are sawblades spinning too fast?

[Tom L]

max torque on a diesel powered woodmizer occurs at 1800 rpm. isn't it torque that keeps a blade in a cut cutting? the machine now at cutting speed runs at 3000 rpm. torque drops off dramatically after 1800 rpm but the machine reaches full HP at 3000 rpm.

would a slower blade speed with twice the torque still remove the same amount of material in the gullet of a sawblade? a gullet can only build up so much material until it clogs and misses material. so if the gullets are clogged at full speed, they don't cut anyway.

so open for discussion , will an 1800 rpm at max torque cut the same cutting speed as a 3000 rpm motor at half torque. what do you think?

this does not apply to gassers which develop max torque at max rpm.

if you could run at 1800 rpm, the engine would last longer.  don't most circular saws run at 500 rpm?

[ladylake]

 Max HP will cut the fastest, I've always had a hard time thinking the gullets get too full on a bandsaw.  Some people blame cutting wavy in hardwood on shallow gullets yet the same shallow gullets will rip through softwood nice and straight while making twice as much sawdust. Steve

[GeneWengert-WoodDoc]

A saw 48" to 60" in diameter will typically run around 500 rpm.  The saw is tensioned, hammered or dished (same thing) for a specific rpm.

[Delawhere Jack]

The the band wheels act like flywheels, "storing" torque. It takes a lot of torque to bring the wheels from a standstill to operating speed, but then hp is needed to maintain the speed...... (?)

There are three things (at least) that I have resigned myself to never understanding:

1) Calculus.
2) The diffence between hp and torque.
3) Women.

[Delawhere Jack]

Regardless of engine speed, band mill bands are engineered to work most efficiently at a specific rate, around 4,500-5,000 ft/min.

[beenthere]

 

[drobertson]

If I were to guess, and only guess, I would say it has something to do with the strength of the blade in relation to rpms, meaning, (my thinking) that the band blades would not beable to handle the stress of the cut at a slower rpm, ?   If the bands were thicker and more rigid maybe they would?

[Tom L]

If I were to guess, and only guess, I would say it has something to do with the strength of the blade in relation to rpms, meaning, (my thinking) that the band blades would not beable to handle the stress of the cut at a slower rpm, ?   If the bands were thicker and more rigid maybe they would?

good point, it is an interesting thing. bandsaw blades. the thicker the better in most all cases.
I have always been told by blade manufacturers in cutting steel that you should never have more than
7 teeth hitting your work at one time. the thicker the material the less teeth per inch. just because of gullet  clogging. this is where I can't understand how a blade with a tooth every 7/8 or 1-1/8 can cut successfully in a log that is 26" wide. by theory the blade should have a tooth spacing of a tooth every 4". and a big tooth at that lol

does anyone know the blade speed and tooth spacing of the big commercial mills, the ones with the 30 ft blades you see in the videos?

[ely]

i think the first post asked a question about circle blade speed, i know on my manual band mill when i push it too fast, i can feel the gullets clogging and spilling over and i can also see it in my tach.
del jack is correct with the speed of a band mill designed around the sfpm measurment.
i find the same principles apply to my sharpener stone while running the band thru it. the stone will only cut so much so fast before things start backing up on you and burning the metal.

[tyb525]

Slowing down the blade speed means reduced cut quality and more stress on the teeth. Max power is achieved halfway between max torque and max hp.

Ely, I have found if I cut too slow, sawdust is left on the boards.

How do you know the gullets are overflowing? The reason for your engine slowing down would be from increased load at higher speed I would think.

Oh, and the difference between hp and torque:
Hp is the ability to do a certain amount of work at a certain speed.
Torque is a measure of rotational force.

A 1hp and 10hp engine can produce the same amount of torque, but the rpms they produce it at is different. The 1hp would have to spin much faster than the 10hp to do the  same amount of work
Or, if the 1hp is geared very low, it can do the same amount of work at the same rpm, but it will take longer to do it.


The high band/blade speeds allows us to use lower hp motors. The inertia of a fast spinning blade, and the smaller bite per tooth means less load on the motor.

[Ianab]

will an 1800 rpm at max torque cut the same cutting speed as a 3000 rpm motor at half torque. what do you think?

Well to keep the maths simple, assume 3600 rpm, and your torque is 1/2. Then arguably yes. Assuming you geared the drive to give the same band speed, the torque AT THE BLADE would be the same. The blade can't tell if it's being pushed by an 1800 rpm engine, or a 3600 rpm engine, through a 2:1 reduction drive. Horse power applied to the band is the same.

But to get the most cutting power at the blade you would run the engine where it produces max horsepower, then gear the drive down to increase the torque. This value is generally somewhere higher than the max torque.

For a normal engine the torque and hp curves will start low, with the torque going up first, especially with a diesel. As the revs rise, the horsepower naturally increases. At some point the torque curve flattens off, but the hp continues to rise. At some higher point the torque actually starts to drop quickly, and the extra revs don't gain you any more HP, so that also falls off. Some time after that you reach the rev limiter, are restricted by valve timing etc, or the engine flies apart, depending on the design.

Now there may be valid reasons to "De-rate" an engine, and run it at less than it's max HP. Longer life and better fuel economy for example.

But then you change the gearing to keep the blade speed the same.

And yes large circle mills usually run the blade at about 500 rpm. This depends on the desired tip speed where the tooth meets the wood. A smaller saw will run a higher rpm. So my mill with a ~19" blade runs closer to 2,000 rpm. This is geared down from the 3,000 rpm engine speed, same way the bigger mill may be geared down from a 1800 rpm engine...  But tip speed on both mills is similar.

Now could you run a band saw at a much lower blade speed? Probably, something like a water powered sash sawmill runs at much lower blade speed. But you would need to redesign the blade geometry to work properly at the lower speed. Then the band would need to be much heavier to cope with the extra torque, and bigger gullets to handle the extra sawdust. So it's not really a practical thing to do.  Likewise increasing the blade speed could theoretically cut faster, but you run into all sorts of nuisance factors like friction, and thhngs tending to fly apart.

Hence most similar mill end up running similar actual blade speeds, and this need not have any relationship to the engine rpm.

Ian

[Ron Wenrich]

540 RPM is what the slower circle mills run at.  I've run one at 700 RPM, and we cut it back to 640.  At 540, a 54" saw is running at about 7600 sf/min.  At 700 RPM, we were running close to 10,000 sf/min. 

The 19" blade running at 2,000 RPM is running at the same speed as a 54" at 700 RPM. 

[ladylake]

Slowing down the blade speed means reduced cut quality and more stress on the teeth. Max power is achieved halfway between max torque and max hp.

Ely, I have found if I cut too slow, sawdust is left on the boards.

How do you know the gullets are overflowing? The reason for your engine slowing down would be from increased load at higher speed I would think.

Oh, and the difference between hp and torque:
Hp is the ability to do a certain amount of work at a certain speed.
Torque is a measure of rotational force.

A 1hp and 10hp engine can produce the same amount of torque, but the rpms they produce it at is different. The 1hp would have to spin much faster than the 10hp to do the  same amount of work
Or, if the 1hp is geared very low, it can do the same amount of work at the same rpm, but it will take longer to do it.


The high band/blade speeds allows us to use lower hp motors. The inertia of a fast spinning blade, and the smaller bite per tooth means less load on the motor.

Right, for sure no trouble with the gullets spilling over with a small hp mill.  Maybe with a 50hp diesel with a 2" band in softwood. 

[GeneWengert-WoodDoc]

Medium and larger band saws and most circular saws are designed to remove a section of wood 0.11" wide with each tooth.  This is called the bite.  So if we know the rim speed or tooth speed for a saw and the tooth spacing, we can calculate the feed rate.  Plus if we know the density of the wood and the depth of cut, we can calculate the HP required.

Once we know the speed of the saw, we can determine how much dish, hammering or tension is required.  With large saws, there is a "straight edge" that is about 10" long, but the one edge is actually curved rather than straight and this curve is the curve that the saw needs when stationary, so that when the saw is running, the centripetal force will flatten the saw and the saw will cut straight.  If we did not have this initial curve or tension, then the centripetal force would cause the saw to develop waves or wobbling when running and the cut would wander.  This is also why we put tension on a band saw...to change the resonant frequency of the blade so it will run without waviness.  It is also why a hot saw will get wavy...the heat expands the metal and removes tension or dish or hammering...and why cooling a saw makes it run better.  It is also why thin blades wander...we can not add enough tension without breaking them.  It is also why we have one wheel of a band saw with a bit of crown, rather than being flat and why when cutting a dense wood, which generates heat, that the saw will wander.  Etc.  The Univ of Calif at Berkley had a week long class on saws that I took; they have the math and formulas for saw performance, rather than "magic" or "experience."

[Tom L]

Gene, that would be something good to know, optimum feed rate for a certain blade type (tooth spacing and profile) running through a certain type of wood, would help for sure with some problems of too much feed for a certain type wood. or too little feed rate

[chet]

  they have the math and formulas for saw performance, rather than "magic" or "experience."

Don't get me wrong, books and formal education are great. But all them formulas and math always seem to be set up under perfect conditions. That's where "experience" comes in, you tweak and adapt to get the best result given the circumstances and problems you encounter.

[Magicman]

:P .......rather than "magic" or "experience." 

There is nothing wrong with "magic or "experience".

[drobertson]

there is no doubt that all the figures that are calculated are of some importance, and a good reference, but there is no subsitute for experience, in any cutting operation.  As far as magic goes, well, sometimes we all have stratched our heads say_what  and just kept on going,   david

[chet]

:P .......rather than "magic" or "experience." 

There is nothing wrong with "magic or "experience".

Somehow I KNEW you had ta respond to da magic part.   ;)

[5quarter]

Gene...you wouldn't happen to have any of those materials handy, would you? I have plenty of experience, but wouldn't mind a little math on my side if I can improve my saws performance... ;)

[beenthere]

:P .......rather than "magic" or "experience." 

There is nothing wrong with "magic or "experience".

"Magic" and experience is what the Forestry Forum is about.  8)

[SPD748]

I joined here and found both... AND... I'm better off for it!

-lee

[GeneWengert-WoodDoc]

I am in Myrtle Beach for a few weeks, and my books from the class are at home in WI.  Next month I can look and share.

My comment about magic and experience referred to the class (read the post again and you will see that) and not to how we fine tune our sawing operations using experience.  Sorry for the misunderstanding.

There is no doubt that experience is important, and perhaps some magic for some of us indeed.  The theory of the class helps to explain why some things we do, or others do, help to make a saw run better.  In also gave ideas for changes in the future.  The class was developed from the techniques learned from sawing cedar blanks into pencils with minimal sawdust (thin saws) and also sawing pineapples to fit into a number 10 or 5 can.

[ely]

when referring to the gullets filling up , ive seen what you are taking about when sawing at slow speed, it does leave alot of dust on the log, same principle when sawing to fast, the gullet fills to the max then spills out, i can see the difference in the amount of sawdust exiting the cut.
my manual mill is designed to cut a certain amount of wood at certain speed with a certain tooth spacing, when i tweak or change anyone of those variables i see a change instantly.
i feel what my saw is doing or not doing long before other folks have any idea its going to happen. my dad sure thinks its magic :D. he runs an lt 40 and says dang the band is dull cause it is walking on me.
at a certain point you have to make drastic changes in your equipment to get much more production out of it. such as wider or thicker bands, more HP etc.

[ladylake]

Don't think the gullets are getting full ,  with more set sawdust will spill onto the cant and less set will leave a lot less on the cant.    Steve

[Ron Wenrich]

On a circle mill, when you get the gullets too full, it hangs the saw. 

[ely]

it will do the same on a bandmill., if i slow up on the push the sawdust is expelled properly, if i slow too much it spills out. if i overspeed my push it will stall.
as i stated earlier, there are several varibles at work simultaniuosly, alter anyone of them and it changes the others at the same time. a person knows when his mill is running at full tilt and cutting well.

[Magicman]

Yup, kinda like that magic and experience working.  You can feel it when it is right.   

[Ron Wenrich]

I can usually tell a circle mill just by listening to it.  I don't even have to be running it.  Must be magic. 

[tyb525]

Equations and calculation on goes so far, then you need experience and an ear and feel for the saw. I know I developed a feel for my manual mill.