Sawmill blade maintenance
The advantages of bandsaw cutting technology in comparison with traditional circular saw benches are well-known: Cutting accuracy, Mobility, Multi-functionality and Economy. Nevertheless bandsaw cutting with all its undoubted advantages is connected with one inevitable factor, - the bandsaw blade. To be more precise, the fact that the blade will break or become too narrow to use anymore after repeated sharpening. Bandsaw blade manufacturers understand this problem and try to act in two directions. On the one hand, they should produce a blade as durable as possible in order to increase its lifetime. On the other hand, they have to reduce its price to the client, to be as cost effective as possible.
There are not many causes of premature blade breakage. The most important thing is to know these causes. Then, after analysis, you will be able to eliminate the causes of blade breakage and save money.
Let’s proceed from the assumption that you use quality blades from well-known manufacturers, for example, the Wood-Mizer Company. This allows us to exclude breakage due to low-quality raw material. So, your blade is of high quality, but – it breaks. What is the matter?
Belts and rollers
The first thing to check is the condition of the blade guide rollers and pulley wheel belts. The roller surface, contacting the blade, should not have cracks or flat spots. Flat spots cause vibration during sawmill operation. This leads to the blade tension changing, and these changes happen with high frequency.
The B57 belts should not have any signs of damage or have an accumulation of sawdust. If, while tensioning the blade up to operational tension, the distance between the blade and the wheel is less than 0,8 mm, then you should replace the blade wheel belt. It is best to replace both belts. The problem is that worn belts allow the blade to contact the wheels and the blade lifetime decreases.
Then you should check the clearance between the back of the blade and the roller flange. This clearance should not exceed the recommended clearance for your type of sawmill (for the LT series sawmill – 3 mm). This parameter should be equal for both rollers.
You should also check the blade tracking on the wheels. People often forget about this alignment or align only the idle wheel. In practice up to 60% of sawmills that have been used for one year or more require blade drive wheel alignment. Certainly this alignment is not so simple as the idle wheel alignment. If it is rather difficult for you, you can ask for help or advice from the equipment manufacturer.
Sawing with dull blades decreases the tension. Some operators struggle against this negative phenomenon by increasing the blade tension more than is recommended. As a result they increase the rate of wear on the wheel bearings, roller bearings and belts. But the most important issue is that cracks appear in the blade body. They subsequently grow and cause blade breakage.
A common cause for blades becoming dull is cutting dirty logs – sometimes it is enough to cut just 15-20 cm. To solve this problem you should use a debarker or clean the logs manually. You should never exceed the recommended volume of timber to be cut by one blade before sharpening.
The operator and his methods
If all the items mentioned above are alright on your sawmill, pay attention to your operators’ sawing technique. Perhaps, in order to go over the quota, he saws too fast. Such sawing practically always leads to the blade overloading and production quality reducing.
It is also important how the blade enters a log. Entering the cut too fast is not recommended. The entry of the blade should be smooth, and only after the blade is fully in the log should you increase the cutting speed according to the type of wood you are cutting. The blade is also overloaded if its parameters (tool angle, setting, etc.) do not correspond with the type of wood.
Once again, the human factor
So, you checked the full alignment of your sawmill. Now you are sure that everything is 100%. Now it is necessary just to check blade sharpening. In fact, wrongly sharpened blades is the most important cause of premature blade breakage.
Let’s say a few words about a a suitable blade maintenance operator. You should be careful in your selection of candidates for this post. He should be an assiduous and conscientious trained worker, who religiously executes all the important points of sharpening. If you have such a person, then you only know of the problem of premature blade breakage through hearsay. If you cannot find such a worker, then we advise you to buy a sharpener with the CBN grinding wheel – this will reduce dependence of successful blade sharpening on the human factor.
Correct blade sharpening
The first point you should check is the sharpener alignment (position of the grinding wheel relative to the blade). The Wood-Mizer sharpener has a special tool for this purpose. If you have no such tool, you should check that the grinding wheel is perpendicular to the blade, and more important – the relationship of the grinding wheel to the blade body.
It is important to sharpen blades not only to sharpen the cutting edge, but also to remove numerous cracks on the blade body. Mostly they occur in places that are between the teeth. For the Wood-Mizer blades this place corresponds with the narrowest part of a blade – the gullet. While sharpening the blade it is very important to remove all the micro-cracks in the gullet. The metal should be removed evenly over the whole tooth profile. Failure to do that will increase the risk of premature blade breakage and the blade parameters will change (tooth profile and height).
If the blade has worked longer than recommended by the manufacturer, you should increase amount of metal to be removed during one sharpening. While doing that it is important to control the grinding wheel pressure on the blade. It should not be excessive. Otherwise annealing of the blade body will occur, which will lead to a reduction in its strength characteristics. And it is absolutely impossible to sharpen the blade without special coolant. Sometimes, even if you only remove a small quantity of metal, annealing occurs anyway. In this case you should clean the working surface of the grinding wheel with a special dressing wheel. This operation should be repeated after the first sharpening cycle. After the blade has being sharpened, check the result (profile, geometry), using a new blade as a template. If the blade was sharpened correctly, its profile will not change, the tooth height remains permanent, there are no rough traces of the grinding wheel in the gullet or on the tips of the teeth, no traces of metal annealing, burrs are completely removed, the cutting edges of the blade are sharp and have the correct shape.
Sometimes you can hear that it is not obligatory to remove all burrs after sharpening. It is not correct. The point is that burrs distort the set of the blade and this can result in micro-cracks appearing in the body of the blade.
Let’s say a few words about the grinding wheel. Using cheap grinding wheels you waste money by increasing blade breakage. The problem is that cheap grinding wheels wear very quickly and as a result cannot provide an even grind even during the course of one working cycle. Blades sharpened with such wheels are easy to distinguish – tooth height and profile differ from one tooth to another. By the way, sharpener adjustments changing during sharpening lead to the same results. Keep one simple rule: having made necessary adjustments, sharpen one circle of the blade and only after that make corrections.
Wishing you successful sawing,
Author: Andrew Fiodorov
Wood-Mizer East, St.-Petersburg, Russia