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Veneer Slicing and Peeling
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There are three methods of cutting veneers, sawing, slicing, half round slicing, and rotary peeling. Sawing thin veneers is seldom used today. Sawing veneers does not require pre-heating of veneers and is most suitable for logs to hard or brittle for slicing. This technique however, has a low conversion rate and difficulties are encountered in maintaining saws for precise cutting.
Slicing veneers accounts for approximately 5% of veneer production and is mainly used to produce radially orientated, fine figured, hardwood veneers for furniture or wall panelling face plies.
Veneers were not always sliced, before the industry revolution veneers where cut by hand using large frame saws and a good deal of skill. Mechanisation ushered in the development of veneer sawing machines but this was relatively wasteful as the saw removed as much wood as it produced veneer. The advent of the veneer slicers in the early 1900's revealed the true potential of timber veneerBaldwin (1981) Slicing has been refined over the decades and today there are four primary systems used.
- Longitudinal Slicing
The log is cut into flitch slabs usually between 50 and 100mm thick. The slabs are drawn under the stationary slicer blade end on, and the veneer is paired off down the length of the flitch.
- Crosscut Slicing
Over the years, more decorative veneer has been produced by this method. It is fast and very efficient, with wastage from each bolt being minimal. In crosscutting, the flitch or full log is locked into the machine. Systems have been developed with stationary or moving knives but the principle is the same. The knife moves across the width of the flitch at a slight angle, and because of its short travel, quick cycle times are achieved with high output. As the flitch is sliced away the carriage gradually moves closer to the blade with a pressure bar controlling the knife position.
- Staylog Lathe Slicing
This technique can accommodate the quarter cut slicing, crown cut slicing, semi rotary peeling, or highly decorative difficult timbers such as burls, butts, ripple and fiddleback. Because of its flexibility it can maximise the yield of these timbers and is particular suited to cutting smaller logs such as regrowth.
The log or flitch is locked into the machine arm which rotates on a centre axis like a lathe. With each revolution the flitch contacts the knife, which cuts across the width of the face. The veneer is sliced off in a tangential arc to the centre axis, and therefore produces veneer which is actually wider than the dimensions of the flitch it came from. This is particularly advantageous when using smaller logs. As the flitch is sliced, both the log arm and knife carriage move together to maintain balance and the correct cutting angle. In addition to being an optimum system for small regrowth logs this technique offers another important advantage over conventional slicing. The staylog lathe has the facility to manipulate the position and angle of the flitch in the slicer, thus avoiding or minimising impact of unsound or undesirable sections, or conversely taking advantage of the better sections. There is only one lathe of this type in use in Australia, and is utilised by Gunns Veneers in Tasmania.
- Rotary slicing or peeling
Picture 1 :-Rotary peeler (Courtesy of Dr. P.D.Evans)
Rotary peeling using a lathe accounts for nearly 95% of all veneer production (see Pic 1). It is predominantly used in the manufacture of structural plywoods(Baldwin 1981, International con. etal 1963) . It involves centring the log or bolt onto the lathe chuck. The log is slowly rotated and the knife carriage moved into the log. The knife then converts the bolt into a cylinder. In this process of rounding, various widths and lengths of wood (fishtails) are removed from the log surface.
Sliced face veneers from a given flitch are stacked and bundled in the exact consecutive order in which they are cut from the bolt. Special care is then taken to maintain this sequence of individual veneers during all subsequent operations, so that veneer matching can be more easily attained. This bundle of sliced veneers is also called a flitch.
After rounding the bolt revolves against the knife at speed above 420 km/min and spindle speed up to 200 rpm have been used on large lathes(Baldwin 1981) . At slower speeds vibration is likely to adversely affect the quality of veneer, producing a rough and corrugated surface.
The gears feed the blade towards the bolt as its diameter is reduced, so that veneer is unwinding in a continuous ribbon.
During the process of peeling, lathe checks can develop in the underside of the veneer, as the underside is under tension during the peeling process. The side of the veneer in which checks occur is known as the loose side, whereas the check free side is known as the tight side(Evans 1996) . The setting of the knife and nose bar pressure during peeling is important in reducing the formation of checks, and also in controlling veneer thickness. Each wood species requires certain nose bar pressure to minimise the development of lathe checks, however nose bar pressures are positively related to wood density.
Veneer recovery from bolts is variable ranging from 30 to 95% depending on bolt size and process variables. With the increase of technology the recovery rate percentage is increasing. To increase recovery, output and quality Baldwin (1981) suggests the following should be installed or maintained.
- Hydraulic clamping and telescopic spindles
- Mechanical charging devices
Frequently used accessories to the slicer are overhead cranes, lift tables for stacking, short driveless roller conveyors as waiting places for stacks, and centre cut saws for reclaiming filches with bad heart. Gunns Veneers in Tasmania actually use the centre cut saw for breakdown of the initial log, as well as conventual cross-cutting. Transport within the mill is rarely mechanised and is mostly done by forklift or manual lift truck. The floor is treated to give off as little dust as possible.
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