4x6 blade life mystery

[C-Bag]

Who'd think they need to track hours on one.

This is amongst other things is a puzzle that I can’t figure out. Did the he use the original blade continuously? How did he come up with the blade only lasted x number of hours that led to the original failure?

Blade tension is a double edged sword in that not tight enough it can slip and cause tracking and other problems. Too tight can not only cause the blade to break it can also cause the bearings in the wheels and gearbox to wear prematurely. I don’t remember who or where I got it but always tensioned until the blade made note when plucked. I also run my bandsaws at the lowest speed. For me there is a lot of seat of the pants and historical info(well, we’ve always done it this way) for all bandsaws. Horizontal and vertical with some of it being borderline superstion.

 

[RJSakowski]

deleted

 

[homebrewed]

This is amongst other things is a puzzle that I can’t figure out. Did the he use the original blade continuously? How did he come up with the blade only lasted x number of hours that led to the original failure?

Blade tension is a double edged sword in that not tight enough it can slip and cause tracking and other problems. Too tight can not only cause the blade to break it can also cause the bearings in the wheels and gearbox to wear prematurely. I don’t remember who or where I got it but always tensioned until the blade made note when plucked. I also run my bandsaws at the lowest speed. For me there is a lot of seat of the pants and historical info(well, we’ve always done it this way) for all bandsaws. Horizontal and vertical with some of it being borderline superstion.

The tension(s) used to perform the measurements were in the 25,000 to 15,000 PSI range. They were all 1/2 inch wide, .025" thick bi-metal blades (standard for this saw) so the upper end was still below the maximum-rated tension.

All the tests were done with the blade running continuously until failure. I actually haven't seen his setup so I don't know how he determined the failure point down to the hour. He's a mechanical engineer with a lot of kit so I believe he's accurate when it comes to the time to failure and the applied tension. I guess I need to ask him how he determined the TTF.

 

[francist]

Maybe it’s just me, but I’ve never had a bandsaw blade break

Well all I can say to that is you're missing out on one of the more exhilarating moments a person can have in the shop!

Seriously, I've broken my share of blades but not from horsing the cut or doing something goofy. Way back when, I used to do a lot of freehand cutting of custom signs and the individual numbers and letters to build the signs. Think inside of a hotel lobby or bank where the shiny brass or aluminum letters direct you to the concierge or whatever. Anyhow, I did a lot of time in front of the vertical bandsaw (originally a 14" wood cutting saw converted down-speeded to cut non ferrous metals) cutting them out before filing and finishing for delivery. My blades were always good quality, either made by the local saw shop or bought straight from DoAll when they still had a storefront in our city. And after a period of time (I really don't know how long in cutting hours that might have been) I would start to hear a little "tick" as the blade went round. And gradually the "tick" got a bit louder and just a bit louder until I'd think to myself "huh, this blade is starting to make a funny noise...". And if I had made it to that stage it usually meant about two more minutes before she'd let go...

Wham! The saw frame would jump from the sudden release of tension, I'd jump even higher than the saw from the sudden explosion about ten inches from my ear, and then the broken blade end would come spooling out the top wheel cover usually right around where my right hand would be.

I don't think I ever got cut, if I did it was very minor, as the blade had nothing but its own inertia behind it at that point so couldn't really damage too much. Sure scared the k-rap out of me every time though.

Looking back I suspect it was a combination of things that led to the demise. Hours of cutting for sure, but also a buildup of chips beneath the blade that would embed into the rubber tires (remember, converted wood saw) which probably created a slightly lumpy ride. Every now and again I'd scrape the tires but you know how it is, when you're on the clock for piecework there's rarely time for housekeeping matters. It was just something I came to expect the older the blade got, but that said I still have some of them on the rack years later that have yet to give up the ghost. All I really know is, one day when I least expect it.... Wham!

-frank

 

[woodchucker]

The tension(s) used to perform the measurements were in the 25,000 to 15,000 PSI range. They were all 1/2 inch wide, .025" thick bi-metal blades (standard for this saw) so the upper end was still below the maximum-rated tension.

All the tests were done with the blade running continuously until failure. I actually haven't seen his setup so I don't know how he determined the failure point down to the hour. He's a mechanical engineer with a lot of kit so I believe he's accurate when it comes to the time to failure and the applied tension. I guess I need to ask him how he determined the TTF.

so my question would be, at max psi was the spring still working, or was the spring maxed out? I find a blade that is no longer on the spring cannot cut straight (wood working). I keep putting it off, I need a new spring in my WW bandsaw. I'll be buying a stronger spring.

IMHO if the spring is maxed (compressed), then the test is compromised.

 

[C-Bag]

IMHO if the spring is maxed (compressed), then the test is compromised.

A 4x6 doesn’t have the tension spring like a vertical. That’s the tricky thing about them. That and tracking them correctly.

 

[C-Bag]

Well all I can say to that is you're missing out on one of the more exhilarating moments a person can have in the shop!

Seriously, I've broken my share of blades but not from horsing the cut or doing something goofy. Way back when, I used to do a lot of freehand cutting of custom signs and the individual numbers and letters to build the signs. Think inside of a hotel lobby or bank where the shiny brass or aluminum letters direct you to the concierge or whatever. Anyhow, I did a lot of time in front of the vertical bandsaw (originally a 14" wood cutting saw converted down-speeded to cut non ferrous metals) cutting them out before filing and finishing for delivery. My blades were always good quality, either made by the local saw shop or bought straight from DoAll when they still had a storefront in our city. And after a period of time (I really don't know how long in cutting hours that might have been) I would start to hear a little "tick" as the blade went round. And gradually the "tick" got a bit louder and just a bit louder until I'd think to myself "huh, this blade is starting to make a funny noise...". And if I had made it to that stage it usually meant about two more minutes before she'd let go...

Wham! The saw frame would jump from the sudden release of tension, I'd jump even higher than the saw from the sudden explosion about ten inches from my ear, and then the broken blade end would come spooling out the top wheel cover usually right around where my right hand would be.

I don't think I ever got cut, if I did it was very minor, as the blade had nothing but its own inertia behind it at that point so couldn't really damage too much. Sure scared the k-rap out of me every time though.

Looking back I suspect it was a combination of things that led to the demise. Hours of cutting for sure, but also a buildup of chips beneath the blade that would embed into the rubber tires (remember, converted wood saw) which probably created a slightly lumpy ride. Every now and again I'd scrape the tires but you know how it is, when you're on the clock for piecework there's rarely time for housekeeping matters. It was just something I came to expect the older the blade got, but that said I still have some of them on the rack years later that have yet to give up the ghost. All I really know is, one day when I least expect it.... Wham!

-frank

I know that click very well. But the difference I think is I avoid cutting any kind of sheet metal with my bandsaws as that always kills a tooth or two. I think it’s interesting I can use them for a while like that and not break. Being cheap it usually takes losing several teeth before I get annoyed and toss the blade.

Between my throatless shear, snips and nibblers I can cut complicated shapes and not use the bandsaws. I’ve never done signs/letters/etc so dunno.

On my converted 14” vertical I have several old magnetic toothbrush heads I’ve modded to keep schmootz off the tires. They have totally stopped stuff from embedding in the tires and are easy to mount being magnetic.

 

[woodchucker]

A 4x6 doesn’t have the tension spring like a vertical. That’s the tricky thing about them. That and tracking them correctly.

just looked at the parts diagram and the hf has a tension spring.. part 35

 

[C-Bag]

Yes, it’s a spring(notice it doesn’t say tensioning spring, just spring) but it’s behind the tensioner screw. It’s just there to keep the tensioner knob from flopping around when not tight. It’s not like on the vertical saw where its between the wheel and the tensioner making the whole thing spring loaded. Hopefully I got it expressed right.

 

[KyleG]

Oooh this is a good one.

The behavior your friend observed isn't too surprising. When we look at fatigue due to a cyclic stress, it's a function of two things:

1. The peak stress. More stress means less life.
2. The stress ratio. In general, the higher this number, the longer the life.

Some explanation on stress ratio: This is the minimum stress divided by the maximum stress. Tensile stress is positive, compressive stress is negative.

• A fully reversing load (say 30 ksi tension to 30 ksi compression) would have a stress ratio of -1.
• A partially reversing load (+45 ksi to -15 ksi) would have a stress ratio of -.33
• A unidirectional load (+60 ksi to 0 ksi) would have a stress ratio of 0

As a blade goes around the wheel, the outside sees additional tension, while the inside sees reduced tension or even compression. If you know the wheel diameter, we can compute the state of stress on both sides of the blade. This is the origin of the cyclic stress which causes fatigue.

As your friend added tension to the blade, he increased the peak stress, but he also improved the stress ratio which improved the fatigue life of the blade.

Here's the S-N curve for 4130 as an example: