Fine Points of Bandmill Blade Tension
Blade characteristics, including steel type, gauge, and width, are factors in determining the best tension to apply to the blade in service. January 18, 2011
Question
I'm new to milling. I have been searching for a good base number to use for my blade; right now we are trying 2900 lbs. The blade is 1 1/2", one tpi and we have up to 72" between guides (guides are adjustable in and out). My partner has come up with the 2900# idea, it just seems a lot to me. But, again, I have no experience (nor does he). I am learning as best I can about sharpening them, but nobody has tension numbers. Does anyone have any suggestions?
Forum Responses
(Sawing and Drying Forum)
From contributor G:
One way is to follow the saw specification. I use 2800 pounds, so your number looks reasonable to me.
From contributor R:
Up to 72" between guides – that’s really wide. With that said, the blade should be very rigid. If you have no specs on the machine, and you can’t contact the manufacture, you can remove the blade guides and tension up the blade where you think it should be. Engage the blade, make sure its tracking correctly, bring it up to cutting speed, and then observe the blade between the wheels from behind. In the blade guide area (doing everything safely of course) look to see if the blade is fluttering. If not, while running, back off the tension till it begins fluttering and then tighten back up till it quits. When it quits, tension up a bit more and you should be good. With the numbers you gave I’m assuming you have hydraulic tension.
From the original questioner:
The machine is hand-built, based from a kit but modified to a larger size. It is definitely capable, but of course something home-made generates its own specs. I am hoping to use numbers generated from others' experiences as a starting point, hence this request for the info. I really appreciate the help.
From Gene Wengert, forum technical advisor:
The strength of the steel, the amount of steel (blade width and thickness) and the weld are key factors in the amount of tension you use. The diameter of the wheels is also a factor as smaller wheels bend the blade more and can cause failures as well. The more tension, the straighter the cut. So, as suggested, the blade manufacturer knows what they are making and should give you the advice you require. Any specific advice here is just guessing or is just general info, without all the information about the blade. Note that as you sharpen the saw, its size (width) decreases and then you need to reduce the tension accordingly. It is likewise important not to have any sharp turns in the gullets, as that creates a stress point; everything is curved.
Regarding the guides, they are normally positioned as close to the log or cant as possible to help keep the blade going straight and to prevent catastrophic variations.
In general, if you do not have breakage and you are sawing without variation, you are at the correct level. Appreciate that there is variation in each blade, so you want settings that will work all the time. Also, you will find that lower tension may work for softer woods, but not for dense woods. Tooth sharpness also affects the situation as does feed speed. In fact, the entire blade and sawmill are a system, so one change can affect the system. Each item is not independent.
From contributor A:
Just for reference my WM uses 2300# on average for blade tension when using WM Double Hard blades. But when I tried Lennox blades I could not run over 1900# or the blades got longer and soon I could not adjust the blade out enough. Also I did not see the thickness of the blade. Thinner blades take less tension or they will stretch and thicker blades take a bit more tension so they will bend around the wheels evenly.
If you have 72" of cut width capacity you should look at going to a larger tpi to reduce the stress on the blade in the cut if you cut a lot of wide material. The gullet should be larger to carry more dust to the exit cut and stop spillage which will cause the blade to overheat. WM 7 degree blades have good gullets and do real well in wide cuts. If the blade is not getting longer on you and the cut is good do not mess with what you are doing. Sometimes things just work and are better left alone.
