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From the pages of Fine Woodworking Magazine Setting Bandsaw Blade Tension Learn to set your bandsaw’s tension to ensure cuts that are straight and even
In an effort to avoid adding to the confusion, I'm going to give you some practical ideas on tensioning blades so that you can adjust your saw for accurate cuts. But first I'd like to make some points about bandsaw blades and tension. Finding the right tension Bandsaw blades require tension and lots of it to consistently produce straight, uniform cuts, especially in thick or dense stock. Most blade manufacturers recommend 15,000 psi to 20,000 psi for a common carbon-steel blade. However, bimetal, spring-steel, and carbide-tipped blades are much stronger than carbon-steel blades, so manufacturers recommend a much higher tension: 25,000 psi to 30,000 psi. Why do bandsaw blades need so much tension? For beam strength. The tighter the blade is stretched, the more rigid it becomes and the less tendency it will have to deflect in the cut. You only need maximum tension for the most demanding cuts, such as sawing dense hardwoods or stock of the maximum thickness that will fit under a saw's guides. In simpler circumstances, you can back off the tension a little. All blades, regardless of width, require the same amount of tension for maximum beam strength. The variable factor is the amount of pulling force needed. For example, it takes approximately 200 lb. of force pulling on a 1/4-in.-wide by 0.025-in.-thick blade to create 25,000 psi of tension. Conversely, a 3/4-in.-wide by 0.032-in.-thick blade will require approximately 800 lb. of force to create the same 25,000 psi of tension. Measuring tension
So how do you know when blade tension is correct? The most accurate way is to check it with a tension meter such as the one I used in my tests. But tension meters are expensive -- typically around $300. I know what you're thinking -- is there another way? Yes, but none is as accurate as a tension meter. Other tensioning methods will work, but they're a lot like gauging air pressure in a bicycle tire simply by squeezing it.
Another method is to test the tension by the amount that the blade will deflect sideways. First, I set the upper guides about 6 in. off the table. Then using a moderate amount of pressure from my index finger (obviously with the saw turned off!), I push the blade sideways. I don't want the blade to bow more than 1/4 in. Of course, you'll have to develop a feel for how much pressure is moderate. Although both of these methods work, they are imprecise. But as I stated earlier, in most situations maximum blade tension isn't necessary. I always test the blade tension with a trial piece before making cuts in an actual workpiece. If the blade wanders in the cut (assuming other factors such as blade sharpness and guide setting are correct), I'll gradually increase the blade tension. Blade tensioning for resawing
I remember a situation some years ago when I attempted to resaw a wide board. It was a plank of deep red cherry -- highly figured with truly awesome curly grain. I wanted to make book-matched panels for a door in a cupboard. Since I was in a hurry, I neglected the necessary precautions such as selecting a blade with the right pitch and tensioning it properly. The blade bowed badly during cutting, making one of the planks terribly thin at the end. The stock was thicker than necessary, so I was lucky enough just to squeeze out the thickness I needed from the resawn plank. But I learned my lesson: A blade needs beam strength for resawing. Beam strength, the blade's ability to resist deflection, is achieved by combining several factors, including correct blade pitch, blade width, and precise guide settings. But a key factor in achieving beam strength is applying the maximum blade tension that the blade manufacturer recommends.
If you've purchased a bandsaw with a wheel diameter of 18 in. or more, then you're most likely planning to do serious resawing from time to time. In that case, I suggest that you also spend the money on a tension meter. Large bandsaws have frames that are capable of overtensioning a blade, which causes it to break. A tension meter is the most accurate way of setting the blade tension. If you own one of the many consumer bandsaws with a wheel diameter of 14 in. or less, then I would use a blade no wider than 1/2 in. for resawing and tension it until the tension spring is nearly compressed.
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