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

[DanG]

There ya go, Ian!  That's exactly what I was talking about.  I just don't know how to do those illustrations.

Ron, I definitely have power issues, but that isn't what I'm talking about here.    Ian explained it rather well.  When I say "center of the blade", I mean along a line across the blade from one edge to the other.  My line is down at the bottom edge of the circle.  I think the line most used by your conventional headsaw is more to the center of the circle.   Is there a vertical adjustment on your saw, or does a 6 inch cant ride in the same place as a 12 inch cant, relative to the center of the blade?  Keep in mind that on these multiblades and swingers we are not usually cutting all the way through the log.

[Ron Wenrich]

But, the tooth is what is doing the cutting and that engages the wood at the same angle no matter how deep the cut is.  Where the blade is in relation to the log doesn't really matter.  Your tooth is still cutting wood at the same angle.

I'm also running a vertical edger.  It doesn't make any difference where I engage the log, it still cuts the same.  The edger is an unguided system, and it will cut straight and deep, as long as it is sharp.

Since we are running a carriage, all material enters the cut at the same base.  But, logs are irregular and some are larger than others, so the angle would be different.  They all saw the same, since the tooth angle is always the same.  It is a guided system.  There is no compensation for height.

Do you folks have any lead in your saws?  I even have put lead in my edger saws, and they seem to run better.  It gives you better clearance on your saws.

Most circle saws are designed to be thinner at the edge than at the eye.  Without lead, you saw will rub and heat up.  Lead can be filled into a saw, but isn't recommended.  A swage is also used to widen teeth to help with the eye clearance.  We also run a bigger collar, which helps in stiffness.  May not be as important with a smaller blade.

[Ianab]

But, the tooth is what is doing the cutting and that engages the wood at the same angle no matter how deep the cut is.  Where the blade is in relation to the log doesn't really matter.  Your tooth is still cutting wood at the same angle.

Difference between a big circle saw and a smaller twinsaw / swingblade. With our saws the tooth enters the wood running parrallel with the grain, and then as it spins around the angle changes so it approaches 90 degs before it exits the wood. On a shallow cut it may only be 10 deg off the grain and cutting is easy. On a full depth cut there is much more cross grain.
This is different to how a large circle saw cuts. When you cut a small log the teeth are running close to 90 deg thru the log, the angle to the grain may decrease in the first part of the cut when sawing a large log, but the saw / log geomerty are different.
I see another picture coming 

[Ron Wenrich]

If I could draw, I would have had one up quicker.     If I can find a pic, I'll show you what I mean.

[DanG]

Ron, I think you & I had part of this conversation at Sawlex.  I think we are having trouble understanding each other because; 1. I have never run a mill that cuts all the way through a log on every pass. 2. You haven't run a mill that does not.   While the end results are about the same, the methods used in getting there are different.  I wish we could get together with both saws side by side so we could explore the dynamics of each together.  That would be fun!

Say, for instance, I'm cutting up a log into 2x4s.  I take a 2x4 out of it on each pass.  That board is oriented either vertically or horizontally, depending on where it is in the log.  On the horizontal board, my edger is cutting 4 inches deep, as deep as it will go. The main blade is only cutting one sixth of its capacity.  I'm cutting a total of 6 inches of wood.  On the vertical board, the edger is cutting half of it's capacity and the main is cutting at one third capacity.  I can cut that vertical board almost twice as fast, even though I'm still cutting 6 inches of wood.  On that vertical board I'm actually removing more wood because the larger kerf is on the wider edge of the board.  It's gotta be the "angle of attack" that is doing it.

[karl]

I don't speak well without visual aids- namely my hands!- but DanG said it perfect about the difference 'tween MD, Swing and carriage type saws. Doesnt matter whether it is the edgers or the main blade - shallow cuts throw long "shavings" with longitudinal (?) grain, and deep cuts throw short crossgrain sawdust. 2x4s swan with edgers cutting 4" cutt slower than edgers cutting 2'' and mainsaw cutting the 4". 4x12s take ALL the power my ol' V Dub has and makes mighty fine(small) dust.

[Ron Wenrich]

Here's what the sawdoc at www.sawdoc.com has to say about feed rates:

"  Different saws running at different speeds on different mills have different feed rates.  But every saw on any mill running at any speed has the same rate of feed per tooth.  Most operations I've been to tend to feed too slow.  This is inefficient and can cause saw problems.  So can feeding too fast.  You will need to figure what rate to feed your blade for your mill to get the best production and life out of your saw.  "

You might get into a problem with the MD mills where your feedrate for the headsaw may be different than that of the edger.  But, I would have hoped that was engineered into the design of the mill.

My question on the angle of attack would be that it would change on every cut.  Would that mean that every cut would cut differently?  Afterall, you're cutting a different type of grain, even in various parts of the cut.  But, your angle of attack would change on each cut.  No?

All ripping saws use an angle of about 35 degrees on their tooth.  It makes little difference if it is a carbide bit, or not, or if it has 6 teeth or 60 teeth.  The angle of the tooth is always the same.  

And since you are running that in a circle, it will always hit the wood and start cutting at the same angle.  It can be on the side of the blade, top of the blade, or bottom.  It will still hit the wood at that 35 degree hook.  What the hook does is chip the wood away.

My vertical edger is always buried.  It never cuts all the way through.  I can also do that on the headsaw on a really big log.  I will throw shavings and always figured that when the saw was at the very top (or bottom), the saw was acting more like a planer than a saw.  The hook angle on a planer is also 35 degrees (I think).

DanG, the only mill I ever saw that didn't cut through to the bottom was at Sawlex.  MD and Doublecut didn't have a dimension mill at the meet.  I would have liked to see one of them run.  I still think you ought to run one of those MDs next to a mill carriage.  

[Buzz-sawyer]

Hey Ron
Just as a side comment doesnt it make you feel uneasy when you,,,,,,,,,,,,,,,,,,,,,,"My vertical edger is always buried.  It never cuts all the way through.  I can also do that on the headsaw on a really big log"

doing this with your headsaw....I hate it , but occasionaly do it anyway.

[Ianab]

I will throw shavings and always figured that when the saw was at the very top (or bottom), the saw was acting more like a planer than a saw.

On a shallow cut thats how our small saws are behaving, like a chisel cutting with the grain. This takes much less power than cutting cross grain. When me make a deep cut it's really noticable as we are making the shavings AND cutting the cross grain stuff. The angle of the saw tooth doesn't change of course, but the angle it travels thru the grain changes. Making a 1" cut my mill flys thru the log with cloud of big shavings flying, with an 8" cut I really have to slow down and feed slower than that 'optimum feed rate'. This is a viscous circle, becase the tooth still has to cut each fibre, wether it's cutting 1/8" or 1/64"  So I can make two x 4" inch passes thru the log in about 1/2 the time it takes to make an 8" pass.

I agree with the feed rate per tooth thing as well. My saw only has 4 teeth driven by 8 hp, so it's no speed machine, but each individual tooth is working the same as one on a 50 tooth / 100 hp saw.

My question on the angle of attack would be that it would change on every cut.  Would that mean that every cut would cut differently?  Afterall, you're cutting a different type of grain, even in various parts of the cut.  But, your angle of attack would change on each cut.  No?

The angle of attack (relative to the grain of the wood) changes DURING each individual tooth cut.

And since you are running that in a circle, it will always hit the wood and start cutting at the same angle.  It can be on the side of the blade, top of the blade, or bottom.  It will still hit the wood at that 35 degree hook.  What the hook does is chip the wood away.

When the small circle mill tooth enters the wood it's actual direction of travel is level with the log. It starts cutting like a chisel running with the grain and then curves around toward the cross grain. On a shallow cut it exits the wood right away before there is much cross grain cutting done.
When your saw makes a shallow cut it effectively cuts crossgrain the whole time, it's only in a full depth cut that the teeth ever travel with the grain. 

2 saws that on the surface seem very similar actually have a very different operation and properties when you analyse it. 

Hope that makes sense 

Cheers

Ian

[Fla._Deadheader]

 Ian, when you do the "Double" shallow cut, do you shut the mill down to run the sawhead back for the second cut??  I'm curious if the sawhead will try to "RUN" back on it's own, with the saw turning

  I worded this as confusing as I know how 

[Ianab]

 Ian, when you do the "Double" shallow cut, do you shut the mill down to run the sawhead back for the second cut??  I'm curious if the sawhead will try to "RUN" back on it's own, with the saw turning

  I worded this as confusing as I know how 

With the chainsaw powered mill the saw is throttled back when not cutting, so the blade is either stopped or just freewheeling as it's pulled back thru the cut. I think Lucas recommend doing a similar thing on their 8" mills as the thinner blade has issues doing a full depth cut in some timbers. (And it's probably faster to do 2 cuts anyway ) As you are running the saw back thru the kerf you just cut I haven't had a problem with it trying to climb out.

Cheers

Ian

[Ron Wenrich]

I always thought that slowing down in deep cuts was more a function of gullet capacity.  I always noticed depth of cut affected feed rates as does wood properties.  That's why I don't look at feed rates as gospel.

Most head sawyers I've talked too have listened to the saw to keep the saw speed constant.  Otherwise, the saw can start doing tricks on you.

Buzz

Now that I have vertical edgers and laser lights, I can see how far I can go without buring the saw.  The edger cuts the slabs in easier to handle pieces.  The biggest problem I used to have was the return stoke.  You have to hope that the log or the saw didn't move any.     

[sigidi]

 Ian, when you do the "Double" shallow cut, do you shut the mill down to run the sawhead back for the second cut?? I'm curious if the sawhead will try to "RUN" back on it's own, with the saw turning

coming back to do the second 'half' of a full depth cut, I always keep the blade running full speed, two reasons I do this;

first it lets me have a good gauge on how straight/true my cut was, and sawdust = bad first cut

Second if for whatever reason there is a small amount of something in the way, it cuts it out.

Like Ian mentioned I haven't as yet had any issues with climbin out of the cut either, but I don't fully exit the log on the first 'half' either, I make sure the riving knife is still in the cut plus a small amount of the back of the blade, in this way I hope to stop any collapse which would close the kerf gap and stop me from coming back in ( just cause I cut ugly logs which most often have a heap of tension)

[Fla._Deadheader]

 Thanks guys. I guess there has never been a time when the saw would catch and try to run away from you while backing up  

  The exposed blade does not really worry me, but, I would give it plenty of respect, and the saw trying to pull away would be a concern ??

[karl]

The riving knife must help a bunch- if I stall/run out of gas in a log and disengage the drive on my MD to restart, I have to back out of the cut and/or wedge the cant to allow the saw back in without deforming the cant OR prepare for the saw to CHARGE back out of the cut on restart. (Note to self-STAND TO ONE SIDE!!!)

[DanG]

Karl, do you have a way to lock the feed mechanism in a neutral position?  Mine has a little hole with a pin to hold the handle in a position where the saw doesn't move.  I use this if I have to restart the engine in a cut.  Much safer than disengaging the drive!  It is recommended that you NEVER disengage when the engine is running.

Another tip along these lines;  I disconnected the spring that pulls the feed back to the reverse position.  It makes it much, much easier to handle large boards by myself.  Now, I just let the board return pull the board back almost to the balance point, then advance the saw a little and release the handle.  The saw will just sit there while I deal with the board, and I don't have to fight to get it out of the way in time.   Pulling the handle is easier on my shoulder, too.

[karl]

DanG
Hmmmm? Don't think so.... Do you by chance have hydrostatic drive? Mine is all belts, have to take a closer look when I can get back outside. Sure would be handy to be able to stop in reverse like you say.

[DanG]

Oh yeah.  I got hydrostatic.  I ain't sure just how those belts work.  With the hydro, the mill moves whichever way you put the lever.  There is a little hole in the frame, and a pin hanging on a chain.  Stick the pin in the hole, and the lever is locked in a neutral position.  Since I removed the spring, I don't need the pin anymore.

[woodbowl]

DanG and Ian,  (reply #7 & # 28) You've got something there! It surely seems that the larger diameter blade is more efficient than the smaller blade when cutting the same depth, same feed speed, same tip speed and same hp/torque coefficient. This leads me to another question. What's with a small edger blade anyway? I realize that it is not the primary cutter and only edges a few inches. I wonder if that's why they call it a edger blade? Doesn't it make sense to put the same size blade on the edger too? Then DanG could cut 2x4's or 4x2's without seeing any difference. With all this said, concidering the full range of the spectrum, the most efficient cut is that of a large blade in a shallow cut, the most inefficient cut is that of a small blade in a deep cut.----------Relatively speaking, the cutting angle is the same on a 20" blade cutting 4" of wood vs a 10" blade cutting 2" of wood. So................would both blades now cut at the same speed and same load?

[NZJake]

Gidday guys, swingers have a very efficient way of cutting. Both vertical and horizontal do not have forces contributing against the blade.

Thats the key to our 10" saw. The horizontal blade has no force coming from the other blade and like wise the other way round. As we all know, when you push a circular blade beyond it's feed rate the blade can wonder around knots(goes for bandsaws aswell). Push it further in it's tendancies it can anly get worse. The twinsaw must have a very resiliant blade in one dynamic to resist these forces of the other blade (ussually twin saw edgers are very thick steel blades with a much larger kerf).

Our 10" machines do not have this factor as an equation. What we do however have is gravity against the blade so the horizontal does tend to have a little more dish to it. We compensate with a slight lead. A 10x10 on our machines is as easy as cutting a 10x1.

I completely agree with Ianab on his explanation on the small cuts vs the larger cuts thing. Efficiency is long peelings of sawdust. 

[NZJake]

My thoughts are that the smaller blade under the same horse power with the same amount of teeth and same tip speed will cut more efficient. It's just that in the twinsaw edger circumstance, forces are being dragged from the larger 'flywheel effected' blade. Could be the difference?

By the way you'ld have to push the circulars blades pretty hard to get it to steer around a knot, typical scenerio means that our blades bog down in the wood before it goes drifting around anything.

Just thought I'd throw my opinions in.

[Fla._Deadheader]

 Your opinions are always welcomed, Jake. Wish I had gotten to see an ASM at Sawlex. Looks like you got the engineering down to a science.