Surface feet per minute

[Opiki]

Hi all
I was hoping to borrow someones formula to calculate SFM of of a 30 inch blade.
What I am wanting to do is repower my Mahoe mill with a newer engine.
Althoungh the newer engine has less H/P it can operate at higher revs and I was hoping with running at a higher speed and then gearing down to the drives, I would stilo end up with enough power to run the mill.
Ultimately I am looking at how to work out pulley sizes to achieve optimum blade speed.

P.S Any ideas as well on what the sfm of a 30 inch blade and a 16 inch blade should be

Thanks in advance Ross

[Gary_C]

If you assume the blade is running at the proper speed now, all you have to do is change the motor pulley (to a smaller diameter) to compensate for the higher speed of the new drive motor. The easiest way to do that is by the ratio of the new rpm to the old rpm. No need to calculate all the ratios and figure the actual SFM.

You need to look at the torque curve for the new engine to tell if it will be OK and determine the speed to run the engine. If the torque curve peaks at a lower RPM than the max rated, you will just shorten the life and probably increase fuel consumption by running it at full rated speed. The only thing you will gain is a little inertia to get you thru nails or hard spots. Since it will be belt driven and you adjust the blade speed with the pulleys, you do not need max speed, just max torque.

If the engine does have less torque, you will just have to cut slower or get a larger motor.

[J_T]

I hate to sound dumb But is this a circle mill or band or

[DanG]

Circle.  I'm pretty sure the Mahoe is a multiblade dimension saw, like the D&L and Mobile Dimension.

[J_T]

Thanks DanG Just don't rember hearing SFM refered to on a circle mill Maybe my sometimer's is gitting worse

[DragonsBane]

To get SFM, multiply the blade dia. by 3.14 (pi), then multiply by the blade speed, then divide by 12.

Ex. 30 inch blade times 3.14 equals 94.2.   Take 94.2 times 1000 (ex. blade speed). That equals 94,200. Now take 94,200 and divide it by 12 to get your SFM. In this case 7850 SFM.

The recommended SFM for hardwoods is 7000 to 9000 SFM. For softwoods its 10,000 to 11,000 SFM.

I've been researching alot of things for my swingblade and run across this. There is also alot of info on minimum and maximum feed speeds too.

Later all........

[Fla._Deadheader]

  Good info DB. Thanks for posting that. 

  How's the Swinger progressing

[DanG]

This brings me to a question I've been wondering about lately.  Are there any figures out there on  depth of cut relative to blade diameter?  The reason I'm wondering is, I have noticed that it takes a lot more power to cut 4 inches deep with my edger blade than it does to cut 4" deep with the larger main blade, even though the kerf is smaller.  It seems to me that the difference is in the angle with which the teeth attack the wood.  With the little blade cutting at max depth, the angle is almost perpendicular to the log, while with the bigger blade on such a shallow cut, the angle is closer to parallel to the log. 

I wouldn't think this would be noticable on a conventional headrig, since most of the cutting on small stuff is still done near the center of the blade.  On the multiblade and the swinger, however, the cutting is done near the edge, hence the  big difference in angle of attack.

Any of you circle saw gurus care to comment?

[DragonsBane]

Hey all,

 I dug up the links to the info I posted.

www.jedc.org/wood/Spring_2000.pdf

www.jedc.org/wood/Spring_2001.pdf

www.jedc.org/wood/Spring_2002.pdf

www.jedc.org/wood/Spring_2003.pdf

Looks like alot of really good info on balancing a saws performance. Its definitely worth the read.

Fla. Deadheader,

 I've made some progress on my swinger but I am having some difficulty in locating a suitable gearbox that won't break the bank. I may just fab something up. I'll need to make a trip to the stell store soon though as I need to really get moving on this.

Later all.........

[Opiki]

Thanks all for the info
Once again the knowledge depth proves itself.
Gary I am going to go through all calcs again just to confirm I have the right blade speed.
If I understand you right then my only concern is the max torque and no amount of gearing is going to increase that !!  H/P is actually irrevelant
J T yes it is a multiblade dimension saw. If you are intersted check it out here
http://www.sawmills.co.nz/super.html
DB thanks for that info I will try and digest it all when I get home tonight

Thanks again Ross

[Minnesota_boy]

Torque is the force necessary to make the blade turn.  Horsepower determines how fast you can turn it with that amount of torque.  More horsepower means faster sawing.

[Ianab]

It seems to me that the difference is in the angle with which the teeth attack the wood.  With the little blade cutting at max depth, the angle is almost perpendicular to the log, while with the bigger blade on such a shallow cut, the angle is closer to parallel to the log. 

Dang, I think you are correct, I notice the same thing with the swingblade. It seems to be the angle as you suggest. When the teeth are ripping with the grain in a shallow cut it's super easy, but as it moves to the teeth cutting across endgrain things slow up. With an 8" cut my Peterson I really have to slow down and it's often quicker to make 2 x 4" cuts. With more power you dont notice as much of course, but with only 8hp it makes a big difference.

Cheers

Ian

[Gary_C]

Opiki

Your are exactly right. Horsepower is determined by multiplying Torque times RPM. Most engines will hit peak torque and then the output flattens out and increases very little or even drops slightly. So an engine can be rated at more horsepower at a higher RPM but it does no good to run it at max speed when there is very little or no increase in torque.

[Don P]

I'll agree with Ianab and DanG on how the blade enters the cut. For most work on a tablesaw or skillsaw I set the depth to maximum to get the teeth in and out quicker, the cut length is shorter. The only reason we were taught to keep the blade shallow on the tablesaw was safety, not sawing efficiency, I think the same principle applies here.
With most larger circular blades the blade should be taking 1/10-1/8" per tooth. If you lack power slowing down without also dropping the number of teeth produces finer dust, the chip becomes smaller than the 1/10-1/8".  The chips are supposed to gather in the gullets and be expelled from the cut as the tooth comes out of the wood. Fine dust falls out of the gullet and drags alongside the sawplate while still in the cut creating friction, heat, blade distortion, more heat...cursing sawyers...and on and on.
I've never seen more rpm's and less power gain anything, I've tipped the balance when I've run fewer teeth. I don't know if that will hold true for you, but it has for me on several other kinds of round blades  .

[J_T]

Ross neat saw  I try to look and learn all I can . If I could just rember it  all

[DanG]

Hey, Ross!  Welcome to the forum!   Now we can add another brand name to our list, eh?   Sure didn't mean to hijack your thread....it looked like the ? had been answered and that would be that.  Oh well, since you're a multi-blader, it'll probably work out for the best anyway.

[f350ca]

DanG,
Your right its the angle of attack of the teeth that makes the difference. Think about a wood chisel slicing along the grain vs cutting across end grain. With the shallow angle your slicing along the grain and parting the cells vs shearing through them.

[Ron Wenrich]

Let me throw this into your mix.  If it would be the angle of attack, then when you turn the log, the head saw is attacking from the same angle as the edger saws had been.  Would the head saw then die down?

The angle the edger saws are attacking the wood is the same as the headsaw.  All things are relative.

I know sawdocs have tried to make convincing arguements that you can saw any size log at the same speed.  They wonder why sawyers slow down on larger logs. 

With increased diameter, your gullets fill up and take longer to unload.  Too fast and you will hang the saw or the dust will spiill out of the gullets and push the saw.

As for cutting on the edge of the saw, all saws cut on the edge.  On a big headsaw, we have saw guides that are close to the edge on smaller logs.  I don't know if you guys are running guided saws.  On larger logs, you're cutting basically with an unguided saw to some extent.

As for the edger slowing down, that might come from 2 different sources.  The amount you are cutting with both the headsaw and your edger blades.  Also, edger blades don't unload the same way as your headsaw.  Could be some friction involved.

How does it work when you have 2 saws edging a 2" piece vs 1 saw edging a 4" piece?  Are they both the same?  If so, then its a power related thing.

My 2 cents

[RSteiner]

I have a Mobile Dimension saw which is some what similar to the Mahoe mill in principle of blade design having edger and main blades.  I assume they both use an inserted tooth blade that have 6 to 8 teeth. 

On the Mahoe mill do both blades turn so each one at the rim is truning the same surface feet per minute?  It could be that the main blade being a larger diameter but rotating at the same RPM as the edger is moving at a higher SFM.  This would make the chip load on each blade different even though they are moving through the log at the same speed.

I know on the Mobile Dimension mill the edger blade chips or saw dust are more course than the chips from the main blade, both have 6 teeth each but there is a huge difference in diameter and SFM between them.

Randy

[Ianab]

A visual aid please....



The cutting angle we are talking about is where the teeth start to approach 90 deg angle to the grain as they are cutting. This really loads the saw up. With a larger blade cutting the same depth the angle is no where near as steep, so it's not as noticable. With a shallow cut any size saw is mostly splitting the wood out with the grain, large sawdust or slivers and easy cutting. With deep cut it's all got to be cut cross grain, finer sawdust, more load on the saw.

Cheers

Ian