This report, the second in a two year project to question the existing “rules” for saw designs and real operational limits, investigates three aspects of circular saw performance. The first area of investigation was to determine how operation of new saws in a mill environment changes their cutting behaviour, the second was aimed at determining the operational “sweet-spot” for circular saws of various thicknesses operating at different feedspeeds and finally, the effect of side board stiffness on saw wedging was investigated.
The results from the mill operation portion of the study showed that:
Operation of new saws in a mill did not improve their performance
Tooth geometry errors are a major cause of saw wedging
With the new saws, a strong correlation was found between saw wedging and radial clearance angle differences (error) as well as tangential clearance angle differences.
After operation in the mill, a strong correlation was found between saw wedging and side clearance differences as well as tangential clearance angle differences.
Close attention must be paid to the initial tangential angles ground into new/re-tipped saws to avoid low, negative or unequal tangential angles that cause excessive wedging, particularly as saws are repeatedly sharpened.
The results from the operational “sweet-spot” portion of the study showed that:
Errors in tip geometry are the primary cause of saw wedging. Even as feedspeeds are reduced, wedging can remain high for saws with grinding errors;.
For feedspeeds up to 500 fpm, there was not a significant difference in the wedging between the 60, 70 and 80 plate saws.
For feedspeeds up to 500 fpm, there was only small differences in the top edge standard deviation between the four plate thicknesses
Total Sawing Allowance reduced as saw plate thickness (and kerf) reduced. This should allow for mills to achieve target size reductions, provided machine alignment and cant control are tightly controlled.
The investigation into the effect of side board stiffness showed that:
Wedging increases slightly for sideboards that separated from the base cant due to their lower stiffness vs. those that remained attached
This report summarizes the results of laboratory tests, conducted during the period April 1/03 to March 31/04, that were designed to assist mills in implementing super-critical speed sawing. The general characteristics of saw behaviour in the super-critical speed region is discussed and measured saw deviations are presented that illustrate the effect on cutting accuracy of increasing saw and feed speeds. Results are presented for guided spline-arbor saws with diameters of 17", 19" and 24". In each case it was found that feed speeds currently in use could be increased by increasing blade speed.
Thin circular saws can suffer from a vibration phenomena called critical speed instability. At the critical speed, a resonant condition occurs where a saw can snake slowly from side-to-side producing unacceptably large sawing variation. To avoid this problem, most circular saws operate at 10 to 15 percent below the first critical speed. Unfortunately, this practice limits reductions in saw plate thickness and corresponding improvements in lumber recovery. Recently, several mills have been able to operate guided splined-arbor saws above critical speed. These supercritical speed saws offer significant sawing performance improvements by allowing both high recovery and high production rates. The use of these saws remains rare, however, and very little knowledge is available to guide mills that want to investigate this promising area. This report describes laboratory tests involving two supercritical speed saw configurations which are operating successfully in industry. Detailed descriptions of these configurations are given and guidelines for use are described. Idling and cutting tests were completed to characterize vibrational behaviour and demonstrate sawing performance levels. Tests were done to investigate the effect of changes in saw tensioning, saw tooth design parameters, saw and feed speeds and saw plate thicknesses. Test results confirm that for the sawing configurations tested, operating speeds can be found in the supercritical speed region where stable idling behaviour allows successful sawing. As with conventional saws, sawing accuracy is best at lower feed speeds. However, by operating at supercritical speeds, acceptable sawing accuracy can be achieved at higher feed speeds than are possible using conventional saws. Saw tensioning allows further increases in saw and feed speeds, but is not essential for supercritical speed operation. Changes in saw thickness strongly affect sawing performance levels and lumber recovery. Thinner saws have higher sawing variation and must operate at lower feed speeds than thicker saws. Curves showing the relationship between saw plate thickness, feed speeds and sawing accuracy are presented which can be used to assess the economic benefits of the supercritical speed saws that were tested. Supercritical speed circular saws offer considerable potential to improve sawing performance and increase lumber recovery. These saws allow reductions in saw kerf widths while maintaining high feed speeds and acceptable sawing accuracy.
To document the current state of sawing performance in Canadian sawmills, a survey was carried out in 47 mills located in the major lumber producing regions of Canada: B.C., Alberta and Quebec. The survey was restricted to sawmills producing softwood lumber products, and to band and circular saws used for primary and secondary breakdown. Data characterizing sawing performance, as well as detailed information on saw and machinery specifications, were collected during visits to each participating sawmill. The results of the survey are presented in this report. Detailed specifications are given for band and circular saws, and sawing performance levels are discussed. The results of the survey show that considerable opportunities exist for improving the performance of both band and circular saws. The large range in saw kerf widths identified in the survey indicate that lumber recovery could be significantly improved by reducing kerf to the levels of the best machines. The performance of these machines demonstrates that thin kerf widths can be achieved while high production rates and small sawing variation are maintained. Specifications for the thinnest kerf machines are presented in the report and opportunities for improving sawing performance levels are also discussed. This report will interest sawmill managers, sawfilers and other mill personnel who want to know how the sawing performance of their mill compares to that in similar mills included in the survey.