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Author Topic: Stability of Cantilever Head Design  (Read 1132 times)

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Offline jeepcj779

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Stability of Cantilever Head Design
« on: October 30, 2019, 09:50:57 PM »
  During the research phase of buying a mill, I looked at several different manufacturers, but only two main designs, four-post and cantilever. I was only looking at thin-kerf band saw mills, not the swing mills. Early on in my research, I saw several videos where I could see the head bouncing on the cantilever mills, and I wondered how on earth that design could cut straight lumber. I also saw many videos where the wood that was cut had waves or ridges in it. I attributed that wavyness to the bouncing cantilever head and reinforced my (zero-experience, uneducated) opinion that a four-post mill was better, so I decided not to even consider the cantilever mills. Since I said as much in one of my earliest posts when asked by WV Sawmiller why I was not considering cantilever mills, I thought I should share what I have discovered/surmised after a few months of deciding what mill to buy.
The short story is, I decided I am getting a mill with a cantilever head, for many different reasons. One of those reasons is they cut straight lumber. I determined that the head-bounce I saw in the videos happened when the blade clutch was engaged or disengaged because the entire engine (heavy) is rotated to do it, and the blade and band wheels are static. When the blade was moving in the cuts, I did not see the bouncing. I'm not an engineer, but I think it could have something to do with the gyroscopic stability provided by the spinning band wheels when the blade is moving at design speed. I don't know the mass of the wheels or the RPM they turn, but that is what I came up with.
When I went to the Woodmizer Carolina open house, they sawed a bunch of logs with cantilever mills that all had straight cuts. Also 1000's of mill owners who say the cantilever cuts straight lumber cannot all be wrong. If cantilever mills made wavy lumber, they couldn't do this:



 

That is a 1/16th slice of poplar, I think. Nice looking with the sun in the background.

If anyone else knows what might impart stability to the cantilever design, please chime in. I'm not trying to kick a hornet's nest, but I wanted to clear the air from my months-ago statement, and I just like to know how/why stuff works.

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Re: Stability of Cantilever Head Design
« Reply #1 on: October 30, 2019, 09:59:52 PM »
   Congratulations. I am no engineer and am not going to try to describe why it works. I am just here to verify it does cut good, straight lumber and when I saw a 4 post mill in operation (it was a WM BTW) it just looked like more trouble to set up and off load from.
Howard Green
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Offline Haleiwa

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Re: Stability of Cantilever Head Design
« Reply #2 on: October 30, 2019, 10:04:12 PM »
The slack between the frame and the head doesn't matter if there is constant tension in one direction only.  Since the blade only engages the log in one direction it provides the tension.   If you look at a two way circle mill, the frame and carriage have an enormous amount of steel and bracing dedicated to maintaining stability in two directions because the blade cuts on both the up and down sides.  Bandmills don't have that challenge.
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Offline Magicman

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Re: Stability of Cantilever Head Design
« Reply #3 on: October 31, 2019, 07:19:02 AM »
jeepcj779, I believe that you have already answered your own questions.  :)   I am not an engineer/designer, only a user.  ;D

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Offline doc henderson

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Re: Stability of Cantilever Head Design
« Reply #4 on: October 31, 2019, 07:43:34 AM »
I am also not an engineer, and I worked all night so did not even stay in a holiday inn express...  :) with a vibration the head can move a little up and down, and the blade on either side of the wood, but changing the cut pattern of the blade takes time, and if the wobble is fast, it prob. cancels itself out and the net result is not wave.  I agree with the tension pulling to one side helping to stabilize.  as you may recall, I own a timberking, but I agree the proof is in the pudding (results).  I have great respect for the sawyers with Woodmizer mills, and the  rep I had lunch with at the pig roast.  I think you have made a good decision!!!
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Offline Crusarius

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Re: Stability of Cantilever Head Design
« Reply #5 on: October 31, 2019, 07:51:52 AM »
The constant torque produced by the engine preloads the cantilever head and will actually hold it further down than when the engine is not running. as long as the drive belts are not slipping the head will stay right where it goes to when the engine is revved up. 

This is why woodmizer actually has an alignment that I believe is 1/16" off parallel to the bed. When the blade is engaged the head pulls down that 1/16".

When I built my mill I built it 2 post. I definitely learned the benefits a cantilever head mill can and does have. Off loading is the #1 benefit. I have to make sure my head is clear of the log to offload anything.

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Re: Stability of Cantilever Head Design
« Reply #6 on: October 31, 2019, 09:21:51 AM »
I cant speak to the engineering that generates the stability but the cantilever design is stable. There is some shake of the head when engaging the blade.  The solution is to start the blade and wait a couple of seconds before entering the cut and allowing the head to stop shaking. 

Once you start using your sawmill, stability observations will be reinforced.  First hand experience also will reveal that there are several ways to make wavy lumber.  Those ways have more to do with the blade, species being cut, build up on the blade, sawing speed, blade tension and blade speed.  Wavy lumber has less to do with design of the mill and more to do with blade operating parameters.

First hand experience will also make clear the difference among actual waves in the lumber and the surface ridges caused by a tooth or teeth out of set and the surface ridges at the beginning of a cut caused by blade chatter.  
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Offline Woodpecker52

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Re: Stability of Cantilever Head Design
« Reply #7 on: October 31, 2019, 09:51:58 AM »
Woodmizer makes both types now cant and 4 post.  I have a Lt15 which is a rail rider and one post on the idle side which cuts great.  I do think they should be two rail riders on each side however, like on the lt15 wide.  The one post has a tendency to give an annoying  catch as the one roller moves over the two rail section seams and yes I have them flat against each other and level.  This is my only grip with this model.
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Offline YellowHammer

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Re: Stability of Cantilever Head Design
« Reply #8 on: October 31, 2019, 10:38:46 AM »
Heres my two cents.
Basically, with the cantilever designs, there is a decent amount of slack and flex in the design, especially when the head is in the full up position, and the track bearing are unloaded.  Steel will flex, unless it is braced to eliminate deflection (four post vs cantilever design). I can grab the head of mine, and rock it back and forth, because everything is unloaded.  Thats typical of lots of types of machines.  However, there is a huge difference bettween unloaded stiffness and operable stability. Once the log is on the bed, and the band is in the wood, everything changes, and the system gets loaded and gets very stable.  All the slack disseapears and everything is rock steady.  Not only that, but since the steel frame is well into its elastic stage, it can overcome small irregularities in the logs and such, and acts somewhat like a shock absorber and actually smooths out the head movement.  If you watch carefully, when sawing fast and hitting a knot, you can see the head deflect backward minutely from mast twist, then ease and recover back to it normal position. This gives a significant shock absorber effect and reduces the instantaneous peak loads on the engine.  I really like this property. Since the system is stable, it will still be cutting in a straight line.  

Groscopic precession also helps load the system.  There are lots of flywheel like components on the mill.  Gyroscopic precession keeps bicycles upright and motorcycles going down the road even after the rider has fallen off.

Put all these factors together, and you have a very stable and flat cutting sawmill.



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Offline terrifictimbersllc

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Re: Stability of Cantilever Head Design
« Reply #9 on: October 31, 2019, 10:48:06 AM »
Also the faster one saws the greater the pulling force on the head. 
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Offline Bruno of NH

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Re: Stability of Cantilever Head Design
« Reply #10 on: October 31, 2019, 08:07:18 PM »
After going from a 4 post to a cantilever head I would never go back to a 4 post.
Lots of advantages to the cantilever head.
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Offline tgabby1968

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Re: Stability of Cantilever Head Design
« Reply #11 on: November 02, 2019, 09:25:55 AM »
Jeep, I was in the same quandary, then same as you reading info here and talking to several people, going and watching the mills operate I went with the cantilever. I was also able to find a used saw with the big diesel at a decent price.

Offline barbender

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Re: Stability of Cantilever Head Design
« Reply #12 on: November 02, 2019, 11:52:31 AM »
The cantilever design saws flat lumber. If your getting waves, it's due to something else. If your blade is pushing up in the cut, you have a blade issue and nothing is going to hold it down. This issue is raised from time to time and it boils down to lack of understanding of how the cantilever mill operates, or dishonesty. I've seen other mill manufacturers literature suggesting that the cantilever bounces up and down and has an unstable cut. They should be ashamed of themselves for making that claim, imo. There are valid differences between WM mills and others, where another manufacturer may have a superior design. The cantilever head is not one of them.
Too many irons in the fire

Offline Stephen1

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Re: Stability of Cantilever Head Design
« Reply #13 on: November 02, 2019, 08:51:03 PM »
The WM as a cantilivered head and single rail allows for what I believe is an easier set up when portable. I only have to make sure I am fairly level, side to side and end to end, I use the water bottle on the mill. I like if it is downhill into the cut. It is also eaiser to saw pretty/ugle logs into table tops. Lots of time I am able to get past a fat spot in a log to take off a notch, or side of crotch. I can then turn the log and get more of it. No outside post to worry about.
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Offline ladylake

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Re: Stability of Cantilever Head Design
« Reply #14 on: November 03, 2019, 03:08:28 AM »
 With the hugh clearance of a TK 2000 you could run a odd shaped 52" log through by turning the hump up.  The blade goes 36" above the deck with 16" above the blade.  Steve
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Offline jeepcj779

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Re: Stability of Cantilever Head Design
« Reply #15 on: November 04, 2019, 12:43:36 AM »
I liked the TK2000 and 2200 mills when I was doing reseach. They are inherently stable due to the 4 post design, and you are correct that milling a tall oval shaped log with a TK is possible due to the height of cut plus the depth of cut. Whatever you can fit between the posts is possible. You can mill some pretty big and odd shaped stuff on a Woodmizer also. I was not knocking TK with my post. Honestly, I just wanted to correct a misconception I put out in another thread by mistake (due to a lack of knowledge) about bouncing cantilever heads. TK seems to make good, capable mills, but I mainly went with Woodmizer because of the proximity of the dealer and ease of setup for portable ops.


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