Photo: American hardwood export council
A veneer is a thin layer of coarser wood to be glued to an inferior wood to improve its appearance. The lower layer is usually one of lesser decorative or aesthetic value. Veneer refers to the thin sheets that are sliced off from a hardwood log by using different processes.
The machines that cut out veneers with differing grain patterns are basically of two types: ROTARY MILLS AND SLICING MILLS. Whereas rotary mills complete the entire veneering process at one location, the components produced by a slicing mill have to be further processed at another location. However, the log preparation procedure for producing the veneer faces is similar, though the type of cut is different.
Different slicing methods are used to produce different visual effects. Logs of a given species, cut by different methods will produce a variety of appearances.
There are many ways to slice veneer and each method produces a unique look in the pattern of the grain.
Here are the most common cutting styles for veneer production: rotary cut, flat slicing, quarter slicing, half round slicing, rift-cutting, lenghwise slicing.
- ROTARY CUT, LATHE PEELING, FULL ROUND ROTARY SLICING
It is the most economical method of cutting. In rotary cutting, the log is mounted centrally in the lathe and turned against a blade, as if unwinding a roll of paper. In order to achieve veneer sheet sequences, we score the log or burl at one point. The modern lathes use a computerized method to determine the true center of the log in order to optimize the yield. This produces sheets that begin wide and gradually get narrower as the blade cuts closer to the logs and burls heart. Since this cut follows the log’s annual growth rings, a wide, bold grain pattern is produced. This method of cut is also used for very exotic woods like burls and uniquely figured species like, bird’s eye maple, sapele pommele, and macore mommele.
The rotary cut veneer has a wild and variegated grain because the veneer is cut at a broad angle by the knife parallel to the growth ring (which is shaped like a tall slender cone). Veneer surfaces that have been rotary cut are best suited for components that make cabinets and smaller pieces of furniture. Rotary cut veneer is in demand in some markets because it is cheaper.
For all veneers that will not be rotary sliced, the logs are sawn into halves, thirds, or quarters or more.
- FLAT SLICING, PLAIN-SLICED VENEER, FLAT CUT, CROWN CUT
Prior to slicing, the bark of the log is removed, sawed in half longitudinally (roughly) and these sawed up halves or quarters (called flitch) are placed in vats of hot water or steam. The log is sliced by clamping the halves into slicing machines with the back or the flattest side against the slicer plate. The slicer alternately raises and lowers the sawn half at an angle between a fixed knife and a pressure bar. Whereas in some machines the slicing happens during the “down” stroke, in others it happens in the “up” stroke.
Once a veneer is cut, the log half or quarter is moved closer to the knife automatically by the machine. As the cut veneer falls off, it is stacked in pallets in the same order as it was cut. This order is maintained so that all the faces that are manufactured from a given sawn log of wood are alike. The cut veneers, nevertheless, display gradual changes in appearance and character.
The veneers are usually cut in a manner that best uses the width of the log, and the result is bundles of veneers of varying widths. At this point, veneer samples are sent to prospective buyers for evaluation. Typically, the samples from three locations within the flitch stack are sent to the buyers: first one at a one-third distance from the top of the stack, second one at half the distance from the top, and the third one from two-thirds into the stack. From a small stack, usually only two veneers are sampled. This helps the buyer to understand the character of the veneer from the face to the back of the log.
The outer parts of the veneers are then clipped to remove the sapwood, juvenile wood, knots, and discoloration. The clipped veneer lines are not parallel and as a result they are not ready to be spliced at this point in time. The clipped veneers can be packed neatly in a bundle and these bundles are shipped to a veneer splicer who processes the dried raw veneers into faces and backs.
As in the case of rotary cut veneers, sliced veneers are also passed through a dryer to reduce moisture content to about 6 to 12 percent before they are subjected to any further processing. Some slicing machines have additional capabilities (like splicing) to convert the veneers into panel faces.
A plain slice is a cut that is tangential to the center of the log. The growth rings are conically shaped and, therefore, the cut at the bottom is parallel to the growth ring whereas the cut at the top is nearly at right angle to the growth ring. In a plain slice, the pattern of the grain assumes the shape of an inverted “V” (sometimes called a cathedral on some veneers). Many a time, the cathedrals are of the same height on most of the veneers and they are centered. A panel that is made up of such veneers has the design of a king”s crown on them. For the same reason, these veneers are also called “crown cut”. Clipping before the veneers are spliced can sometimes result in a partial cathedral component on the sheets. If these are matched, the resultant appearance is a split-heart cathedral. Plain sliced veneers may also alternately have a rift cut, quartered or rotary cut appearance. This is dependent on the individual log and the angle at which it is cut when sliced.
When a log is sliced, it displays a more pronounced grain pattern and for this reason sliced veneer is preferred to be used in the manufacture of wall panels, cabinets, furniture and cabinets of higher quality.
- QUARTER SLICING
The quarters from a log are usually cut with the help of a plain slicing machine. The log is cut into four longitudinal pieces instead of the usual two. The quarter log is placed on the slicing machine such that the blade, at every stroke, slices the flitch in a direction perpendicular to that of the growth rings and in a radial direction to the log”s center. The appearance of the grain is very straight in this case. Wood rays, found in all species, but more commonly in wood such as red and white oak, are exposed by quarter slicing and in these cases are referred to as fleck or quarter flake.
In cases where only the straight gain appearance is required minus the flake, the log is rift cut to eliminate the flake. For this, the flitch is placed on a machine called the stay log or half round. This machine functions like a rotary slicer but is different in that the log passes under the knife only for a minor part of the revolution. The log is cut perpendicular to the rays but not the growth rings. Rift cut machines make the appearance of flakes less prominent on veneers than quarter-sliced ones.
Plain sliced logs sometimes take on the appearance of quartered veneer or rift-cut veneer because, at the end of the slicing exercise, the machine cuts the log at right angles to the growth rings and thereby exposes the juvenile wood of the tree. This portion is softer, weak, has knots and is discolored. These are clipped off and the remaining straight grain from the sides of the log is visible. Such veneers are also called “bastard quarters”. They are few in numbers and are narrow.
The bastard quarters of red and white oak that contain flakes are sold as quarter sliced and those without flakes are sold as rift cut veneers. However, they should meet basic standards for the specified grade. Panel face components made from such veneers are also likely to be narrow.
Using a stay log machine is yet another method of cutting veneers from logs. This is commonly used on the wood of maple (it can be used on any wood species) as it produces lighter wood (more sapwood) than does a conventional log slicer. Keeping away from the dark heartwood of maple is a desirable characteristic. It works much like a rotary log slicing machine. A flitch is placed with an offset on the lathe in a manner that the knife comes in contact with the flitch for only half a rotation. The resultant cut resembles a plain slice with a broader grain appearance. Such veneers are sold as half round or plain sliced.
Whatever be the method of cutting the veneer, preparation of the log is a very important factor. The log should be adequately cooked. Under-cooking does not loosen the fibers and it results in the veneers displaying a ruptured appearance. Over-cooking, on the other hand, causes excessive breakdown of the fibers and the veneers tend to have a fuzzy appearance. This may also lead to red-colored discolorations in parts of the log.
Another important factor is the condition of the knife on the lathe. If the knife has a nick, it leads to formation of gouges on the veneer surface all across the grain. These marks are more obvious in wood that is hard and those that have a high mineral or ash content like hickory. They can, however, occur in any species. Lighter knife marks are removed by sanding the panel surface. Those that cannot be removed serve to reduce the grade and value of the veneer sheet.
In this context, it is important to note that any method of cutting the veneers produces different patterns of grain on the veneer surfaces. Whereas rotary cut wood sometimes appears like plain sliced, half round cut produces both the patterns. Quarter sliced and rift cut veneers have straight grain regions on the sides. The Hardwood Plywood and Veneer Association and Architectural Woodwork Institute standards therefore have a note under the veneer heading tables which clarify that the headings refer only to the patterns on the veneer face and not the method by which it is cut. The method of cutting is the prerogative of the mill unless otherwise specified by the buyer. An example would be “plain sliced to be cut on a either a vertical slicer lathe or a half-round rotary machine”.
- HALF ROUND SLICING
Half round slicing is a variation of rotary cutting. Veneer is sliced on an arc roughly parallel to the center of the log to achieve a flat cut appearance. The ‘cathedral’ pattern can have more rounded tops than the pattern produced by flat slicing.
Segments or flitches of the log are mounted off center on the lathe. This results in a cut slightly across the annular growth rings, and visually shows modified characteristics of both rotary and plain sliced veneer.
Rift slicing also achieves a straight grain pattern, but avoids the appearance of “flake” that occurs in some species when quarter sliced. Rift slicing uses a “stay log lathe,” which cuts with a rotary action. A quarter of the log is fixed to a plate on a turning stay log. As the flitch is rotated, the blade and angle can be varied so that the wood is cut exactly to produce the very straight rift grain. Most often, this method is used with oak. Other species such as rift-cut maple, walnut and cherry can be specified to be rift cut to achieve wider sheet widths. Since rift grain is generally the straightest and free from cathedrals and variations in grain, it is used to enhance verticality, and is easily sequenced and matched.
- LENGTHWISE SLICING
A board of flat sawn lumber is passed flat over a stationary knife. As it passes, a sheet of veneer is sliced from the bottom of the board. The width of the veneer and figure produced with this method depends on the width and figure of the sawn lumber and is typically variegated.
WHICH OF THESE METHODS OF VENEER MANUFACTURE IS USED IN YOUR SAWMILL?
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