Saw blade tension and work force.

Too much tension on a band saw will add unecessary stress to the blade. I use the "bow-string tuning" method. You get used to plucking the blade while tightening until you reach a nice medium-high note. The idea when free cutting on a vertical band saw is to let the blade do the cutting and not force the issue.
 
My 4x6 setup is probably incorrect and non-productive, but I'm after smooth/accurate/square cuts. I spent a lot of time tweaking the guides and clamp backplate to get precise 90° cuts.

I have the blade tight and the initial cutting pressure light so as to avoid any twist/bow/tilt of the blade during the critical start. As the cut progresses, the blade is guided by the kerf and the down force increases.

To get this setup, the spring tension gadgetry was modified to get a lot more spring tension, so much so that the bandsaw arm will stay in place (hover) when lowered to about 45°. When lowered to the workpiece, the blade is probably pressing down only about a pound or two.

I also get better cuts by precleaning the blade. I'll press two wood sticks against the sides of the running blade, sandwiching the blade between them, holding the wood sticks tilted upward about 20°, so that the top edges of the sticks scrape and clean the blade.

I'm in no hurry, and do other things while it's cutting.

And, yes, it took a lot of experimenting to get to this setup.

Since adopting this method (and changing over to bimetallics), I no longer experience blade breakage...
 
I found out a lot of things I never knew, doing my research on saw blades. According to Starrett, Lenox, and a few other companies , the tension on a bimetal blade up to 1 1/4" should be 25000 to 30000 lbs. For me, that is popping a vein tight. Carbon flex back and hard back blades are only 15000 to 20000 lbs.

I am using bimetal , 1/2" wide, .025 thick with a 6 /8 tooth combo. These cut good for most metals and last a long time.

While I wait on my hydraulic feed parts, I will be making a blade tension gauge. I have the plans, the materials, and the perfect gauge ( 2 /12" diameter and reads in tenths ). 25000 lbs. is .00345 stretch at 4inches.

As usual, I will post a write up on the guage. It only has two parts plus the indicator, but a lot of machining. Since my mill is torn apart right now, this may take a while. Fixing the mill is top priority.

Mark Frazier
 
I'd be willing to bet that those 15000, 20000, 25000, 30000 numbers are actually PSI.

Our 0.025" x 1/2" blades have a cross section of 0.0125 SqIn, or 1/80 SqIn.
30000 lbs on this would be a section tension of 2,400,000 psi, far beyond yield strength.

A section tension of 30000 psi would be a 375 lb tension.
A section tension of 15000 psi would be a 188 lb tension.

That's about what my blade tension feels like...
 
you said your blade guy knows his blades, whats his advise on the subject? i have no advice to offer, im still hacksawing by hand.
 
You are correct. I'm sorry, i knew that in my head but didn't type it that way. It is PSI :sorry2: .


SG51Buss said:
I'd be willing to bet that those 15000, 20000, 25000, 30000 numbers are actually PSI.

Our 0.025" x 1/2" blades have a cross section of 0.0125 SqIn, or 1/80 SqIn.
30000 lbs on this would be a section tension of 2,400,000 psi, far beyond yield strength.

A section tension of 30000 psi would be a 375 lb tension.
A section tension of 15000 psi would be a 188 lb tension.

That's about what my blade tension feels like...
Click to expand...
 
X-RAY said:
you said your blade guy knows his blades, whats his advise on the subject? i have no advice to offer, im still hacksawing by hand.
Click to expand...


Yes, as a matter of fact, he does know his stuff and he makes dozens of blades a day for various industrial customers. He has advised me on tooth combination selection, and how to look at the chip from cutting and determine if more down force is needed or the speed is too fast or too slow. And he makes a good blade that doesn't break like the ones I used to get. They always broke on the weld. I have been experimenting with the knowledge I got from him and my saw does perform a lot better.

Mark Frazier

P.S. he also recommended using a tension gauge if I had one because the proper tension range is important to blade life and cutting straight. Yes you can guess at it but that is not guaranteeing you are even close.
 
I have started a tension gauge . it will be posted in members projects if anyone is interested in following along

Mark Frazier
 
i wasnt questioning you or your blade guy, i was interested in his thoughts. i read your build thread and now i know. good luck with the project, ill be watching and learning.
 
X-RAY said:
i wasnt questioning you or your blade guy, i was interested in his thoughts. i read your build thread and now i know. good luck with the project, ill be watching and learning.
Click to expand...

:allgood: Mark
 
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