Sharpening, tooth setting and tensioning of sawmills blades

A bandsaw blade is a highly stressed machine part. Three main phenomena have an impact on its lifespan.

The first factor that comes to mind is fatigue. After a certain number of hours of use, the radial bending undergone by the blade as it passes through the wheels and the lateral bending caused by the thrust in the wood will eventually break the blade. The blade is then at the end of its life and it becomes unrecoverable so there is no need to think about soldering it.

The second factor is the shape of the teeth. A tooth base whose radius is too small will significantly and locally increase the stresses at this location.

This phenomenon will promote crack initiation and possibly breakage of the blade.

It is therefore important that the radius at the base of the tooth is as large as possible.

The third factor is more subtle. The high speed of the blade in the material causes cyclic impacts on the teeth. Over time, these impacts can create microcracks at the base of the teeth that will eventually cause the blade to break.

When well done, blade sharpening and tooth setting will increase the life of your blades and here's how:

1- A sharp blade moves more easily through the material. It will therefore do fewer laps for a given production. The penetration force will also be less or even negative. Indeed the angle of the tooth can cause it to seek to penetrate the material itself. The web of the blade will then not undergo much lateral pressure.

2- The angle of attack (rake angle) must be negative (about -10 degrees) and the bottom of the tooth must be rounded to help in rolling the chip. This will allow the tooth to convey more wood. Finally, the back of the tooth must have adequate clearance so that the tip does not have too little or too much load.

Sharpening should take all these factors into account. The tip of the tooth is filed down to make it sharp and the gouge is filed down to remove microcracks.

3- A normal tooth set must ensure that the blade make a passage equivalent to twice of its thickness. A 42th blade, for example, should be set 21th on each side. It will then make a kerf of 84th. For a horizontal sawmill, if the blade tends to descend into the material, the top tooth set is deficient. If the blade tends to go up in the material, the bottom tooth set is deficient. Tension has little to do with these behaviors. If the blade becomes hot, the whole tooth set is defective.

This shows the importance of sharpening a blade, but there is still an important factor to consider and that is the tension.

Blades manufacturers specify the nominal stresses to which the blades must be tensioned. If we cut the blade, we can see the surface of the section which is called the web of the blade. This surface as well as the quality of the steel define the tension that can be applied to the blade. A normal grade blade can take a stress rating of 17,500 PSI (pounds per square inch) while a premium blade can take a stress rating of 27,500 PSI. It should be remembered that sharpening reduces the web in question. It must therefore be admitted that the tension will have to be reduced accordingly in order to remain within these stress values ​​and to avoid sooner breakage of the blade.

In short, a new blade can be significantly tighter than a blade that has undergone several sharpenings and your sharpener should provide you with this tension value after each sharpening. However, do not confuse blade tension with wheel pressure. The latter has about twice the tension of the blade. Thus, a normal quality 45th blade with a 1-3/8 inch web should be tensioned to 1024 pounds. It is then necessary to adjust the pressure for a thrust on the axis of the wheel of 2088 pounds. These are average values.

The best way to adjust the tension in the blade is therefore to measure it directly. This can easily be done with a tension gage which will give a measurement of deformation according to the tensile force. 1200W Inc. offers a tension gage adapted to the reality of portable sawmills. Inform yourself and in the future, ask your sharpener for the tension adjustment values. If he cannot provide you with this value, contact us at www.1200W.ca with your dimensions and we will be happy to answer you.

Conclusion :

Sharpening, tooth setting and the appropriate tension are key to the performance and longevity of your sawmill blades.