Is there something called tension in a log?

[Glenn1]

I am working with a sawyer who I really like, and I asked him about the variable thickness of the cuts.  We were shooting for 1 1/8" but I noticed that sometimes it is more like 1" and other times 1 1/4".  He said that the tension in the log can effect the depth of cut.

Can someone explain this principle to me?

[POSTON WIDEHEAD]

Is the various thicknesses on the ends of the board?

[Glenn1]

Is the various thicknesses on the ends of the board?

Yes, it seems to start out thinner than the desired thickness

[drobertson]

Log species and opening cut can surely cause this outcome.  And then, some logs will eat your lunch before noon, even if you do it (right).
This is a challenge many times, whether folks admit it or not.  As forum member ladylake has mentioned numerous times, trim cuts are very necessary for true cut green lumber, and a board or two at times may be lost,   It really depends on the where the lumber is going, and what the target is, and how it will be handled after the green chain.  Sounds simple, but when there is a custom saw job, with a desired material list, and barely enough logs to get the job done,, it make things very challenging.

[Quebecnewf]

A lot of logs have tension. We use a local term called ( box) . Some logs will move so much as you saw them that it becomes very difficult to make any thing other than slabs out of them.

If I have a log that starts to move and twist on me as I saw I try to make a 6x6 or something like that out of it.


That being said there are logs that you will saw that were better of left in the woods on the stump.

It's that saying " you can,t make a silk purse out of a sow,s ear". There are some species of trees that are more prone to stress than others but it is present in all trees in one form or another.
Quebecnewf

[Glenn1]

In this particular case, we were cutting walnut logs.

After you make a couple of cuts, is it something that you can see on the log before the next board is cut?

[Percy]

There is alot of info  here/on the net, about log tension. Some logs are so wound up, there is more tension in them than the head table of a Jewish/Catholic wedding. :D

[drobertson]

Typing with a previous post so, not sure what they said, but yes, Kinda, when I saw I pick a pattern for the desired job and cut list.
So, if the second cut does not react like I thought it would then the third and forth face cut will determine my pattern.  It can burn you, but, mainly the stress in the log will reveal a pile in the first few cuts.  Some logs go both ways, I've found cedar to do this many times,  and it is brutal.  The best we can do, is what we do, most folks sawing want good lumber, while others want bd/ft'd.  So I cant' tell you without being there,  but it can be brutal, and time is money, for me it is more getting the most useable lumber, and it is what it is.  I will say, most is quite useable with minimal waste, just a lot of trims at times.  Useable is the target in my world, because most of it goes up green.  Flooring and kiln dried stock has tolerances that if held in the range can be used.

[Magicman]

Me too David, so here goes....

Carefully watch the cant ends as you saw.  If the cant ends lift up from the sawmill bed, then flip the cant 180° and resume sawing.  Many times only one or two boards can be removed before it has to be flipped again.

I very seldom saw much deeper than 1/3 of the way through a cant without turning, even when no stress is seen.

[Glenn1]

I guess that I am still a little confused but I do appreciate the posts.  If the initial part of the cut is shallower than intended, does that mean that the back of the cant has raised up?  My concern is getting boards closer to 1".  I skip plane every board so that extra 1/8" is important.

When the initial part of a cut is shallower, should the sawyer intentionally make the next cut 5/4 to compensate?

[WDH]

I, like MM, turn often just on principle.  I also look at every board coming off the mill, and when tension (growth stress) rears its ugly head, I turn the log 180 degrees to try and balance the tension.  If you ignore it, and saw to the bed, the last board can be hourglass shaped, thick on the ends and thin in the middle. 

If you are sawing a board, and as you move through the cut, if the end of the board where the blade first entered begins to rise off the cant or bend to one side, there is tension in the cant.  The only way to release it evenly is to take sequential boards off opposite faces, and even that will not work everytime.  I see it bad in yellow poplar sometimes, and bad in hackberry sometimes.  Pecan and hickory can drive you crazy as they are the  when it comes to growth stress.

[Magicman]

There is much more to sawing than making sawdust.  :P

[dgdrls]

I guess that I am still a little confused but I do appreciate the posts.  If the initial part of the cut is shallower than intended, does that mean that the back of the cant has raised up?  My concern is getting boards closer to 1".  I skip plane every board so that extra 1/8" is important.

When the initial part of a cut is shallower, should the sawyer intentionally make the next cut 5/4 to compensate?

Quick response No,

see if this helps

https://forestryforum.com/board/index.php?topic=73605.0

Dan

[Quebecnewf]

With the fir and spruce I saw I can usually look at the log and tell if there is stress in it. That being said I can also think a log is good and when I saw it goes wild.

Even if you dog the log down hard and saw nice same thickness boards out if it these boards will sprung and warp so much that they are not much good for anything.
You hold the cant straight and the boards cup and twist . You let the cant move and you get boards that are 1" thick on each end but 1 1/2" thick in the middle.

Bottom line if the log has enough stress you can turn and saw whatever way you want but you will get nothing but junk out of it
Quebecnewf

[drobertson]

for a walnut to go south sounds weird, unless the opening face is way wrong, or the log(s) have massive sweep, and the opening face is wrong and not corrected.  Multiple flips are most definitely required.  It take time and patience, and a goal for quality not quantity.  Some folks go for it as the log lays, this often time does not fair well for straight lumber or flat lumber.  I'd say, if you are not paying for them, live and learn, and make the pay.  Just Saying when you buy them it is a different ball game, but the fact remains quality rules in walnut and care should(must) be taken, not being there, well there is no way of telling what is going on. 

[Magicman]

When I am sawing a customer's logs, it is my job to read the logs and to get the most usable lumber from each log.  The customer is there and it would be highly unlikely that he would ever discard a log. 

[terrifictimbersllc]

I guess that I am still a little confused but I do appreciate the posts.  If the initial part of the cut is shallower than intended, does that mean that the back of the cant has raised up?  My concern is getting boards closer to 1".  I skip plane every board so that extra 1/8" is important.

When the initial part of a cut is shallower, should the sawyer intentionally make the next cut 5/4 to compensate?

The basic phenomenon you are asking about: the cant under the board being sawn, as that board is being sawn,  will either lift up in the middle, or on one or both ends.  If you go on to cut the next board without changing anything, that next board will be in the first case, over desired thickness in the middle, or in the second case thicker at one or both ends.

[longtime lurker]

The ability to consistantly saw straight, dimensionally accurate timber is what seperates professional sawmillers from wannabes. If you can do it then you produce saleable timber - if you can't then you starve. And understanding log tension and how to deal with it is a key part of that production process.

The best way I can explain log tension is for you to visualise a tree standing. There are two forces acting on that tree - compression, which is the weight of the tree bearing down to the ground and is generally located on the heart section; and tension, which is the effect of gravity trying to drag the tree apart and get it onto the ground.

When a tree falls compression forces tend to dissipate rapidly. It is a rare and joyous thing to see compression pulling boards into the heart, because in my experience it only occurs to a noticeable level in really big logs, or occasionally if the logs are very fresh.
Tension however tends to hang around for a good long while, and the effect of tension gets worse for a considerable period after falling due to the establishment of a moisture gradient between the sapwood and heartwood. Sapwood cells are purpose built to transport water and they drain it fast. The subsequent cell shrinkage increases the tension forces.

The best way to picture this is to imagine a straight line from side to side of the tree intersecting the heart as a rope. Tension forces are like men on that rope having a tug of war. Before you start sawing the forces are in stasis... put a cut into one side of the log and its like taking one man off one side of that imaginary rope. The tree bends because of the reduction in tension. Take another man off and it bends further.

As previously mentioned one of the best ways to deal with tension is to roll the log. By rolling the log during cutting and taking slices off opposite sides what we're doing is the same as taking one man off one end of the rope, and then another man off the other end of the rope. In this way we can keep the forces balanced out. Failure to do this results in what you're seeing... boards that are nominally 1" but are actually 7/8"on the ends and 1 1/8"in the middle. This is because the reduction in tension causes the log to "hump up" in the middle during sawing. Another phrase you'll hear is "pull to the sap", because in terms of what the boards (rather then the logs) are doing they tend to bend the opposite way - towards the sap. This results in bow in backsawn lumber, or spring in quarter sawn. On a real bad day you even get both.

So heres some pictures of a log being sawn with a Lucas mill, which might help you to see whats happening. I took them for a mate a while back who was struggling with the same issue.

Log is a Rose Gum Eucalyptus grandis. As a timber its what I'd call moderately springy ( you really dont ever want to deal with what I call springy), and this particular billet is a top log... ie up in the branches. In this case per my records we'd taken a 40 foot butt log and another 40'center log under this... so this piece was between the 80 and 100 foot height, swaying in the breeeze on a windy exposed slope. Top logs like that are always going to be interesting...



 

We're running a through and through pattern on him... x2 and x3 backsawn framing, and 1" quartersawn flooring stock out the middle section.



 

Cut in about a third and ready to turn.



 

Cut in to the point where its carrying enough quartergrain to shift to flooring stock. You can see the crack opening up in the end of the log which is the result of tension. By cutting from the top and bottom of the log we've now put it in a position where the tension left and right far exceeds the up and down tension - hence the log is trying to tear itself apart.



 

Cut down the middle. This cut allows the tension left and right to release.  Cutting big bits and resawing is the other way to deal with tension--- let it spring then get what you can out of it. This is why eucalypt mills run on 30% recovery!!! The gap between the two halves is solely how far its pulled itself apart during the splitting cut to drop tension ( as I said you dont want to ever deal with what I call a springy log). From the point of view of running a bandmill, that bend is just how much itd be pulling itself around in your horizontal cuts.



 

Tension dropped we can now saw straight accurately dimension boards from the flitches.



 

19'quarter sawn select grade flooring stock coming out of a top log... straight and accurate. You only ever get out of a moulder boards as straight as what goes in so being able to get this right matters. Not bad, even if I do say so myself.

[4x4American]

Here's an example of trim cutting. 
Sawyer is making a cut at 12" and notices that the cant has raised off the bed in the middle.  Sawyer flips cant 180° and makes a cut at 12" again, knocking off the thick ends, and watches as the blade goes over the middle of the cant, to see how much of a gap there is.  This will tell sawyer how thin the middle is, and help decide whether it is good enough to drop and make a board or to trim a little more.


At this past Boonville show, I was observing old Davey Crockett sawing a red pine log with that Baker 3665-D.  As he was sawing, he did flip the cant a few times to relieve stress.  The guy next to me asked me why he did that, so I told him about stress relief.  He told me that he's been a commercial sawyer for 27 years and has never seen anyone do that.  He said that guy (Baker's Sawyer) is wasting time, instead of taking 1m30sec to saw that log it took him 3 minutes. 


Which proves Drobertson's point, about how some want quality, and some want quantity.  That's the beauty of a portable sawmill.

[Ron Wenrich]

Depends on the mill.  The big industrial carriages will hold many logs so that the spring doesn't come up in a log.  Think heavy hydraulics.  The spring will come up when you turn, if you cut too hard into one side.  But, the lumber will be consistent.  I had a minimum of 2 dogs on every log.  When I got to where there was overhang of a couple of feet, the spring in the log was evident. 

I was at one mill that had a large tong dog in the middle of the carriage and the sawyer used that instead of the dogs on the headblocks.  His lumber was junk, as there was no way of holding the log to prevent much of the spring in the log.  The boards were thin on the ends as the center held the stress with the tong dog, and the ends curled up.  The tong dog should have been used to hold big logs, in addition to the other dogs.

My finding on turning to relieve stress is that when you turn back, the log has had movement and needs a shim cut to square up the face.  Without the shim cut, you will still have a poorly produced board. 

I haven't been around the smaller band mills to see how you dog your logs.  So, your results may vary.

[drobertson]

My mill just has the one clamp some where near center of the log,  the way I deal with it is just watch the board as I am going through the cut, looking for movement, there is either none, which is nice, a slight raise as you cut, and the side ways action, this is where I like to set my second and forth face cut be, and then there is the raise in the middle at times, this can cause two bad boards at times, depending on the severity of the stress.  I know folks have seen these jump off the mill at some point,   I think this is what our outback neighbors call super springy ;D  watching the cant, watching the board, looking for rises and heaves is just part of it, knowing when to flip becomes second nature at some point, this is the main reason I have always stressed to new sawyers to simply take notes, and for sure mental notes.  for custom sawing when a customer has limited stock that is bumping the material cut list, speed just goes out the window many times.  flipping and trimming flitches to reach the target goal just takes a little more time than through and through, but most times yields a little better finish product to build with.

[Glenn1]

This is all extremely helpful and I am getting a better understanding.

Do homeowner sawmills such as the LT series offer three dogs to help keep the log still?

I have seen that sawmill companies offer lasers as an option.  Is this used to help see where the log is raising up or is it easy to see just by eyeing it?

[longtime lurker]

Theres an unofficial policy here in euc mills: "we'll give you your choice of either bow or spring free with every board. If you want neither let us know and we'll charge you extra for it"

Its part of the reality of trying to balance a reasonable recovery rate against useable timber against price of production: to shim cut (here we'd call it a face cut) enough to remove both the bow and the spring drops recovery rates back that far that the cost of production significantly increases. I don't think anyone in the industry likes it but... we do what we can with what we've got. Generally speaking from a construction viewpoint bow is preferred over spring because you can pull the lumber back with a clamp. And you've got to remember that houses are built differently here, which is probably a design response to the reality of timber that wants to move like it does, in the lengths the industry wants to use which are long to very long compared with US construction norms.

On a carriage mill a good operator here learns to advance and recede his headblocks against the linebar, in addition to frequent turning. This does exactly that... it allows us to cut a board thats pretty consistant in thickness without loosing half the log to face cutting, but at the expense of having some bow in the resultant boards. The other commonly used technique here is to break the log into large flitches, drop the tension out that way, and then recover what we can from the sprung cants. This results in far straighter lumber but recovery goes down unless youve got large logs.

Attached is  a page from a FWPA paper "Processing methods for production of solid wood products from plantation-grown Eucalyptus species of importance to Australia" that illustrates the headblock technique that we tend to use to help us deal with this.

[Delawhere Jack]

Lurker, Thanks for that description in your reply (#17). That is a great way to conceptualize how tension acts on a log. I'm going to re-read that a couple times.

[Magicman]

Do homeowner sawmills such as the LT series offer three dogs to help keep the log still?

I do not see the need for more clamps.  Orient the log properly for the first face opening and watch the cant carefully as you open the other faces and remove boards.  I would rather see the tension released and saw accordingly.