Types of plywood construction

I have mentioned plywood a number of times, and it occurs to me that you might possibly be laboring under the illusion that "plywood is plywood," and that one is like another. Nothing could be further from the truth.

Plywood is constructed in several different ways, and there are various grades and classifications. All plywood constructions have

Fig. 521. Cut-away view of the internal construction of y4-inch veneer-core plywood.

this much in common: they consist of a number of layers of wood bonded together to make up the full thickness, whatever that may be, of the resulting panel. Starting from one side and going through the thickness of the plywood panel, you will find that all plywoods are made with the grain of each layer running at right angles to the previous layer.

Within the framework of these general similarities, there are several different ways of making plywood. The first and most common method results in a product called veneer-core plywood. Since most of the material you will be using will be y4-inch plywood, the illustration (Fig. 521) shows the construction of 34-inch veneer-core material.

The construction consists of two layers of veneer on the outside, crossbands beneath each layer of veneer, and three layers of core in the middle; making a total of seven plies altogether. This is the usual method, although you may possibly run into some nine-ply material. The number of plies is always an odd value, giving a balanced construction on either side of the center ply. This arrangement is intended to reduce any tendency to warpage —and it does.

For material of less than %-inch thickness, the basic construc tion remains the same, but the number of plies is reduced. For example, the 14-inch material, which you will use for control panels, will have only three plies. If you use any l^-inch material, it will generally be five-ply. Plies are always added or dropped in pairs on either side of the center. And, customarily, the core plies are thicker than the veneers or crossbands.

The next important construction method results in a product called solid-core plywood (Fig. 522). The veneer and crossband layers, top and bottom, are similar to comparable layers in veneer-core plywood. The difference lies in the core. In this case the core

Fig. 522. Solid-core plywood differs from veneer-core in that the core material is composed of bands of solid material edge-glued to form the sheet of core material.

consists of 3-inch wide strips of solid wood running lengthwise through the panel and edge-glued to each other. Whichever way the annular rings of one strip of core material are curved, the rings in the strips on either side of it will curve the other way.

Solid-core plywood is more expensive than veneer-core, but is preferable. It will take edge veneers better and tends to have a smoother surface. That is to say, it has less tendency to telegraph ripples from underlying veneers and is more warp resistant than veneer-core.

Counterfront is a type of plywood that you will probably have little occasion to use except for built-in work, but you should be able to distinguish it from solid-core. Counterfront, like solid-core, consists of two veneer layers and two crossbands plus a solid core (Fig. 523). The core consists of strips li/2 inches wide instead of the 3-inch strips in solid-core. The grain of both the coring strips and the surface veneers runs across the short dimensions of the panel, rather than down the length. This material is used mostly as an architectural plywood, rather than as a furniture material.

A relative newcomer to the field is plywood constructed on a core of chip or particle board. Chip board or particle board con sists of wood chips or wood shavings mixed with a bonding agent and pressed into a panel. Using this board as a core, plywood is made by adding the crossbands and surface veneers. The major claims made in favor of this material are comparable warp resistance and smoothness at a lower cost than solid-core. Since the core is grainless any tendency toward warping is sharply reduced.

One thing to watch out for with these chipboard core materials is the density of the core. There is considerable variation between different chipboards. If you are going to use a plywood with this type of core be sure it is strong enough to make good, solid cabinet joints. Some are; others are not. It depends on the purpose for which they were made. Some light-density chipboards are manufactured primarily for tabletops or sliding doors, where joints are not required and where stability against warpage is the primary consideration. Such a material is likely to be unsatisfactory for cabinet purposes.

On the other hand, denser-chipboard cores are made primarily for cabinet purposes.

Graining standards for veneer faces

One aspect of plywood quality is governed by the type of coring specified, another is covered in the grading of the face veneers. A plywood panel will have two face veneers, one on each side. These face veneers will be of the same thickness, but they will not necessarily be of the same quality nor even of the same material.

The price of plywood varies depending upon the grade of face veneers used. Use a good grade on any faces that will show. On the other hand, it is rather silly to go to the expense of paying for good grades on faces that will not show. For example, it is ridiculous to use good two-faced plywood for building speaker enclosures, where one of the faces will be inside and never seen. On the other hand, a swinging door is adequate reason for the use of good two-face material.

In general, then, specify one good front face. Specification of the backing face will vary depending on the cabinet; that is to say, on how much of the backing face will show.

For your convenience, we list here the basic Hardwood Plywood Institute Grading Standards governing faces and backs of hardwood plywoods. Most of the plywood you encounter will be governed by these standards, as the major American manufacturers are members of the Hardwood Plywood Institute.

Custom grade

This grade includes special selections and types produced by individual mills, or panels of a grade description agreed upon by buyer and seller. Architectural plywoods, technical types and matched-grain panels for special uses are also included.

Good grade (1)

For natural finish—the face shall be made up of tight, smoothly cut veneer, containing the natural character markings inherent in the species. If made of more than one piece, matched at the joints to avoid sharp contrasts in color and grain. A few small burls, occasional pin knots, slight color streaks or spots shall be permitted. Knots (other than pin knots), worm holes, splits, shake and doze shall not be permitted. Sound grade (2)

For smooth paint surfaces—the face shall be free from open defects to provide a sound, smooth surface. The veneer is not matched for grain or color. It may contain mineral streaks, stain, discoloration, patches, sapwood, sound tight knots up to inch in average diameter, sound smooth burls up to 1 inch in average

diameter, hairline splits or open joints up to a maximum of 1/64 inch. Rough-cut veneer, brashness, shake or doze are not permitted. Utility grade (3)

Discolorations, stain, mineral streaks, patches, tight knots, tight burls, knot holes up to y4 inch in average diameter, worm holes, splits or open joints not exceeding 3/16 inch and not extending half the length of the panel, cross creaks and small areas of rough grain shall be admitted. Brashness, shake or doze are not permitted.

Reject grade (4)

The veneer shall be unselected for grain or color. Knot holes no greater than 2 inches in maximum diameter and no group of knot holes in any 12 inch square exceeding 4 inches in diameter and splits no wider than y2 inch shall be admitted. Splits y2 inch wide at widest point may be one-fourth panel length; those not more than ys inch wide at widest point may be one-half panel length; those not more than y4 inch wide may be full panel length. Mineral streaks, stains and discolorations not associated with rot or doze, shims, plugs, patches, knots, burls, worm or borer holes and other characteristics are permitted, provided they do not seriously impair the strength or serviceability of the panel.

Fir plywood

In addition to the grading standards used for hardwood plywoods, another set of standards has been set up for grading fir plywoods.

In spite of the fact that its exterior appearance and finishing properties suffer by comparison with the hardwoods, fir plywood, for our purposes, is structurally just as sound as a plywood with a hardwood veneer. And, in view of the cost factor, it is, for many applications, the material of choice. It would be absurd, for example, to make the backs, bottom and baffle boards for speaker enclosures out of hardwood veneer. It would be equally silly to make the internal partitions in a horn out of hardwood veneer.

Fir plywoods are made entirely in veneer-core construction and are of two types, as well as several surface-appearance grades within each type. The two types of fir plywood are differentiated by the kinds of glue bond between plies. The exterior type is bonded with a completely waterproof glue, and it is required by the standards that the glue bond test out to be stronger than the wood itself. Interior type is made with a moisture-resistant but not waterproof glue. It will stand up under occasional wetting, but is not intended to withstand soaking.

In each type there are several grades, which refer to the surface appearance of the plywoods, similar to the grades used in hardwoods. Altogether there are seven grades of exterior and nine of interior type. However, not all of them concern us.

Grading of fir plywood is by letter rather than by the numbers used for hardwoods. The following standards for face veneers in fir plywoods are quoted from official publications of the Douglas Fir Plywood Association, which sets the standards in this industry. The standards given are minimum for each grade or, to put it another way, the defects mentioned are the largest defects that will be allowed in each grade. The largest percentage of the material exceeds the standards given for the respective grades.

Veneer grade A

Must present a smooth surface free from knots, open splits, pitch pockets and other open defects. The veneer should be well joined if more than one piece is used. Grade A admits discoloration, sap wood and pitch streak averaging not more than 54-inch width and blending with the color of the wood; admits maximum of 18 veneer patches in a 4 X 8-foot sheet; admits shims and neatly made panel patches. The shims may not be used over or around any type of patch and multiple repairs must be limited to two patches. All patches and repairs must run parallel to the grain. Grade A also admits approved plastic filler in splits and other minor defects up to 1/32-inch in width; in small splits or openings up to 1/16-inch in width if not more than 2 inches long; in small chipped areas or openings not to exceed i/8 by 14 inch long.

Veneer grade B

This grade must present a solid surface, free from open defects, except splits not wider than 1/32-inch.

Vertical ambrosia beetle borer holes are permitted if they do not exceed 1/16 inch in diameter and averaging not more than one per square foot; also horizontal tunnels 1/16-inch across, 1 inch in length, 12 in number in a 4 X 8-foot panel, or proportionately in other dimensions.

Veneer quality C (repaired)

This is used for underlayment grade only. Tight knots can be up to li/2-inches in greatest dimension and worm and borer holes or other open defects may not exceed l/4 X i/£-inch. Splits not to exceed 1/16 inch wide are allowed. Solid, tight pitch pockets, ruptured and torn grain, minor sanding defects and sander skips up to 5% of the panel are allowed.

Veneer quality C

Knotholes shall be 1 inch in their least dimension; pitch pockets not wider than 1 inch; splits 3/16-inch (must taper to a point). Worm or borer holes % X ^A inches; tight knots—\/2 inch; plugs, patches, shims and minor sanding defects are permitted.

Veneer quality D

This veneer is used only in interior type plywood. Knot holes —21/2 inches; pitch pockets—2 X 4 inches; splits, widths at widest point: 1/2 inch up to quarter panel length; 14 inch up to half panel length; 3/16 inch up to full panel length; all must taper to a point.

Plugs, patches, shims, worm or borer holes and minor sanding defects are allowed.

Veneer quality N (natural finish)

This is a special grade. It presents a smooth surface, 100% heart wood of yellow or pink color without stain. It must be free from knots, splits, pitch pockets and other open defects.

If joined, not more than three pieces of the veneer should be used, joints to be well matched as to color and grain and all joints to be parallel to the edges of the panel.

A maximum of two shims is allowed, not to exceed 6 inches long at the end of the panel. A maximum of four well-matched small patches not to exceed % X 2i/£ inches is permitted.

All repairs must be parallel to the grain of the panel; neither overlapping of repairs nor plastic filler is permitted.

With these various veneer grades, different grades of plywood panels are made; the grading of the panels depending on the quality of the front and back face veneers. For example, AA grade of either exterior or interior type is good two-face. AB is good one face with the second face not quite as good as the first. On grades AC and AD the back face becomes progressively less attractive in appearance. By the time you get down to CB, you're using a utility grade. Structurally, it's just as sound as the others but its appearance is not as good.

In a situation where, for example, Formica type plastic laminates are going to be used for surfacing the cabinet, why pay for A faces when they're going to be covered? BD is a perfectly good grade for this purpose. No one is likely to stare for hours with rapt attention at the back or the bottom of a speaker enclosure. Again, this is a perfectly good use for BD grade.

Special surface veneers

In addition to standard hard or soft-wood panels, several companies produce specially surfaced decorative panels primarily for use as wall surfacing. Among the various decorative effects available are striated surfaces, embossed surfaces, simulated tongue-in-groove and parquet effects, actual tongue-and-groove strips and raised-grain effects. These special-effect panels are not desirable for a free-standing cabinet but, where a built-in wall installation is intended, some very beautiful results can be achieved by integrating the installation into a paneled wall.

I am sure that you are all familiar with at least some of the laminated plastic surfacings that are available. These appear under a wide variety of trade names: Formica, Micarta, Textolite, Conso-weld, Pionite, Nevamar, etc. You are probably familiar with these sheets in the form of the rather grisly patterns used for kitchen work surfaces, bathrooms, luncheonette counters and tabletops. These same panels are also available in excellent wood grains and in a number of solid colors. Some very exciting combinations can be achieved by the judicious use of solid colors; for example, a white top or white fronts on a walnut cabinet.

The great advantage of these materials, of course, is that they are virtually impervious to the assaults of alcohol, cigarettes, small children and pets.

Walnut grains as well as dark and blond mahoganies are very attractive in plastic laminates; oaks or birches less so.

Hardware

The hardware, or lack of it, employed in your furniture can practically make or break the design. Choose it with great care.

  1. 524. Several examples of available knobs and pulls. The variety of hardware is so great that the best thing to do is browse through a shop or catalog until you run across something suitable.
  2. 524. Several examples of available knobs and pulls. The variety of hardware is so great that the best thing to do is browse through a shop or catalog until you run across something suitable.

You won't believe, until you have browsed through a really well-stocked hardware store, what a tremendous variety is available. There are literally hundreds of different knobs and pulls, as well as numerous kinds of hinges, catches, ferrules casters, gliders and various types of decorative metal trim.

Your choice of knobs and pulls is strictly a matter of taste. Look at what is available until you find something that strikes your fancy. A few types are shown in Fig. 524.

With hinges there is one practical consideration over and above appearance. Whether you use the completely concealed type such as the Soss, the semi-concealed pivot hinges, piano hinges, butt

Fig. 525. Hinges are also a matter of taste. These are but a few of the available types.

hinges or any decorative hinges, be sure that they are capable of carrying the load of the door, lid or drop front you plan to hang on them. Fig 525 shows just a few of the available types.

As far as weight-handling capacity is concerned, the piano hinge is the strongest. The butt and Soss hinges are next. The weakest of the lot is the pivot hinge, so be careful as to how much weight you plan to load on it.

If you have any pullout drawers, for example, housing record changers or tape machines, you will need drawer slides (Fig. 526).

Fig. 526. Drawer slides of several types are available for equipment that is mounted in pullout drawers of any kind.

Make sure that the drawer slide will be able to carry the weight of the equipment. The drawings at the left show ballbearing catches—a few of the many different types that could be used.

Fig. 527. Door catches of various types are also available in many sizes, shapes and forms. The bullet and magnetic types are the least conspicu-

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Fig. 527. Door catches of various types are also available in many sizes, shapes and forms. The bullet and magnetic types are the least conspicu-

The standard type will be perfectly adequate for your record changer but, if you have a good-sized tape machine in a pullout

Fig. 528. Casters must be chosen with a view as to how often the cabinet is going to be moved and how much weight it wilt have to carry. The one illustrated here is a good general purpose type.

drawer, you will probably find it will be necessary to look for some heavy-duty slides. Check the weight of the unit to be drawer, you will probably find it will be necessary to look for some heavy-duty slides. Check the weight of the unit to be

Fig. 529. Ferrules for wooden legs are available in brass. They are primarily designed for round tapered legs.

mounted in the drawer, and be careful to get slides rated to carry, say, at least 5 pounds over this weight.

Some slides are made for undercarriage mounting (that is, to be used under the drawer) while others are made for side mounting between the drawer and the side of the cabinet. Usually, either is satisfactory (and they'll still work perfectly well) but, if you use a slide in a manner different from the one for which it was intended, you'll change its weight-carrying capacity. So when you check the loading capacity, be sure you know which way the slide is to be mounted to give that capacity.

Of the various types of door catches available, (Fig. 527) the most inconspicuous is the bullet catch. The only trouble with it

Fig. 530. An example of an all metal leg. Many types are available in brass, wrought iron or plated metal.

Brass seems to be the most attractive material.

is that it's not too well constructed for the most part and sometimes tends to jam after its been in use for a time. You won't have this trouble with the spring-clip types, but they do show more.

A rather nice, recent innovation in this area is the magnetic catch. It consists of a magnet mounted in a carrier in the cabinet and a plate of ferrous metal mounted on the door opposite it. No moving parts, nothing to wear or jam. Very lovely indeed.

Casters (Fig. 528) are another item to which you have to give a second thought as to the amount of weight that is going to be placed on them. If you've got a unit that is designed for casters so that you can roll it around, be sure that you've left a good solid area in which you can attach the caster, and also that you use a large enough caster to handle the weight. Remember that the larger the caster wheel, the easier it will roll over carpets and minor obstructions such as door sills and the like.

The trouble is that by the time you've got a big enough caster so that it will roll easily, it's also big enough to be a trifle unsightly. You'll probably wind up having to make some sort of a compromise between the size that would be functionally best and the size that would be esthetically pleasing.

If you want brass tips or ferrules on the legs of your furniture, you'll find a good selection available in round ones, but not much in square ones (Fig. 529). So if you're going to use ferrules, figure on using a round tapered leg. If you want to use a leg in metal throughout (Fig. 530), there is wrought iron, which has the advantage of being very inexpensive, but brass or brass plate is far more beautiful. In brass and brass-plated legs, sizes range from 3 to 30 inches in height, in round, tapered shapes either straight or angled.

A small number of straight square legs in brass are available, but not nearly the selection that you can get in round tapers. Also, a few decorative brass legs and some decorative brass feet are available for use with period designs.

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