Band Saw blade fatigue questions

[David Ragan]

I a MM-16 with 1 1/4" Resaw King blade from Laguna. I love it.

Also, I have a Starrett bandsaw blade tension gage (anyone want to buy it?) I routinely tension to 30-35K. (Now, I know the finger push or flutter method is perfectly fine.)

My WW is on wkd only-and light duty then.

The blade was/is detensioned @ end of every workday.

About 3 years ago, I left the blade tensioned all night. Came back down the next day, and it broke. Awful racket. Blade was 2-3 yrs old.

Sent it back, couldn't be repaired, so I bought another one. I figured that it was my fault for leaving the tension on all night.

Some time ago, I read that there is no need to detension the blade----I have been a little put out with Laguna ever since I read that. They never asked me how it occurred, just said it couldn't be welded and that was it.

Besides the debated tension issue, did I do too much back and forth de/tensioning and cause metal fatigue?

Do I have a beef with Laguna?

Whats the deal?

[John TenEyck]

As long as you are below the fatigue limit adding/removing tension should have no impact on when the blade failed. 30 to 35K seems awfully high, however, but I don't know what Laguna recommends. Still, I'd be suspicious that tension level is the reason it failed.

John

[Barry Richardson]

Haven't used the Resaw king that much, it got dull and I put it aside and have never got around to resharpening it. However, I use a Laguna 16 and find that hard back blades like lennox dimaster will often fatigue and break before they are dull. I have read somewhere that 16" (and even more-so 14") wheels are too tight of wrap for big blades and the can fatigue prematurely, I think the theory has merit.

[David Kumm]

Tensioning and detensioning wasn't the cause. I'd look for hairline cracks at the gullet. Sometimes the sharpening can cause stress at the gullet that eventually fails. Spinning the back bearing fast can heat the band and lead to premature failure too but at 30-35K tension that probably didn't happen. That tension sounds OK although I run the trimaster which has a thicker band. I run 1/4: bimetal at that tension and it has lasted until dull. Dave

[Erik Loza]

My entirely non-scientific opinion: De-tension at the end of the day. No idea why the OP's blade failed but I do recommend de-tensioning after use.

The amount of tension your average 14" saw is capable of exerting on a blade nothing like the pressure a big, steel-framed bandsaw can. Or at least one of our steel-framed bandsaws.

Erik Loza
Minimax USA

[ian maybury]

From an engineering perspective the factors that determine the fatigue life are the stress level (typically at the most highly stressed point), and the number of reversals. That's stress on/off cycles.

15 - 20,000psi working stress/tension is often recommended for carbon steel blades, and 25 - 30,000 psi or more for stronger alloy/bimetal steels. Tension gauges infer the stress from the strain. (ins stretch/in length) By a quirk of the maths they don't need to know the width or thickness of the blade to do so - but they do presume all steels have the same Young's modulus value which may be a slight approximation. The notching for the tooth gullets probably also introduce some variability.

30,000 psi is a pretty high level of stress. Heat treated higher carbon content and alloy steels can be multiples stronger, but mild steels can often be close to starting to deform/take a permanent 'set' at this stress level - so it's not insignificant. Meaning that blades tensioned to this level are subject to fatigue effects/working in a zone beyond the fatigue limit. i.e. will eventually break if the load is cycled/the stress reversed enough times.

Graphs or fatigue curves record test data which says how many cycles a steel can handle for a given stress level. Fatigue doesn't arise below a certain minimum stress level (up to the fatigue limit), but as the stress level is increased beyond that failures can occur. The number of cycles needed to cause failure is initially very high, but reduces initially slowly and then progressively more quickly as the stress level increases - until eventually the piece will fail in a few cycles or just by simple overloading.

e.g. too small band wheels (below about 400mm) can for example cause premature failure in some (thicker) blades as a result of the tighter than recommended bend required to get around them shoving the reversing bending stress level (which adds to the stress already caused by the tension applied to the blade) high enough so that fatigue failure quickly follows. Correctly set up blades will if run for long enough likely eventually fail through fatigue, but should at least be set up so that they wear out before they have been through enough stress reversal cycles for this to happen.

It's tough to predict fatigue life in reality, but if a blade is failing too quickly when run on a suitable saw at the recommended tension/stress level then several possibilities come to mind (among the very many that could be in play):

1. The blade is overtensioned for some reason. (gauge problem?)
2. Some sort of defect like a nick at the bottom of a roughly cut gullet, a weld or material defect in the steel is resulting in a stress concentration/notch effect. These can easily multiply the local stress by a factor of 2, 3 or more - greatly reducing the number of stress reverals required to cause failure.
3. The metal has at some point been made brittle (tolerates fewer cycles) or much weaker (suffers fatigue effects at lower than intended stress levels) as a result of e.g. previous bending and straightening/work hardening, overheating, or disturbance of the heat treatment or composition during brazing or welding.
4. Something about the saw set up is adding stress to the blade, or increasing the number of stress reversals. Maybe even some sort of bending or vibration is happening in a heavy cut - less likely.

For my money if a blade that hasn't suffered damage/been abused is failing prematurely on a saw that's been correctly set up and suited to the blade from the start then it's hard not to suspect a blade defect. One imponderable is the blade history - if it's been re-sharpened several times and done lots of work, or if it's been run for a while overtensioned or on an unsuitable saw then there's no putting back the clock even if the set up is subsequently sorted out...

There's a nice write up on fatigue failure here, including an introduction to visually recognising a fatigue break: http://www.sv.vt.edu/classes/MSE2094_NoteBook/97ClassProj/exper/meyer/www/meyer.html

[David Ragan]

Wow, thank Ian--now that is the kinda answer I like, even if I cant understand it all.

I had suspected that the blade was overtensioned, then the off/on cycles sealed its fate.

I'll call Laguna and post what they say, hopefully this week.

[ian maybury]

Describing the possibilities in general terms based on theory is one thing David, but reaching a definite conclusion on the specific cause of failure for a given blade is another - it'd take a lot more knowledge than I have nd probably a metallurgical lab. Even then… So unless we get lucky and there's something in the area of the break (e.g. its failing at the welds suggesting it wasn't properly annealed, or cracking) that's a dead giveaway it's likely to remain a bit of a mystery/trial and error deal. Or unless you know something about the history/any mistreatment of the blade.

It seems unlikely that tensioning/removing tension after use is a factor - the number of cycles at one per day would be tiny over even a long period of use.

A closer check of the numbers since the above post suggests that its quite likely a bandsaw blade is not in fact really meant to be stressed past the fatigue limit (the stress level above which fatigue failure becomes a possibility) in normal use, because even the 10 exp 6 cycles or so that the curves suggest that steel handles once stress edges over the fatigue limit is a fairly finite life. (1,000,000 cycles amounts to only about 17 hours of running time at 1,000 cycles/min) Once a blade is tensioned even more - that's even further past the fatigue limit - the cycles the balde will tolerate/the life reduces very quickly indeed. (exponentially) This is probably why a nick or stress concentrating defect in especially a gullet (which can multiply the stress at a point concerned several times) as above can matter so much.

A high carbon steel saw band might yield at about 90,000psi, while it seems (very roughly from S-N curves) that the fatigue limit might (?) be around 60,000psi. This being the case it's unlikely that blade tension of itself (max 35,000psi say) will be a problem. It's when stress due to bending and the effect of startegically placed nicks/stress concentrators, welds and heavy use are added on top that the problems will start.

I'm not sure how it might play out in practice (i've not had blade failure problems), but as well as not overdoing the tension it's probably important to make sure that the rollers (especially the back/thrust roller) are well adjusted. If the blade is being bent back hard when cutting because the support is too far back and the wood is being horsed through the cut that likely would significantly increase the bending and hence the stress along the tooth gullets - and hence significantly shorten the life of the blade. Point being that the stress stuff like this causes ends up added with stress caused by the tension - which could easily tip it past the fatigue limit...

The converse of what I said above is that while there's likely a defect if a properly specified and used blade fails prematurely, its equally the case that there's plenty of scope for mistreating a blade - so it's likely that a supplier will presume the latter unless failures are clearly attributable to a specific manufacturing related cause...

[Brian W Smith]

Probably shouldn't respond...muddying up the waters and all that.

First off,we TIG up ours/others blades....and do it very well(NO THIS IS NOT A SOLICITATION)....we've had factory welded blades break within minutes of use.In these cases it's always "at the weld".....more specifically at the HAZ(google it).Then we'd re-weld it and go about the biz.But far more frequently we find that stress risers in the gullets,usually from regrinds as the killer of BS blades.But it isn't limited to "regrinds".....as posted above the stresses involved with fwd/bkwd movement,as small as they may be can be a factor(blade hasn't been reground).Trying to figure out these causes in EVERY case can be frustrating....we chalk "some" of the failures up to,"the price of doing business".Best of luck,BW

[ian maybury]

That fits well Brian. A reground blade (depending on the type) as well as risking nicks in the gullets will likely have more fatigue cycles/use on it too and for both reasons be more likely to fail - the gullets are anyway always going to be the weakest/most heavily stressed points on the blade. Brittleness at improperly heat treated or imperfect welds has to be the other problem. Set up/mode of use too.

One key distinction as you imply is that if the problem was just the heat treatment of/another defect in the weld area then re-welding after removal of the the very local HAZ metal should be an option. On the other hand if the problem is that through a combination of generalised over sressing, long use and/or e.g. gullet defects the blade has over most of its length reached some sort of fatigue limit then re-welding won't make much sense - it'll just let go at another crack or whatever. Suppliers will for all sorts of reasons probably be reluctant to re-weld a used blade - if only because of the uncertainty that this possibility must introduce.

I've seen some say that on some (?) blades you can often hear a fatigued/micro cracked blade 'creaking' if you roll a bend along it, don't know if it's true...

[Brian W Smith]

Now that you mention "hearing" Ian,I will say that makes sense.I've "felt" it....not while blade is on the machine but when handling/Tig'n them.Obviously hard to put into words......but you can feel they have a crispy feel/sound vs a more clean feel.You get the "feeling" as well when welding....like,this may not work,but here goes,haha.

[Larry Frank]

The real shame of it all...if you had a bunch of money and a well equipped lab, you could analyze the fracture and figure out what caused it. However, the money for a Scanning electron microscope. I would guess that blade mfg have access to that type of equipment to analyze blade fractures.

I would not think that the tension in this case alone would cause a break. There was likely some other defect or small crack that started the fracture and then cycles of running the blade or tension and de-tension would make the crack grow.

[Peter Quinn]

I'm no expert, not an engineer, just a bandsaw user. I really doubt tension, or leaving the tension on over night on an occasion is the culprit based on experience. Way better that thing snapped overnight than during a cut with your hands in play. I've had a few go in use and never had a problem but it sure will scare the pants off of you, big loud bang, wheels still moving, hitting the brake only stops one wheel. They say to drop tension end of day, I've always felt this was more for the saw then the blade. Relaxes the spring, takes stress off the wheels. I almost always do this on the home shop machine. But at work I've seen blades left under tension for 6 months, beat relentlessly, dull to the point they can barely cut fire wood....just keep going. It takes a lot to break a good blade. I will say Ive always used 20" saws at work in the past, presently I'm using a MM16, and I've seen more blades break way earlier than I would expect, never at the weld, always fractures radiating up from a gullet on a wide rip blade. I'm thinking there is a mismatch between the blade stock thickness we are getting from the local blade supplier and the 16" wheels, too tight a turn for too thick a blade, it heats, it cools, always breaks close to a start up after a long run the a cool down and restart, like after lunch. Never had this happen on a 20" saw? And doesn't seem to happen with narrower blades. I almost prefer to resaw with it using a 1/2" 3 pitch than the larger 1" 1/2 vary pitch.

I'd guess every manufacturer has some failure rate that would qualify as below average, all the steel can't be perfect. Put that together with the tight wheels, some start/stop cycles, very high tension under load....takes all of those things IMO to make that band snap.

[ian maybury]

It seems unlikely as above that the correct operating blade tension will raise the stress high enough (close to double the normal max tension in the case of a carbide blade) to lead to fatigue issues - not without other factors of the sort mentioned adding stress on top. Over tensioning is easy though in the case narrower blades.

Depending on the thickness of the band material (which is the main factor that determines how much stress bending a band through a given radius causes) it sounds from user reports that some of the smaller wheeled saws (down around 450mm) running especially bimetal and carbide tipped blades with thicker bands can sometimes run into problems with fatigue failures...

[Larry Frank]

Fatigue issues are almost always associated with some type of crack. It may be extremely small and not important at the beginning. The crack could be there a long time. But with time, a crack can grow with various levels of use and cycles of use. At some point, the crack will reach the critical length and cause a complete fracture.

[David Ragan]

So, the collective wisdom, is: it seems like the 30-35K tension, although high, is probably not the sole cause of the break.

Also, detensioning @ the end of the day is more for the machine (and I fully agree that in fact was in my mind originally, before the blade break.)

So, I had a Rikon 16" before, and then wanted something "better". Researched and got the MM-16 cause of the triple beam spine (Eric, please feel free to correct me, as it has been some time.) As Eric also knows, my only gripe was I had to build a rolling platform, to raise the machine about a foot.

I must say that the main reason I got the MM-16 is the beam strength (correct term?) as i was into veneer at the time. So, you might say I am invested in the concept of having huge amounts of tension on the blade......

Still need to call Laguna and ask them about tensioning.

**Does anybody else actually use a tension gauge?** (I am going to create a new post and ignite the tension question)

[Jim German]

Leaving the blade tensioned overnight wouldn't have been the culprit. As Ian has said fatigue failures are due to cycles not time, so while it may have been a fatigue failure due to being run at too high a tension, being left at a high tension overnight wouldn't matter. The number of cycles produced by the retensioning/detensioning is also highly unlikely to be an issue for a weekend woodworker, as even after 10 years you'd only have accumulated 1000 cycles.

35ksi seems like an awfully high stress level for a blade to me. That is a net section stress and when multiplied by the stress concentration factor caused by the gullet you'll be at 70-100ksi, which is a significant stress even for a good steel. Although the blade will be under constant tension while running, small factors like any camber on the wheels could cause cyclical loading on the blade and cause the fatigue failure.

There isn't a real reason to detension a saw at night for the blade, it may however have some benefit for the bandsaw tires.

[ian maybury]

For sure there's the potential for all sorts of peculiarities (as in much higher than nominal stress levels) to arise once the camber, tooth gullets, tracking, vibration, saw set up and the rest are factored in.

The obvious reason for detensioning is to head off the possibility that the bandsaw tyre might take a set and develop a flat or other irregularity - but quite a bit depends on the stress level and the polymer used, and both may vary a lot. Plastics in general are prone to taking a set/plastic deformation under fairly moderate sustained stress levels. Which again is the principle - but it'd need knowing the specific polymer/plastic and calculating the maximum compressive stress to take an informed shot at whether or not it might be a real issue or not.

I seem to remember some claiming it's not been a problem on their saw, others that it has...

[ian maybury]

PS thought just prompted by the tension gauge thread. Judging by the fits it caused an old lighter duty (still claimed to be 'professional') saw i had due to frame flexing - a deep cut, tough wood, heavy feed and coarse teeth all have the ability to significantly add to the tension in the blade too - this on top of static tension and all of the other factors we've discussed.

There's tables about somewhere on cutting forces in wood which would make it possible to estimate this factor if somebody wanted too.

The bottom line message that comes across has to be that even though 35,000psi static tension is well below the likely fatigue limit of maybe 60,000psi for a tempered high carbon bimetal or carbide blade band that there's an awful lot of not necessarily obvious additional factors simultaneously stacking up on top of each other to push the actual working stress up towards or beyond the latter limit. Especially when stress concentration effects caused by nicks and notches have the potential to multiply the static stress several times...

[Erik Loza]

David, correct: All the Centauros (MM-series bandsaws...) have three steel beams for the spine...

http://i75.photobucket.com/albums/i315/erikloza/Mini%20Max%20Bandsaw%20Works/MM16triple-boxedbeamcolumn.jpg

This is just my opinion, so feel free to agree or otherwise: I feel like the whole "fatigue cycle"-thing may be more an issue on paper than in the real world. At least in regards to Lenox carbide blades and at least in regards to Centauro bandsaws. I say that because I have probably shipped as many 1.0" Lenox Tri-Masters as anyone (figure probably in the 100's...) to MM16/S400P owners, the first thing I tell them is to de-tension at the end of each day, and have yet to hear back from one owner who had one of those blades break prematurely. And that's going back 10 years. In the Austin shop, we had TRi-Masters that went on and off the machine a bizillion times and showed no evidence of starting to fail. I've killed Tri-Masters from cutting too green of wood but never from "on and off the machine a lot".

http://i75.photobucket.com/albums/i315/erikloza/Mini%20Max%20Bandsaw%20Works/Euro-rollersguideposition.jpg

On the other hand, I have spoken to owners who I am pretty sure fatigued their blades and also deformed their tires by leaving carbides under tension for extended periods of time. Bear in mind that my experience is solely in regards to Centauro saws. So, maybe these machines are able to stretch out a carbide blade enough to kill it like that? I snapped an almost brand new Tri-master once from over-tension on an MM16 and it was cutting perfectly until the moment the blade let go. Regarding tires, the Centauro snap-in ones do seem a lot more pliable to me than than the tires on ACM or Agazzani saws, so it's possible that the material they are made of could deform more readily. I've never been interested in testing that, since it's so easy to just de-tension an MM16 at the end of the day.

http://i75.photobucket.com/albums/i315/erikloza/Mini%20Max%20Bandsaw%20Works/MMtongue-and-groovetire.jpg

Keep in mind that I am neither an engineer nor a metallurgist but based on my experience, you should de-tension your blade at the end of the day and also, if you take good care of your carbide, it should last a long, long time. "Fatigue cycle" may indeed be a factor for some machine and blade combinations but I have yet to see it in hundreds of 16"/400mm saws sold with carbides over the years.

Erik Loza
Minimax USA

[David Ragan]

I had not thought about the deformation of the tire. That alone is a good reason for me to detension at end of day. If I was doing BS work every day, I would think this would not be as big a factor.

[ian maybury]

Agree (as on the other thread) that de-tensioning in as much as its necessary in the case of a given tyre type and material is likely about protecting the tyre - polymers/plastics in general have some tendency to take a set if loaded for prolonged periods.

As in most things mechanical there seem definitely to be people about that bust blades Erik, and lots more for whom it never happens - so it's probably a matter of our taking on board that at least reasonable care is necessary.

The one that's an absolute given is that if a saw can through misuse/wrong settings (if only on a narrow one which pretty much has to be possible) load a blade heavily enough to break it, then by definition it can if misused load a blade heavily enough that it can likely suffer fatigue failures.

Another theoretical but likely practically relevant thought. Chances are that frame/saw chassis that's not all that rigid vs the blade that's being run, and especially one that twists out of alignment as opposed to deflecting in the line of the blade will have a greater tendency to break blades/will all else being equal run into problems at a given wheel size before another.

There's the potential for if nothing else vibration in the form of 'nodding' of the cantilever (which would induce lengthwise 'tugging' in the blade), but if the chassis twists out of line as it loads up (but the camber maintains tracking) as well then not only may this occur - it'll probably also load one edge of the blade much more heaviliy than the other. Likely to be especially dodgy if that happens to be the toothed side.

The problem as at the start is to use what is a theoretical possibility to accurately predict reality. The one thing that seems to be pretty clear (judging by comments on forums) is that high end saws like e.g. the heavy duty Italians don't seem to run into problems unless something isn't right about the blade or the set up…....

[Erik Loza]

The most common complaints I hear from owners seem to be related to choosing the wrong blade for the job or trying to make one or two different blades do everything on all species/moisture levels of wood. Those issues far, far outweigh actual defects in either the blade or the machine. At least in my experience.

Erik Loza
Minimax USA

[ian maybury]

You've i'm sure seen a lot more than me Erik, but poor blade choice has to be another variable so far as the potential for blade overloading/fatigue is concerned.

It seems to come up time and again for example that running blade that's of too fine a tooth/too small a gullet for a given depth of cut results in a scenario where the gullet ends up clogged with sawdust before the tooth reaches the end of the cut - with the result that the dust spills out into the kerf/the gap between the saw band and the timber. Leading potentially to very high levels of drag/friction/jamming, and lots of heating - and the possibility of over tensioning/over stressing/overheating/annealing and weakening/fatiguing the blade and even overloading the motor.

The green wood you mention is probably a similar scenario - one where the chips don't necessarily cut cleanly and or get carried out of the cut as easily as say dry sawdust...

There was always a lot of talk of the usefulness of skip tooth blades in this situation, but i guess the very deep gullets on carbide tipped blades like e.g the Woodmaster CT may along with the ability of the block shaped teeth to a degree to plane the sides of the cut may also at least partly explain why they seem to cut so freely.

Another cumulative factor to load up the blade even more, and cause problems which are not the fault of the saw or the blade...

[ian maybury]

Just a PS. These guys (Vollmer) make bandsaw blade grinding machines, and have some interesting things to say about how correct sharpening can extend the fatigue life of bandsaw blades: http://tiny.cc/nt34qx - among them that regular full tooth grinding is advisable. This because it removes material that has already started to (micro) fatigue crack in the gullets. Presumably that in doing so it (?) to some degree anyway zero resets the cycle count on blade...