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.