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I occasionally use (or abuse) a small tabletop band saw to rough cut aluminum plate. I generally having a lot of success and have been able to cut half inch thick plate without any problem using 6 to 10 tpi wood cutting blades.

This weekend I tried cutting some more aluminum and could not get the saw to cut a straight line. The saw wanted to cut at about a 20 degree angle off of straight and no matter how much I tried, I couldn't follow my line. I then tried a thinner (1/8 in.) piece of aluminum and didn't have any trouble at all. I went back to the half inch thick piece and again couldn't cut a straight line.

What would cause this? Is this a blade problem or a tension problem or what? I'm not a band saw expert, but I tried adjusting the saw with no luck. The guides all seem to be in the right place so I think the issue is something else.

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This sounds similar to problems that chainsaws have when only half their teeth have been sharpened on one side. –  Doresoom Jul 12 '11 at 20:03
    
Doresoom: I was thinking that, but I don't know how I'd have damaged one side of the blade making straight cuts. –  Scott Saunders Jul 13 '11 at 13:10
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So, is the cut 20 degreees off in the vertical plane (the plane the blade is in) or horizontal (the plane the fence/table is in)? –  Cody C Aug 15 '11 at 12:35
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Cody: It's off in the horizontal plane. The cut was fine vertically. –  Scott Saunders Aug 15 '11 at 13:37
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3 Answers

Cheaper bandsaws tend to drift in the cut more, but any bandsaw can suffer from it. You can watch the line it is cutting along, and adjust to it, or you can try to resolve the problem. There are a few issues.

If you need to push the material through the cut too hard, then it tends to twist the blade. This will cause the cut to go at a angle. Why might you be pushing?

  • A dull blade
  • Thicker material than normal to cut
  • Impatience

Another factor is the blade tension. A bandsaw has a spring that you tighten to keep the blade under tension. Your saw will probably have instructions for how tight to make the blade. This spring can loose its ability to properly tension the blade after some time, allowing the blade to twist in the cut. Replacing the spring can thus help you to bring your saw back to optimum cutting performance. A sign that it is time to do this is if your saw tends to drift when it did not do so in the past on a similar cut, with a good blade installed.

Next, check the blade guides. If your blade guides are damaged, then they may be allowing the blade to drift off line. Most saws have a set of lower and upper guides. Some are small rolling bearings (on more expensive saws) but on a tabletop saw these are probably metal, held in place with a set screw.

The blade can actually wear the guides away from friction. The metal guides can often be reversed before they need to be replaced, but they are inexpensive to replace. You can usually find a non-metallic (slippery) replacement for those guides, which runs a bit cooler. This is good because less heat means your blade will run more cooly, so live longer before it dulls.

Finally, check the tires. Your saw has rubber (or plastic/polyurethane) tires that the blade runs on. There will be either two or three such tires. After a few years time, these tires dry out. The saw will then start to behave poorly. Those tires can be pulled from the wheels and replaced.

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First of all, I'd make sure you have correctly tensioned the blade. Since it's off in the horizontal plane what you're seeing is called drift. Any band saw blade has a certain amount of drift. Most band saw fences have some sort of adjustment to compensate for drift. 20 degrees seems a bit much for normal drift hence why I think you might not have enough tension on the blade. Also, you can use the tracking adjustment to move the blade farther toward the front or rear of the band saw wheel. Because the wheel is crowned this will cause the blade to cut more the the left/right as you adjust it. Every blade's drift is different so typically you need to adjust for it every time you install a new blade.

Also, if your band saw has markings for tensioning different sizes of blades totally ignore those, they're all grossly understated. If I put a 1/4" blade on my band saw I usually need to tension it to the point that it's the tension indicator is at 3/4".

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A bandsaw will sometimes cut off vertical if the blade is dull. Did you already retest with a fresh blade?

If your bandsaw has multiple speeds you should slow it down. Cutting metal generates a lot more heat which will dull blades much faster than cutting wood.

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No kidding about the heat part - I've had to use scrap metal as a pusher because I couldn't stand to touch an aluminum plate I was cutting before. Aluminum is a REALLY good conductor of heat! –  Doresoom Jul 20 '11 at 20:08
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