Tensioning - one of the fundamental operations for a good functioning of the blades
There are various reasons why sometimes blades don’t operate optimally or develop cracks: tensioning, sharpening, machine condition and settings and not only. In the upcoming articles we will discuss each cause that can lead to a defective functioning in detail and step by step.
For the optimal functioning of the blades the correct tensioning is one of the necessary operations and we will address this subject below.
Firstly, the tensioning must be done correctly, after each use and before sharpening. Before tensioning it is recommended to clean the blade of all wood deposits.
The term of “tensioning” as it is usually used actually refers to 2 distinct operations: levelling and tensioning.
Levelling consists of reducing the deformed areas inside the body of the blade, resulting in minimising the friction between the blade and wood, obtaining a straight and smooth cut. Tensioning and levelling is done after each cutting cycle in order to permanently ensure a high quality cut with no risk of cracks appearing in the tooth gullet or on the back of the blade.
The actual tensioning consists of passing the middle area of the blade between 2 rolls by applying a differentiated pressure so that a curved profile of the blade is obtained, which matches the profile of the bandsaw wheel. The operation can be performed with the aid of a tensioning machine equipped with 2 rolls with spherical surfaces and a plane work surface on which the levelling operation can be done.
Through rolling the middle of the blade is elongated do that a chord shape and value similar to the one of the bandsaw wheel is achieved.
The bandsaw wheels have a curved profile in order to ensure stability while cutting and prevent the blade from gliding off the wheels. If the middle of the blade is not sufficiently stretched the contact between the blade and wheels will only be in the highest point and the front and back areas will be insufficiently stretched, allowing lateral displacements and resulting in wavy cuts and even cracking.
The tensioning operation begins in the middle of the band and is followed by alternating towards the 2 edges, gradually reducing the rolling pressure.
The rolling pressure must not be overly high because this causes blades to become wavy and difficult to subsequently plane.
When tensioning, a great amount of attention needs to be given to the back of the blade. This is done through a few rolling passes at the back edge, using a small amount of pressure. Avoid rolling closer than 20 mm from the back edge since it is susceptible to cracking. The higher the rolling pressure and the closer it is applied to the edge, the faster the curve appears. However, pay close attention and control frequently.
Elongating the back edge accordingly gives stability to the blade due to the inner tensions accumulating 10% more around the tooth line than on the back of the blade. Simply put, the cutting area of the blade is better tensioned than the back.
While cutting the blade will mainly heat up at the tips and area below the teeth, thus elongating the tooth line which has to be compensated through this stretch of the back line.
As the blade is re-sharpened and becomes narrower, the back edge ends up closer to the midpoint of the wheel. Therefore, the tension tends to drop. This tendency must be countered by adequately elongating the back line.
More information about tensioning and levelling can be found on the Services page.