A lathe monitoring system has been developed and successfully tried in a mill. The system can measure the position, the hydraulic driving pressure and contact pressure of the backup rolls, the position and the hydraulic driving pressure of the roller bar, the position and contact pressure of the knife carriage against the peeler block and the driving torque of the spindle motor. Some of the monitored data points required additional sensors which were then connected to and then downloaded directly from the lathe controller, i.e., PLC and VME. The results showed that the lathe parameters vary significantly with time and knife position. The backup roll offsets control the lathe performance and peeling quality, particularly spin-out rate and veneer thickness variation. The best results seemed to come from the combination of tighter outer offset and looser inner offset.
Further work is needed to fine tune the software program for user-friendly data analyses. More mill tests are required to understand the interactions between the backup rolls, the roller bar, the knife and the block.
This report addresses issues about productivity, recovery and quality concerning veneer peeling in plywood mills. It was demonstrated that green veneer can be composed using a stitching technique. The maximum stitching speed was 50 ft/min which was slower than a current veneer composer. Stitched veneer did not have a significant effect on bending properties, but shear strength was slightly reduced which could be caused by the existence of stitching threads between the glueline.
The roller bar diameter size had a significant influence on veneer quality. In general, peeling veneer with a 1” diameter roller bar resulted in the smoothest veneer with the most uniform thickness. The veneer thickness and roughness between 1.0” and 2.56” diameter roller bars were significantly different, but the difference in veneer quality between 1.75” and 2.56” diameter roller bars was not significant. Further, the difference in veneer quality between 1.0” and 1.75” diameter roller bars was not significant except for veneer roughness.
Knife height also had a significant effect on veneer quality. Setting the knife at the spindle center proved to be the best. Veneer thickness at this setting was consistently closest to the target, and had the smoothest surfaces and smallest lathe checks. Average veneer thickness was lowest as well. While higher or lower settings created rougher veneer, higher settings were more forgiving than lower ones. For best results, the peeling knife should therefore be set at 0.0” to 0.015” above the spindle center.
Incisor teeth pattern affected veneer quality. Narrower teeth and a wider gap resulted in better veneer quality in terms of veneer curl-up (flatness) and green and dry veneer thickness variations. However, the effect of incisor teeth patterns on veneer roughness and lathe checks seemed to be negligible.
The validation tests revealed that an optimum lathe setting for the smooth roller bar was the following: pitch angle (PA) =89.50, vertical gap (VG)=0.425” and horizontal gap (HG) = 0.1”, and the optimum lathe setting for the incisor bar was the following: PA=90.50, VG=0.388” and HG=0.1” to 0.11” when peeling 1/8-inch veneer.
The peeling computer program VPeel® was successfully upgraded to allow users to define profiles of pitch angle and horizontal gap. This feature will help the veneer product industry to define optimum lathe settings.
As a result of its fast growth and abundant availability, aspen has become an increasingly important commercial wood species in the production of oriented strand board (OSB) and veneer-based composites such as laminated veneer lumber (LVL). The purpose of the study described in this report was to determine the effect of conditioning temperature on veneer quality using a 5/8" roller bar, and to determine the optimum bar gaps based on results from previous Forintek studies on aspen veneer peeling.
Veneer incising at the peeling lathe, a new technology developed at Forintek, has been increasingly used in Canadian/US softwood plywood/LVL mills. Significant benefits include reduced veneer curl-up and spin-outs and increased veneer recovery. However, veneer incising and peeling is a very complicated process with a number of interactive variables: 1) lathe setting variables including vertical gap, horizontal gap and pitch angle; 2) veneer incising variables including overdrive percentage of the incisor bar and 3) lathe peeling speed. Therefore, optimization of veneer incising and peeling appears critical to mills to achieve good veneer quality and high veneer recovery.
In order to better simulate the industrial peeling lathe, Forintek’s laboratory mini-lathe was significantly upgraded. A statistical method (Response Surface Method) was used to investigate the effects of 5 main process variables: vertical gap, horizontal gap, pitch angle, peeling speed and bar overdrive percentage on the veneer thickness variation and curl-ups. More than 100 spruce and Douglas-fir blocks were peeled using either incisor bar or smooth roller bar. The effects of bar type on veneer curl-up and thickness variation were compared. A series of regression analysis models were generated for veneer thickness variations and veneer curl-up. The important variables affecting veneer quality were identified. The main results were:
1) Significant interactions existed between the main peeling variables. Veneer thickness variation and curl-up were lathe setting dependent. Pitch angle and horizontal gap were identified as the two most critical variables affecting veneer quality in terms of both thickness variation and veneer curl-up, followed by vertical gap, bar overdrive (incisor bar) and peeling speed.
2) There was a trade-off between veneer curl-up and veneer thickness variations. Based on a joint optimization method, the optimum lathe settings for either incisor bar or smooth roller bar veneer peeling have been established and validated for spruce, which can help industry achieve better veneer quality and higher veneer recovery as well.
3) The optimum lathe settings for smooth roller bar peeling and incisor veneer peeling for spruce were quite different, which demonstrated that lathe settings need to be adjusted after changing bar from one to another. The optimum lathe settings for 2.50” smooth roller bar peeling were pitch angle 89.50, vertical gap 0.425” and horizontal gap 0.1”. In contrast, the optimum lathe settings for incisor bar peeling were pitch angle 90.50, vertical gap 0.388”, horizontal gap 0.1” and overdrive percentage level 100.5%. Compared to smooth roller bar peeling, incisor bar peeling allowed a slightly tighter gap (or higher compression) due to applied bar overdrive. By comparing the respective optimum lathe settings, it was concluded that veneer incising at the lathe did not contribute to more thickness variation in veneer.
4) Compared to Douglas-fir, spruce tended to generate more serious curl-up veneer. For spruce veneer, higher moisture in the core contributed to a reduction in veneer curl-up. Using the optimum settings with the incisor bar for spruce, it was found that these settings also produced high quality veneer when peeling Douglas-fir.
5) While the above findings based on the laboratory lathe are useful for understanding the lathe operation mechanism, optimum settings are specific for industrial lathes. It is therefore recommended that studies using similar statistical approaches be conducted in mill trials.