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.
Research into the cutting characteristics of bandsaws has shown that the average cutting path of the sawblade is biased to one side of the ideal cut path. It has also been shown that a strong correlation exists between this cutting bias and sawblade deviation. This report is the second on the topic of bandsaw cutting bias and describes the development and mill trials of a PC based, self aligning bandsaw guide system. This system uses data from the sawblade, and programmable logic controllers controlling the bandmill, to monitor bandsaw cutting behaviour and align the guides to minimize the cutting bias and sawblade deviation. The system is shown to effectively reduce sawblade deviation and unscheduled blade changes and compensate for damaged or poorly prepared blades.
The objective of this project was to develop a new feed speed control system for band saws. This system regulates the feed speed based on the cutting depth measured ahead of the saw. In comparison to other feed speed control systems presently used in sawmill operations, the new system allows maximum feed speeds without overfeeding the saw and also avoids underfeeding. The equipment developed basically involves two laser light generators which place laser lines along the saw cut on the saw entry and saw exit side, two cameras which locate the position of the laser or saw lines, and a microprocessor which processes the information from the cameras, determines the feed speed based on the known relationship between sawing variables and gives the signal to the carriage drive. Considerable experimenting was required to accurately measure the distance between the laser lines on the saw log due to colour variations of the long surface and more so due to the irregular geometry of many saw logs. Also a high working speed of the system had to be achieved to correspond to the high feed speeds used in sawmill operations. The new feed speed control system was tested with Forintek's 5-ft band mill. It was found that the actual feed speed set by the feed speed control, and the calculated feed speed were in close agreement showing that the system is able to effectively control the feed speed based on the cutting depth. Additional work is needed and will be carried out to further refine the system before commercialization can be undertaken.
The effect of bandsaw roll-tensioning and bandmill axial forces on the stresses in the blade, on the interaction of the blade with the bandmill and on the stiffness and cutting accuracy of the sawblade are examined. The stresses due to roll-tensioning are measured and their effect on frequency and stiffness determined. The effect of the blade-wheel geometry on the stresses in the cutting region are analyzed showing how the stresses, frequency and stiffness of the sawblade relate to its cutting accuracy. Empirical relationships are developed that enable the stresses due to roll-tensioning to be estimated. Analytical methods accurately predict the effect of roll-tensioning on blade torsional frequency and lateral stiffness. A model was developed that successfully predicts the effect of overhang on the stress in cutting region of the blade. Both analytical and experimental results confirm that rolling in the centre 60% of the sawblade will increase blade stiffness while rolling outside this region will reduce it. Cutting accuracy is shown to be strongly related to lateral tooth stiffness, and it is also shown that optimum increases in stiffness and improved cutting accuracy are obtained by confining the roll-tensioning to be close to the blade centre-line.
Across North America the amount of tensioning used in bandsaws varies drastically. Sawblades tensioned to fit circle gauges from 28 feet to 80 feet in diameter are in regular use and performing very well. This raises the questions as to what is the right amount of tension and what effect does it have on cutting accuracy. In this study, the cutting accuracy of five sawblades, with varying levels of tension, have been measured and compared. The results show that little change occurs in cutting accuracy once enough tension has been put into the blade for it to fit an 80-ft circle gauge.
This paper investigates the feasibility of increasing bandmill production by proportionally increasing both blade speed and lumber feed speed. A modal analysis of the bandmill and bandsaw was conducted and resonant conditions, likely to impair performance, were identified. Cutting tests were conducted to determine the effect of increased blade speed on cutting accuracy, surface finish and sawdust quality. The tests were conducted at blade speeds of 10,000 fpm, 12,500 fpm and 15,000 fpm and examined the effect of tipped and swaged blades cutting Coastal Hem/Fir and Interior SPF.
