plain sawing and quartersawing. Plain sawing is the more common method, and has the least amount of waste. Such terms as flat and slash are also applied to this method.
Quartersawed lumber is also spoken of as being edge sawed or edge grained, especially when referring to softwood. A log that is to be quartersawed is first cut into four parts as shown. From these quarter-sections the planks and boards are cut in one of the four methods discussed below.
After the logs have been sawed into planks, boards, and timbers they must be seasoned. Seasoning is nothing more than a drying process whereby the moisture content of the wood is reduced. Moisture occurs in wood in two forms: free water and imbibed water. Free water is found in the cells, and imbibed water is found in the porous walls of the cell.
When the drying process takes place, the free water evaporates first; this stage is known as the fiber-saturation point. A piece of wood has approximately 30 per cent moisture at this point. As the drying process continues, the imbibed water is removed from the cell walls. This causes the wood to shrink.
The purpose of seasoning is to shrink these cells before the lumber is used; for if this is not done shrinkage will take place eventually, with the result that whatever has been made of the unseasoned wood will tear itself apart. The drying or seasoning of lumber is done in two ways: air seasoning and kiln drying. The use of a kiln to dry lumber reduces the moisture content far below that obtained by air seasoning.
Air seasoning is the cheapest method of drying lumber, for no expensive equipment is required. It is an extremely slow method, taking from 30 to 300 days to season a piece of stock, depending on the nature of the wood and the size of the material.
The drying time for stock that is thicker is more than proportional to the increased thickness; for example, a piece of wood 2 inches thick will take more than twice the time shown in the table. The drying time shown in the table is based on the average climatic conditions of the regions in which the particular species is native.
Stock piled for seasoning in the spring and summer (the best drying weather) requires a minimum period; material set out for seasoning in the fall and winter takes the maximum length of time.
Seasoning means the reducing of the water content in a piece of wood. The water must be evaporated and carried away from the lumber in the form of water vapor. This can be done only by allowing free circulation of air around each board or plank.
There are several factors that must be taken into consideration when air-seasoning wood. They are the direction of the prevailing wind, variations in temperature, and variations in humidity. The wind is necessary in order to keep the air circulating about the stacked lumber. High temperatures will speed up the evaporation and low temperatures will retard it.
Low humidity will hasten the drying and high humidity will slow it down. The perfect combination would be high wind, high temperature, and low humidity; but these three factors very rarely are found together.
The stacking or piling of lumber for seasoning is of the utmost importance. There are two methods of stacking lumber to allow free movement of air around each piece. In the first the lumber is piled with small spacer sticks, usually measuring 1" X 2", between each board. The spacers should be placed close enough so that there will be no sag in the plank or board.
In the second the boards are standing on end, with the edge resting against a support, and piled alternately. The stacked lumber should be protected against rain, the direct rays of the sun, and dampness from the ground.
At best, this method of seasoning will bring the moisture content of the lumber down to about 20 per cent. Stock having a moisture content this high will be found satisfactory for rough work such as general construction, but for cabinetwork or patternmaking this moisture content is too high.
In order to reduce the moisture content to pro-duce a piece of wood that can be used for cabinetwork or patternmaking, it is necessary to use a forced drying method. This method is known as kiln drying.
A drying kiln is nothing more than an air-conditioned oven in which the wood to be dried is placed. The main difference between the air- and kiln-drying process is that, in the kiln, heat, humidity, and circulation are controlled every minute. In most cases, lumber that is to be kiln dried is first air-seasoned, then placed in the kiln to further re-duce its moisture content.
It is possible for lumber to pick up moisture and thus for the cells to expand again after it has been taken from the kiln. Kiln-dried lumber should be stored with care so as to avoid exposing it to dampness.
Defects may develop in seasoning too rapidly or too slowly. When wood seasons too rapidly, uneven shrinkage takes place. Such defects as checks, honeycombing, warping, loosening of knots, and collapse of the cells are signs of uneven shrinkage. Checks are lengthwise separations of the wood, usually across the annual rings.
Honeycombing is a form of checking which is not often visible on the surface of the wood, as it commonly occurs in the interior, usually following the medullary rays.
Wood that seasons too slowly may develop such defects as molds, stains, and decay, all of which can be traced to the action of fungi.
If proper air circulation is maintained during the drying process, these defects can be eliminated or kept to a minimum.
Seasoning proceeds more rapidly during the late spring and summer and may cause excessive checking. This shows up in the wood as a split or series of splits at the ends of the log. These checks may run the full length of the log.
Trees cut in the late fall and winter season more slowly, with a corresponding reduction in checking. There is very little danger of insects or fungi attacking wood stored out of doors in the cold weather, for insects are dormant, and what moisture is present is in a form that is not conducive to fungus growth. By the time warm weather arrives, the wood is partly seasoned and therefore less susceptible to attack. Since fall- and winter-cut wood seasons more slowly, it is less likely to check excessively.
Selection of lumber
The selection of the proper lumber for any particular job is of prime importance to the woodworker. Each kind of wood has its own characteristics, and these must be known if a wise selection is to be made.
Grades and sizes
Lumber grades are fairly well standardized, officially, by the American lumber standards. Many lumber retailers use a local grade classification or their own judgment; therefore when ordering lumber from the local yard be sure that the standards you have in mind correspond to those your retailer is using.
Softwoods. Softwoods, such as pine, fir, and hemlock, are graded as follows:
The top grades, A Select and B Select, are suitable for natural and paint finishes. These are often sold together, and called B or Better grade. C Select and D Select are suitable for paint finishes only. The ordinary grades of lumber beyond the Select grades are based on the size, number, and location of knots, pitch pockets, and other defects.
Knots and pitch pockets are strictly limited in the Select grades, and therefore have no influence on the strength of the board. #1 Boards and #2 Boards have little difference in strength, and although weaker than the select grades they are sufficiently strong for permanent structures. #3, #4, and #5 Boards permit large knots, holes, and shakes, with a corresponding weakening of the board.
Hardwoods. The two highest grades of hardwood are known as Firsts and Seconds; they are usually sold combined. These gradings are based on factory use rather than on building requirements, and take into account the yield and size of the cuttings with one face or surface clear. Hardwood lumber that is used for building construction is graded the same as softwoods such as #1, #2, #3, #4, and #5 Boards.
Lumber, as ordinarily stocked in the average local yard, is surfaced or dressed smooth on the two faces and the two edges. Stock may also be carried dressed on the faces only. The purpose of the dressing is to make the lumber ready for immediate use, and to avoid paying transportation charges on material that would have to be removed and discarded when the board is put to use.
Boards, when purchased at the lumberyard, are sold according to their nominal size; that is, a piece of wood that is listed by the lumber dealer as measuring 1" X 6" would actually measure 25/32" x 5 1/2" if it was dressed on all four sides. If the same piece were sold rough-dry (seasoned but not dressed), it would measure 29/32" X 5 5/8".
Lumber is bought and sold by a unit which is known as the board Pot. The board foot is actually a cubic measure and means a piece of wood the contents of which is 1 foot square and 1 inch thick. It is obtained by multiplying the thickness in inches, by the width in feet, by the length in feet. The formula would be inches X feet X Pet. A piece of wood measuring 1" X 4" X 10' would contain 34 board feet.
When purchasing lumber, remember that the cost is based on the size of the rough stock, or the size of the original piece that was required to produce the piece being delivered. The purchaser must pay for the waste. Therefore when figuring the board-foot content always use the nominal size of the board or plank.
Bill of materials
When ordering lumber a bill of materials should be made. It should include the number of pieces required and the length, width, and thickness of each piece.
In order to obtain from the lumberyard the piece of wood you have in mind, you should have a thorough understanding of what is meant by length, width, and thickness as applied to lumber. When dealing with lumber, the length of a piece is the measurement taken from end to end, or with the grain; it never changes.
When preparing a bill of materials, there are two methods of arranging the required information, depending on whether standard-size lumber is being ordered or the lumber is to be cut to a specified size at the lumberyard or mill. A bill of material for ordering stock-size lumber would have the necessary information arranged as follows: 2/10' 1" X 2"
Such a bill would call for 2 pieces of lumber 1" thick, 2" wide, and 10' long. The number of pieces of like size are shown by the first figure; the length of these pieces is given in the second figure, and the thickness and width of the material ordered are shown in the third and fourth figures, respectively. The figures must always be written in that order.
When ordering lumber cut and finished to special sizes, the arrangement of the figures is slightly different. In this case the number of pieces would be listed first, the thickness next, then the width and the length. For two pieces of wood 3/4" thick, 2 1/2" wide, and 48" long, a bill of materials, or cutting list as it is often called, would appear as follows: 2 pc. 3/4" X 2 1/2" X 48"
Since lumber is stocked in standard dimensions at the lumberyard, to order lumber you must make up a cutting list first. The cutting list must show the thickness, width, and length of each piece needed, which you obtain from a drawing or plan of the article that is to be made. After the cutting list has been made up it is necessary to determine the required stock-size lumber.
In the list of standard-size stock will be found the various dimensions of lumber carried in the lumberyard. If two pieces of 3/4" X 2 1/2" X 48" lumber are needed, they can be obtained from one piece of standard-size stock 1" X 3" x 10".
Although only 8' are required, it may be necessary to obtain a piece 10' long, as standard lumber is generally cut in multiples of two starting with 10'. Even though an 8' piece could be obtained, it will not be enough to obtain two 48" boards because you must allow for waste or loss in cutting.