Veneer Lathe Backup Roll

Topics

Introduction
Effective Use of the Backup Roll
Maintenance
Roller Bar Setting

Another key lathe feature is an effective backup roll that counteracts normal core flexure at small diameters and also speeds core ejection. The rolls pivot from arms attached directly to the lathe carriage. Three steel rollers come down onto the log at a 15 degree angle, holding the bolt firmly. Variable hydraulic pressure controls the device. Variable pressure control is a way of securing the block to the lathe knife. A trace roll located near one end of the block monitors the peeling process and relays information to the backup roll located near the middle of the block. The resulting pressure is the minimum needed to prevent the block from deflecting and creating thin, buckled veneer in the center of the ribbon as the veneer flows downline.

Two important criteria of any backup roll system are

1. How well it provides firm support to prevent lathe chatter and
2. Rapid core ejection.

The lathe backup roll is designed to counter act normal core flexure occuring at small core diameters (see Pic 1). Through reduction of core diameter, the backup roll can be an effective tool for increasing veneer recovery. Effective use of the rolls is characterised by veneer that is flat and free of buckles to the end of the ribbon. The core will have a slight hourglass shape produced by the backup roll, which does not allow the core to bow away from the knife (Baldwin 1981).

Picture 1 :- Operation of Backup roll (Courtesy of Dr. P.D.Evans)

The appearance of a barrelled shaped core with a noticeable corrugated appearance at mid length indicates that the lathe back-up roll is being used improperly or is not being used at all (Baldwin 1981) . Humpy and broken veneer appearing towards the tail of the ribbon is an additional indicator that the bolt is riding away from the knife at smaller diameters.

The actual design of the device used to counteract the flexure or bowing effect, is simple. An air, or hydraulic activated arm cradles the bolt being peeled. One or more sets of rollers spaced along the length of the block complete the structure. The combination of the pressure arm and the cradle formed by the rollers eliminates flexure and helps to produce quality veneer from small diameter portion of a peeler boltBaldwin (1981)

For full effectiveness of the backup roll it is essential that the lathe is properly maintained, eg loose knuckles, sloppy spindles or worn pitch rails. Considered that veneer thickness is measured in on thousands of an inch, lathes obviously must be designed and maintained for precession work. Moreover humpy veneer is created by variation in thickness of 0.008 inch or more between midwidth of the veneer ribbon, the correlation between a tight lathe and quality veneer is appreciated. Well regulated and consistent pressure is very important. The resistance needed to prevent core flex depends on controlled back up pressure. Extending or retracting the rolls swiftly and smoothly helps achieve veneer quality and increases the speed at which a series of small bolts can be cut.(Baldwin 1981).

Proper setting of the horizontal roller bar is also important. At larger diameters a pressure only one step away from tearing the grain can produce extremely smooth veneer(Baldwin 1981) . The same pressure can increases core flexure when peeling a smaller diameter.

The specific back up roll pressure load applied to the bolt varies with different species and to a lesser extent thickness. It therefore becomes necessary to match the load being applied to varied operating conditions. Gradually increase the pressure until flat buckle free veneer is produced. In adjusting the back up roll pressure to the species, lathe setting or peel thickness, the shape of the core can provide an accurate measure of successful operation.(Baldwin 1981).

Alternatively a tracer roll placed near one or both ends of the bolt measures the "should be position" at midlength during peeling and also monitors back up roll position. The resistance applied by the tracer roll prevents upward deflection. This resistance provides immediate adjustment in pressure to compensate for a range of peeler bolt characteristics.

Effective use of the backup roll can result in increased recovery of smooth, buckle free veneer. A decrease in splitting provides a generally upgrade of the veneer. Core ejection becomes automatic because of the push exerted by the roll. In general overall operating efficiency is improved.