Band Saw Blade Distorted -- No Mystery After All

[Doug_H]

EDIT: I found a crack above the weld to explain the "mystery." So this post is now just for your information. Why this blade would crack after moderate use is a question for Laguna tools to answer.

I have a 3/4" 3 tpi blade on my Laguna 14-12. It suddenly developed what I would call a permanent negative camber over about 12" of the blade, roughly symmetrically on both sides of the weld. The weld looks solid -- no cracks. If I put a straightedge against the back of the band the max distortion is 3/32" . The rest of this 115" blade appears to be fine.

I have never come across this before. Any opinions on what caused this? Might the blade have been defective or was the adjustment of the saw to blame? I don't think the blade ever got too hot. I am puzzled.

The saw is adjusted as well as I can manage -- wheels are co-planar, guides are adjusted. I have been cutting 6/4 oak. Moderate level of use for 6 months. Teeth still seem sharp, it was cutting well until this "event". Now of course the workpiece "bucks" out when that part of the blade hits it. I am shut down until my replacement blade arrives.

Doug

[Arlin Eastman]

Doug

I had the same thing happen and had to throw it away. I never did know how or why it happened.
Hope someone has the answer.

Arlin

[Doug_H]

Hi, Arlin

I finally found the cause. There is a crack part way through the blade a few inches above the weld. I'm going to delete my post but I thought I would respond to you.

Doug

[lkomroff]

Doug, Please don't delete your post. I think it might help someone else. Arlin now knows what probably happened to his blade. If this happens to me someday, I will hopefully remember this post.

Thanks,

[AHill]

It's really hard to tell why the crack developed. Could have been mistreated during manufacturing. Could have been an inclusion in the steel (I majored in Metallurgical Engineering from that school in the same town Doug lives in). Could have been some sawdust on the wheel between the blade and the tire that might have overstressed that part of the blade. Could have been a ding or scratch on the surface of the blade that was a crack initiation site, etc., etc. I'd contact the blade manufacturer and let them know what happened. They may want the blade returned to them for analysis.

[Arlin Eastman]

That is a good idea.

[Ohio Mike]

AHill said:

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It's really hard to tell why the crack developed. Could have been mistreated during manufacturing. Could have been an inclusion in the steel (I majored in Metallurgical Engineering from that school in the same town Doug lives in). Could have been some sawdust on the wheel between the blade and the tire that might have overstressed that part of the blade. Could have been a ding or scratch on the surface of the blade that was a crack initiation site, etc., etc. I'd contact the blade manufacturer and let them know what happened. They may want the blade returned to them for analysis.

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Allan,
Can you comment from an engineering point of view.

Is it possible his blade experienced metal fatigue caused by the small (14") diameter of the wheels?

He said the blade is 3/4" wide so it's likely between .035" and .045" thick. Can blades like that reliably flex around a 7" radius?


Thanks,

Mike

[Peter Tremblay]

It's not

That blade stock is .025"

I have one on my G0457 and I love it.

They make these blades to be able to be used on a 14" saw.

I don't know if the spring on my saw can properly tension it (maybe I want it over tensioned) but it is supposed to work on smaller saws.

[Ohio Mike]

Peter Tremblay said:

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It's not

That blade stock is .025"

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Ahhh, I didn't understand he was using a thin-kerf blade.

As you say, a blade that's .025 thick should have no trouble with 14 inch wheels.

Thanks for pointing that out,
Mike

[AHill]

Ohio Mike said:

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[blockquote]AHill said:

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It's really hard to tell why the crack developed. Could have been mistreated during manufacturing. Could have been an inclusion in the steel (I majored in Metallurgical Engineering from that school in the same town Doug lives in). Could have been some sawdust on the wheel between the blade and the tire that might have overstressed that part of the blade. Could have been a ding or scratch on the surface of the blade that was a crack initiation site, etc., etc. I'd contact the blade manufacturer and let them know what happened. They may want the blade returned to them for analysis.

---

Allan,
Can you comment from an engineering point of view.

Is it possible his blade experienced metal fatigue caused by the small (14") diameter of the wheels?

He said the blade is 3/4" wide so it's likely between .035" and .045" thick. Can blades like that reliably flex around a 7" radius?


Thanks,

Mike

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[/blockquote]

Not likely. A 14" bandsaw is designed to handle a 3/4" blade, even though there are a lot of folks who claim you can't get adequate tension on a 3/4" blade with a 14" wheel size. I say that's bunk. I have a 3/4" blade I use for resawing and it's seen lots of use and never failed. Theoretically, if steel is tensioned to a point below the fatigue strength, you can run it forever and it will never experience a fatigue failure. Fatigue strength is the strength (in psi) where a steel won't fail by reversing loads of tension and compression. It's less than the yield strength, which is the point where the steel will permanently deform. Yield strength is less than the ultimate strength, which is the strength at which the steel will break. Fatigue failures happen when the strength applied to the steel is somewhere between fatigue strength and yield strength. Each fatigue cycle lowers the point at which the steel will deform. The higher the load, the fewer cycles it will tolerate before failure.