With the pressures on our industry at this time, sawmills are using thinner saws to try and improve recovery without reducing current production levels. This has led to washboarding becoming a widespread problem in the industry. Washboarding is a wavy pattern that appears on the surface of lumber due to excessive vibration of the sawblade as it cuts through the lumber. In this study the washboarding behaviour of saws was studied at both the analytical and experimental levels to develop a much better understanding of the problem and lead to a set of guidelines for industry use.
This was a joint project between Forintek Canada Corp. and The University of British Columbia. Dr. S. G. Hutton led the analytical side of the work and Dr. J. Taylor the experimental portion of the work. This report is a portion of the overall research endeavour and presents the results of the experimental work that was conducted.
To compliment and validate the analytical portion of the work a series of cutting tests were conducted to examine the washboarding phenomenon and the factors that influence its occurrence. The effects of wheel rotation speed, strain, tooth bite, and depth of cut were examined and their effects recorded. As small changes in tooth design are known to influence a saws washboarding behaviour, but are not accurately predicted by the mathematical models, we also carried out cutting tests with saws of different thickness and tooth pitch, progressively increasing the length of the tooth face and the depth of the gullet until washboarding on the lumber surface was quite severe. In this manner we hoped to be able to develop some guidelines for the industry.
It was apparent from the initial tests that there were two types of washboarding. We have called the larger diagonal waves that often appear with industrial sized bandsaws, Type I. The narrow more vertical pattern or that on circular saws is called Type II. The results in this report are primarily associated with Type I and show the somewhat insensitive nature of the washboarding instability to changes in the operating parameters. The occurrence of washboarding is more sensitive to changes in the tooth geometry.
This report presents the results of the first stage of an investigation into the feasibility of developing a machine capable of automatically tensioning a bandsaw blade. The present work involves an experimental and analytical investigation of the effects of roll tensioning upon the cutting performance of the bandsaw. In order to understand the role of roll tensioning its effects on internal stress distribution, torsional and lateral natural frequencies and stiffness of the blade have been investigated. The results of strain measurements induced during different rolling patterns and with different thickness of plate and differing rolling pressures are presented and an analytical explanation of the results is given. Experimental results showing how the stiffness of the blade and its natural frequencies are affected by the roll tensioning are also presented. An accurate analytical model that relates the rolling pattern to the lateral stiffness has not been found. Cutting tests have been conducted in which the performance of a blade with no tension is compared with a blade with different levels of tensioning. The results of these tests are presented and indicate that the relationship between cutting accuracy and tensioning is very subtle.
In the laboratory testing two distinct washboarding patterns were obtained. They are labeled as Type I and Type II (Figure 1). For bandsaws, Type I washboarding is the one with he washboarding pattern at about 45 degrees. Type II washboarding is usually associated with small bites and the pattern slopes at 20 to 30 degrees from the path of the saw teeth. Although we obtained some data on the Type II washboarding, this report is associated only with Type I, as this is the type most often encountered in the primary sawmilling industry and is of principal concern to Forintek's members. It should be noted that the same two types of washboard exist for circular saws; however, these are not covered here.
The effect of the many variables listed above on the range of washboarding in bandsaws has been experimentally examined in some detail. The results are presented in this report along with some guidelines for the elimination of washboarding. Although much valuable data have been obtained during this study, more research is required in order to obtain a complete understanding of washboarding. The recommendations given in this document were found to be effective for 16, 17 and 18 ga. blades on a 5 ft. bandmill, but can be expected to be helpful for other sizes of bandmills also.
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.
When large saws are operated at high speeds cutting accuracy deteriorates and very high levels of oscillation occur after the blade exits the cut. These high oscillation levels may adversely affect the next cut. This report describes laboratory tests that were conducted to determine if the addition of extra guides would improve the response of such blades.
Tests were carried out with four different guide configurations: a front guide; a front guide plus a pin guide; a front guide plus a pin guide plus a back guide; and a front guide plus a back guide. The results indicate that the addition of the extra guides has a marked effect upon blade response. In particular, the addition of a back guide results in a significant change in wedging of the cut boards.
The results presented indicate that extra guides could be useful in reducing cutting deviation. Further work is required to determine the appropriate location of such guides and whether the addition of such guides can be accommodated in a mill situation.
In this paper, the finite element method is used to examine the affect of the standard bandsaw operating parameters - such as blade width, blade thickness, span length, tooth size, bandmill strain and roll-tensioning stresses - on lateral tooth stiffness. The results are presented graphically. Specific examples of how changes to the operating parameters can affect tooth stiffness are presented and indicate that, for some conditions, increases in bandmill strain of over 40% are required to regain tooth stiffness lost through reducing blade thickness by one gauge. The relationship between the fundamental torsional frequency and lateral tooth stiffness, for a range of roll-tensioning positions, is examined and shows that an increase in frequency is always accompanied by increase in tooth stiffness. The converse, however, is not always true.
This paper presents the results of an analytical and experimental investigation into the effect of bandsaw roll-tensioning on blade stiffness and cutting accuracy. The results show that rolling in the centre 60% of the sawblade will increase blade stiffness while rolling outside this region will decrease it. The optimum rolling location was shown to be the blade centre-line. A modified roll-tensioning procedure coupled with an increase in bandmill strain was shown to increase blade stiffness and improve cutting accuracy over conventionally tensioned blades. This was accomplished without increasing sawblade stress levels. Benefits of the revised procedure are an estimated increase in lumber recovery of 0.5% coupled with reductions in sawblade maintenance requirements. Mill trials are in progress to determine the long term behaviour of saws with the modified tensioning.
Currently there is not a complete scientific understanding of the reasons for many bandsaw roll-tensioning practices. For the last three years Forintek Canada Corp., in conjunction with the Dept. of Mechanical Engineering at the University of British Columbia, have been involved in a joint research project to investigate the effect of roll-tensioning on bandsaw performance. The objective of this article is to present some of the key findings from the research.
