Struggling with bowing boards/cants

[jovol]

Hey all, I've got about 500 hrs on my LT-50, and for the past few months (can't remember when exactly) I've been struggling with bowed boards. I've called WM a couple times when through this period, and the two pieces of advice I've received are: it's normal, try flipping 180 deg every few cuts to balance the stress in the log; and also it could be a setup issue/improperly set up outriggers. At this point, I don't know. I'm at my wits end.

What I definitely notice as I'm cutting is the first couple feet of the board getting cut off will "peel up", then start to settle down as I reach the middle of the log. Sighting down the cant, it doesn't look like it's cutting flat. I've checked my blade to bunk heights many times over this time period, and they've always been consistent. I check my blade guide angle regularly and it is generally flat +/- 1/16" over 12" length. Today I made a concerted effort to set up the sawmill with the 4 main bed outriggers taking all the weight (tires off the ground), and the front/rear outriggers just barely touching the ground, so as to confirm the main bed is well supported. Even still, I was having 4/4 poplar cut from an 8' long straight well-behaved log slightly "peel up". I've attached some pics of what I mean... let me know if you need more info to help me out.

Help!!

Thanks,
John

 

 

 

 

 

 

 

[GAB]

Jovol:
In 15 years, I have sawed ONE poplar log that did not do that.
Most had a lot more curl than what is in your picture.
I'm going to be interested in reading what the experience of others has been.
GAB 

[jovol]

Here's a couple more photos, this time white oak. I know I had a snowballs chance in hell at getting 4 square 6x6 splitting the cant like that, but I didn't expect that level of bow.

 

 

 

[Magicman]

It is very normal for (some) boards to react such as yours.  You are relieving the stress that is built up within the log.  Also the outer boards may contain both sap and heart wood and will curl toward the bark side.

Wood-Mizer gave you some good advice regarding flipping the cant 180° to balance the stress being relieved. 

Keep your tires firmly on the ground when you set the sawmill up. 

You described some boards raising up and then settling back down.  You will also have some that will bow up in the center and others that will move sideways as they are sawn.  Again, you are relieving stress that is contained within the log.

There is much that can and does happen during the log's life as a tree.  Maybe it grew next to an opening causing the limbs to grow on the open side and suppressed on the shady side.  Maybe it was on a hillside and spent it's entire life trying to stand upright.  Any of these plus much more can cause stress to be contained within a log that is released when it is sawn.

That last picture of the cant being split into 4 is exactly what you should have gotten. 

[YellowHammer]

What you are experiencing is the difference between a guy who just cuts wood and a SAWYER.  First, you have taken steps to properly align and fix you mill.  Good job.

Now you have noticed that wood behaves as if it's a real, organic thing, and you would like to understand and correct it.  Good job.  This is the point where many people who cut sliced bread on a sawmill stop being inquisitive and stop learning.  So don't.  These people drive much business my way, and I get lots of customers say they've never seen straight lumber come off a band mill.  Except mine.  

As others have said, this log stress is normal, and it must be predicted and corrected while sawing.  Depending on what species you are sawing, and what product you are sawing for will dictate the sawing patterns required for that particular log and how to either minimize the stress, or use it to your advantage.  

For example, if you saw 4/4 walnut with a high sapwood fraction on one face it will bow up like a carrot peel.  The same with cherry.  Not so much with basswood.  If you are sawing grade hardwood, you rotate the stress 90° so the board doesn't bow up, it curves or slips sideways instead.  This results in a flat board but must be edged later to take out the curve.  But it produces a flat board.  

Others sawing dimensional lumber prefer bow rather than curve so rotate 90°.

[Southside]

Definitely log stress in that Poplar and split cant.  Try flat sawing a Sycamore - it just might curl up and whack you in the back of the head. 

On a serious note, as was mentioned, you want and NEED the tires on the ground, they are actually an important part of the outriggers.  WM actually makes a special kit if you want to remove the axle, so reduce the down force and get them back on the ground.  Set your front and rear outriggers down solid by running the head to the opposite end and have the others just touching the ground when there is no log on the mill.  

I see hills behind you, so that might be a part of your issue, side hill trees with off set pith love to move.  Find a nice, clean 4 sides pine, or even a decent cedar, with a centered pith and see how your lumber does.  Basically eliminate the log issue and check the mill.  

[barbender]

Every picture you have shared shows stress in logs, that has nothing to do with the mill itself, IMO.

[longtime lurker]

There's a lot of "depends" about how I approach logs with tension - and mostly my logs are heavily tensioned - and what I'd consider acceptable in terms of boards not laying flat.

With framing timber I prefer bow over spring.. thing being that it's easy to pull bow out of an 8 X 2 with a clamp when building, but trying to pull it down when it's sprung is another matter. Bow is my normal in backsawn patterns, and my framing is almost always backsawn. I turn the log as required but I'm not OCD about it.

Bowed green lumber will mostly dry flat with careful stacking too.

Cabinet or joinery timbers I tend to prefer spring over bow. You can edge spring out, or take it off with a jointer once dry. The fact that 90% of cabinet and joinery work is in lengths under 3' long is also a factor... a wise tradesman will dock his rough sawn lumber to length before finishing and what looks horrid over 16' is pretty mild over any 4' section of it. Mostly I quarter saw those species so it springs rather than bows.

When I want dead straight - such as your 6x6 posts - I either overcut and straighten/edge them both ways. Or saw from the inside out ( break the log into flitches which bend under tension and then cut straight boards from my flitches.

YH is right: Good straight boards sell themselves. One of the things I struggle with ( my OCD / perfectionist tendencies) though is knowing when it's good enough and straight enough. "Fit for purpose" means knowing when to make gunbarrels and when good enough for a chicken coop will do. There's not much premium in gunbarrel straight lumber with the attendant costs of production when all the customer wants is to make a chicken coop.

[terrifictimbersllc]

Your learning to sight down the board or cant is also an important technique. Also sighting the sawmill rail for straightness of setup. 

The cut which you said did not appear flat might well be the bottom (cant) bowing, giving the impression your saw cuts in a curve which it probably does not-but it could if the mill setup is such that the rail has a curve in it because it's not jacked straight in the setup. 

[Magicman]

You will encounter cants that the ends will rise up as boards are removed thus producing boards that are thick on one or both ends and thin in the center.  Then you will encounter cants that will rise up in the center.  I had one to pop out of the clamp Tuesday. 

Carefully watch your cants as they are sawn and make cant adjustments such as flipping before you loose the entire cant.  Yes, cants can and will do bad things.

[moodnacreek]

Your learnen.

[jovol]

You described some boards raising up and then settling back down.  You will also have some that will bow up in the center and others that will move sideways as they are sawn.  

I've yet to see the middle peeling up during cuts, it's almost always the front of the log. I'll pay attention to it today, should be cutting some other stuff other than poplar. I've definitely see the sideways peel.

Definitely log stress in that Poplar and split cant.  Try flat sawing a Sycamore - it just might curl up and whack you in the back of the head.

On a serious note, as was mentioned, you want and NEED the tires on the ground, they are actually an important part of the outriggers. Set your front and rear outriggers down solid by running the head to the opposite end and have the others just touching the ground when there is no log on the mill.  

So how would you mill flat sycamore? Are you only expecting flat lumber from the heart?
Interesting you say set the front and rear outriggers solid. Fella from WM yesterday said to make the 4 middle outriggers solid, then have the front and rear outriggers just taking some weight (slightly moveable if you give then a good kick). Have you tried that? Mind you, I have the fine adjust outriggers... I never gave this so much thought when I had the LT40 with normal outriggers haha.

If you are sawing grade hardwood, 

Could one explain what "grade hardwood" means, and what the alternative would be labeled?

Bowed green lumber will mostly dry flat with careful stacking too.

Cabinet or joinery timbers I tend to prefer spring over bow. 

When I want dead straight - such as your 6x6 posts - I either overcut and straighten/edge them both ways. Or saw from the inside out ( break the log into flitches which bend under tension and then cut straight boards from my flitches.

If I have a cant today that's bowing, I'll try giving it a 90deg turn and see if it translates to curved boards.

One of the 6x6s I cut (not from that cant I split) I milled to 7x7 and trimmed it 1/2" a time to 6x6 and still couldn't get it true. Maybe the mill wasn't well set up that day as I was on soil. Especially challenging getting fl at 16 footers.


I'll do a fresh setup of the sawmill today. I'll plan on doing what the WM guy said yesterday, using the 4 middle outriggers to get everything set up level with tires still touching the ground, then button down the front/rear outriggers to just taking weight. I've got to say, I don't really see the point of keeping the tires on the ground except for giving better protection from a roll-over... keeping leaf springs in the picture sounds like an unpredictable variable when it comes to evenly supporting the bed frame.  say_what

[terrifictimbersllc]

You can have the outriggers firmly sit on the ground without them having lifted all the weight off the spring. The springs in that case aren't really part of the mill rail support. In other words when the head comes along with all its weight, if the jacks are firmly set the weight is on them, the head weight is not compressing the spring more as it passes by. The point of leaving some weight on the tires is so they act as outriggers to stabilize the meal against tipping over when the head is all the way up. Imagine not having any weight on the tires, the head all the way up, and a very heavy log rolls forcefully against the back stops. In that case there will be unnecessary movement until the tire is able to start taking up significant weight. If something happened to the Mills footing during such an event,Under the wrong conditions it could be very less than ideal.

I wouldn't know where someone is coming from if they say not to set outriggers one and four along the rail firmly. The rail should be rigid in a straight line as the head goes from one into the other, or you have a recipe for sawing a long arc Which will leave a rigid cant thick in the middle and thin at the ends.

The two out Riggers on the idle side are the ones maybe being referred to which are just adjusted so they are taking a little weight. But most here would agree that you adjust them either level or a little bit higher than the inside so that Logs will roll easily towards the stops.

All of my comments here are about Woodmizer cantilever set up not other mills.

[moodnacreek]

The most trouble is the jacket boards and the heart. After a slab a 1" board [jacket board] is taken. Next could be a thick cut like 2".Flip 180 and do it again, the 1/4 turn; slab, 1" and last turn ditto. Now you would cut 1 or 2 2", turn 180 and dog center hard and finish. Just an example of cutting a hardwood log. You get what you get even with a good sawing pattern. In general don't cut longer logs than you need [other than trim]. Clear butts may be the worst for sking up. Staggered/ opposed knots can equalize the tension and saw straight lumber where grade is not important. The more you saw the better. Then you will come to expect the good or bad results.

[Magicman]

I've yet to see the middle peeling up during cuts,

 
Here you go. 

[jovol]

Milled some straight, round white pine into 6x10s and it came out true. Live sawed some walnut, came out true. Did some more tulip poplar 4/4, some true some not. Some small chestnut oaks with uncentered piths, most bowed. 

Does definitely seem to be the logs not the mill. That being said, my fine adjust outriggers definitely do loosen (especially the three on the rear) through use (I'm guessing loading/turning logs, with the weight of the head all the way at the front, leaves the three rear outriggers susceptible to jostling/loosening). Anyone have good ways of mitigating that?

[YellowHammer]

If you are sawing grade hardwood,

Could one explain what "grade hardwood" means, and what the alternative would be labeled?

Grade hardwood is formally defined by the NHLA (National Hardwood Lumber Association) and generally defines classes and defects of hardwood lumber.  In a general sense, the grade hardwood can simply be labeled "high grade" hardwood, which is generally FAS and Better, and is usually parallel bark sawn.  However, it can also be defined as the stuff people use to to build furniture.

In reality, grade hardwood is the stuff where you cringe if you screw up a cut, it's wood you make sure you are sawing right, and trying to produce the best and flattest of boards.  In my mind, which is just my opinion, grade wood is the stuff you want to make an "A" grade and not an "F" when sawing whatever species. :D :D

[Magicman]

I have been using FAO's since 2012 and I have not seen them loosen.  At one time I thought so and bought a gadget from  Wood-Mizer that locked the top nut into place.  What it taught me was that the outriggers were settling in as the log's weight is applied and I needed make the rounds and snug them tight.  I always have to re-tighten the back 3 after moving the head to the front.

[terrifictimbersllc]

There are brackets that cost six dollars a piece that sit on top of the jack and hold the nut from turning. I have had Faos loosen. For example if the mill is set up on asphalt or on boards were you know it's not them sinking in. Part number 059577 $6.34.

[WDH]

You can stand some bow in construction and framing lumber.  a stud in a wall can have some bow but still be flat to the wall face.  You cannot stand bow in furniture grade or grade hardwood like when making a table top.  Customers do not like bowed tables.  You cannot stand side bend in framing or construction lumber.  The edge has to be straight to the wall face.  You can stand side bend in grade hardwood because you can use a machine to straighten the edges the make the curved board into a straight board.  So, how you deal with log stress and tension depends on what the lumber is destined to be used for. 

[longtime lurker]

Technically speaking there are two forces at work in every tree - tension and compression.

Tension is the result of longitudinal stress due to the maturation of cells in the growth area of the tree... the vascular cambium layer just under the sapwood.
Compression is the result of gravitational stress on the tree and generally speaking is at it's strongest in the centre of the tree (note that's centre of tree, not necessarily heart of tree)

These two forces are in stasis in the tree.... tension around the outside of the tree holding the tree up, gravity trying to bring it down. When we turn tree into log it plays with the balance of things, and a couple things happen:

Compression stresses tend to dissipate, particularly when logs lie on the ground for extended periods because gravity isn't trying to pull it down any more. Longtitudinal stresses (tension) tends to increase.... this is why log ends crack ok.

Dealing with it is best described like this: think about a log in cross section and you've got a point C in the middle which is the compression axis, and around it you have a whole heap of point T's which is the tension axis pulling against each other to hold the tree together.

We take a slice off a tree and we play with the balance BUT - it's mostly predictable with experience. Now you'll have to forgive me, I'm trying to explain a complex relationship with bad drawings and poor penmanship.... I normally do this verbally with a few crayon squiggles on the end of a log :D


 

 


Thing is I've spent a lot of my life cutting extremely tensioned eucalypt logs, and I am rarely suprised any more by log movement.... I can look at a log and assess it and know which way and how much its going to pull, and knowing that I can plan my saw cuts to counteract the effect of tension. Thats about experience, but also knowing visual cues around grain and deflection and off center hearts/ egg shaped logs ( see egg shaped logs on exposed windy sites a lot). And there are end of log tells... you got a nice smooth looking log end with a patch that looks rough, the rough area is under high tension ( cell retraction due to tension release - same thing they measure with a Cirad -foret tool or a Huggenburger tensotast tool.... the wood fibres have pulled away from the cut due to tension release.

One day I swear I'm going to write this out all neat with diagrams that make sense and show what happens at each step of the way... one day. :D :D :D But this is why I can cut straighter boards than the guys with 20 million dollars worth of gear.... my eyes tell me whats going to happen before it happens and I cut to stop it happening rather than trying to clean up a mess later with an edger.

[moodnacreek]

Yup, that's why you don't take a thick board first.

[Wattwood]

Longtime Lurker: Those are important and really helpful diagrams. Thanks for doing that. If you feel inspired, a couple sketches of logs with an off-center pith condition, due to lean or edge conditions, and what to expect with those tension/compression issues would be really helpful. 

Cheers:Rob

[Beavertooth]

I haven't read all the comments so this may have already been mentioned but one thing that definitely helps to keep the cant from bowing up in the middle or up on the ends as you are cutting is to leave all of your boards in place until you are completely through sawing the whole cant. This is extremely helpful if you are sawing 20' or 21' long lumber where you don't have bed support under your cant. This is mainly helpful as you get lower into the cant because of all the weight that is still on top as it keeps the cant from bowing up in the middle and down on the ends and also keeps it from bowing up on the ends. Works with 1 or more cants at a time. I just sawed 20' 2x6s all day a couple days ago and it  makes a world of difference. Takes you longer as you have to raise up above what you have already cut everytime but definitely worth it to keep straight lumber.