Bandsaw blade tension

[Hornbeam]

Can anybody explain why M42 blades require more tension than standard blades of the same dimensions

 

[Ollie78]

Probably a "harder" or "stronger" metal so it requires more power to get the same tension as a softer steel.
A metalurgist will no doubt be along shortly to tell me I am wrong and its to do with the formation of the crystals and stuff.
I await the full answer.

Ollie

 

[deema]

I’m actually researching this at the moment, as it was a question that has intrigued me. M42 has a Young’s modulus very similar to HSS, so, it’s ‘stretches’ roughly the same amount under a given tension as carbon steel. However it’s does have a higher ultimate tensile strength, so can withstand a higher tension than carbon steel and remain in the elastic boundary. My theory at the moment, and I’ve written to a few blade manufactures and asked the question, is that they make use of the ability to increase the tension to give the blade a higher beam strength. With a higher beam strength a blade will cut straighter

 

[Bojam]

This question also intrigues me. In addition to why M42 blades require greater tension than regular carbon steel blades, I also would like to know why carbide tipped blades require even more? I use a 20mm (3/4") 3TPI M42 blade which I can tension fine and cuts straight and true. But the 20mm carbide tipped blade I have is more problematic. It is on thin band material (thinner than the M42 blade) though the teeth have a wider kerf. It has a vari-tooth 12-14-16 pitch. It would be great if someone could explain why this blade requires higher tension. I've been having difficulty getting it to cut straight as it wants to lead away from the fence at the start of the cut.

 

[Hornbeam]

I am a metallurgist.
Ultimate tensile strength is not critical, but the higher yield stress is as that is the point at which the blade effectively permanently stretches. I think that Deema is correct as the higher yield stress will allow higher tension, reducing deflection. On that basis, if you are cutting deeper thicknesses where deflection is more likely due to the increased distance between guides then M42 would offer a real advantage both in straightness and tooth wear, whereas for thinner sections the deflection is likely to be much less so the real benefits are only in tooth wear

 

[deema]

@Hornbeam brilliant, a metallurgist; a person who will know! May I ask, I had tried to find generic data on M42 yield points from where elastic changes to plastic elongation. However, all I could find was Young’s modulus and ultimate tensile strength. I have assumed that the higher the UTC would correspond to a higher yield point. Is that a reasonable assumption? Is yield point data not something that is published for materials, or am I just being a numpty and not looking in the right places? Sorry, I’m an electronics engineer originally, so I may not have the technical terms correct.

I believe that the yield point reduces with temperature, so I’m assuming that they expect the blade to heat up and have made an assumption on what is likely as the blade dulls to arrive at a recommended tension. I initially thought that it could be due to consideration of stress raisers due to the teeth being stamped in, however the tension seems generic and not specific to any tooth form. Does that sound reasonable?

Lastly, there are hard back and soft back saw blades, does hardening of the teeth / back of the blade affect the Young’s modulus and yield point?

 

[--Tom--]

I’m also intrigued, always assumed it was because it could be tensioned more so manufacturers say why not and publish the higher figure.

I managed to snag a Starrett tension gauge on eBay that wasn’t accurately described a while back. Is an absolute luxury but is really nice to be able to take the guesswork out

 

[woodieallen]

I just whack the tension on mine up until it hits the right note.

 

[Bojam]

I just whack the tension on mine up until it hits the right note.

Are you musically literate? I know some people do pluck the blade and listen for a specific pitch. But I wouldn't have the first clue.

Or do you just mean you tension the blade until it cuts well?

 

[woodieallen]

I have a good ear, that's true. But could I state the pitch ? Not a chance.

 

[Hornbeam]

I cannot find any further information on M42 steel grade and tensile properties. In general increasing UTS also increases yield stress but it depends on how the material is treated and this is not always true. If the material is cold worked then it will work harden, increasing yield stress but reducing ductility, creating a more brittle product.
The stress in the blade from the tension applied will b be the same regardless of the blade material assuming the same blade cross section
The friction caused cutting wood is not unlikely to raise the blade temperature to a point where blade softening would occur
My conclusion at this stage is that the blades do not need to be run at higher tensions, however the ability to increase tension due to the higher tensile properties will improve cutting performance, without the risk of blade breakage/stretch due to exceeding the yield point