Several types of superior wood products have been developed by modern technology. The majority are wooden “sheet” materials – flat sheets of laminated or compressed wood products that are generally stronger, more flexible, and less likely to split than ordinary lumber. They expand and contract less than common lumber and are available in large sizes (usually 4 by 8-foot sheets) that would be impossible to mill and distribute in the wood’s natural form. They are sometimes available in half sheets (4 by 4 feet) or odd sizes. All of these products fall under the categories of either plywood or hardboard.
In effect, plywood is a sandwich panel of real wood, made from an odd number of thin sheets of wood laid with the grain running in alternate directions and bonded under pressure, glues, and sometimes heat. There are several types available to the home craftsmen that are excellent for the construction of furniture.
Douglas fir plywood is the least expensive and most commonly found type. It is available in two types – interior and exterior – depending upon the glue bond and grade of veneer.
The inner piles of exterior plywood are all of high quality and bonded together with completely waterproof glue. This type of plywood will resist time and weather – even when boiled in water it will not decontaminate. Interior plywood is water-resistant but not waterproof, so should not be included in projects for permanent outdoor use.
All fir plywood’s (particularly rotary cut types) have a tendency to check when used outdoors. These should never be used outdoors with only a stain because of the maintenance problems. Checking can be reduced by careful painting with a conventional three-coat paint system. Edges should be daubed with a heavy coat of primer or a thick mixture of white lead and oil. Warping may become another problem, but is less likely to occur if the panels are stored in a dry place and finished identically on both sides.
Standard fir plywood thicknesses are ¼, 3/8, ½, 5/8 and ¾ inch. Some large lumber distributors also carry 1 1/8 inch.
Quality of face and back panels determine the grading of fir plywood. Letters A, B, C and D indicate the different grades. A is the best quality, B is smooth (excellent for painting), C has knotholes and splits, and D is the poorest (often used for inner ply of interior plywood). Two letters are used in the grading of a sheet of plywood – one for the face and one for the back. Where only one side will show, it is economical to use A-D, with one good side.
The biggest job when working with plywood is usually cutting the large panels to size. If you will be cutting several panels from a full-sized sheet of plywood, it’s a good idea to draw the sections on a piece of paper and transfer the marks onto the plywood. Always double-check measurements to be sure they are correct, and don’t forget to allow for the saw kerfs (width of cut) between pieces. If you plan to cut the piece with a handsaw, power radial or table saw, mark on the better face of the plywood and cut it with this side up. If using a portable power saw, mark and cut the panel from the back side. Plan to reduce the sheets to workable size with the first cuts.
Power saws are great for cutting plywood. Radial, table, and portable power saws are extremely helpful in cutting straight lines, jigsaws, band saws, and saber saws cut curves easily. If you cut plywood by hand, choose a keyhole or coping saw for curves and a handsaw with 10 to 15 teeth-per-inch for straight lines. Special plywood blades with fine, shallow-set teeth can be purchased for power saws, to aid in making clean cuts. Be sure all saw blades are sharp.
There are several methods used to help prevent plywood undersides from splitting-out when being cut. Both sides of the panel can be scribed through the top veneer along the cut-off line with a sharp knife or chisel. Or, you can carefully tape along bottom side of cut -off line with cellophane tape. When hand-sawing, put a piece of scrap lumber under plywood and saw both together.
To keep wood from splitting-out when boring holes, either clamp a piece of wood on the back of the plywood or turn it over as soon as the point of the bit appears through the backside and finish drilling from the back.
To plane plywood edges (a job that isn’t usually necessary), work from both ends toward the center using a sharp, shallow-set blade. If possible, cut a tiny bevel at each corner to help prevent edges from splitting-out.
Sanding surfaces of plywood is not necessary – it only removes soft grain. When sanding edges, use finer sandpaper.
Source by Protechwood