A cut above 2015: Latest tech for the filing room
By Bruce Lehmann
June 6, 2015 - Around 1970, a new type of circular saw was invented that started to appear in sawmills. Instead of the saw blade being held onto the shaft by a collar, it used guides to support the saw directly below the cutting region. This resulted in much more accurate sawing and allowed for the use of thinner saws and faster feed speeds.
By Bruce Lehmann
Since thinner blades produce less sawdust, and therefore, more lumber from the logs, and higher feed speeds result in lower production costs, the next question posed in the saw blade optimization equation was, “How thin and how fast can we make them?”
This question has been answered through trial-and-error, but the results are generally specific to the conditions in a particular sawmill. For this reason, sawmill equipment manufacturers, saw makers and university researchers started developing an engineering model that can predict saw behaviour.
The first models for circular saws were designed for collared saws. The models worked well for predicting certain rotation speeds, called critical speeds, at which the saw has a dramatic loss of stiffness that causes it to wildly deviate when cutting.
The design rule for collared saws to avoid sawing problems is to avoid rotation speeds within 15 per cent of a critical speed. Models for guided circular saws were also developed to predict critical speeds, but the theory and the practice did not match as well as for the collared saws.
Cutting experiments at the FPInnovations Sawing Laboratory in Vancouver showed that guided saws cut very well, even if the rotation speed was at a critical speed.
There is also rotation speed for each blade at which it will start to vibrate wildly, even while idling. It is not possible to increase the rotation speed to get beyond the flutter zone, as it is called.
As a result of the uncertainty in the model regarding critical speeds and an inability to predict the onset of the flutter vibration, sawmills had to send saws to the Sawing Lab to be individually tested if a speed increase or change in plate thickness was wanted.
The cost of the change not working is significant, so these tests provided certainty that the new saws, or saw parameters, would work when implemented in the mill.
Ahmad Panah, a wood-machining scientist in Vancouver who recently completed research work for his PhD in mechanical engineering, has developed a refined model for guided circular saws that correctly predicts the onset of the flutter vibration and the good cutting performance at the critical speeds.
A set of charts and tables has been created for mill personnel to evaluate their saws.
The critical speed charts published by FPInnovations are still needed because worn and grooved arbours may create the conditions for a guided saw to perform poorly at a critical speed. A TechNote will be published that contains all the tables.
This step in refining the design guidelines for guided circular saws allows FPInnovations to bypass the step of having mills sending us saws for testing. That said, if a mill wants test run related to other aspects of the cutting process, such as tooth designs, saw deflection, sawing accuracy and horsepower, we are more than happy to accommodate them – we can do full-sized cutting at the lab up to 700 feet per minute and up to about 4,000 rpm.
If a mill wants to push the envelope, keep in mind that circular saws are like race cars, the faster they go, the more perfect everything else has to be. If the saws and the feed system are not maintained and aligned, the cuts won’t be straight and expect downtime.
Bruce Lehmann is the associate research leader for the machine and saw performance group at FPInnovations. For more information regarding the new guidelines for circular saws or available tables, contact Bruce Lehmann at FPInnovations at Bruce.Lehmann@FPInnovatations.ca.