Thread: Log Tension by Timboz

Handling tension. Small logs have high stress gradients and it is easiest to counteract this by sawing patterns that balance tension. When whole, a log has its tension in balance. The centre is trying to elongate and the outside is trying to shorten. It is like pushing down all around the edge of a stiff rod while the centre is trying to push up - all is balanced. Now if you lay the log on your mill bed and skim off the top as a flitch or a series of boards you have unbalanced the log - there are more fibres (cells) at the bottom trying to shorten than the top so the log will bow up in the centre. More if the stress gradient is high. This is why you place bearers under the very ends of logs being swing sawn as it allows the weight of the log to bow down the middle of the length and counteract this upward bow. Imagine the face you have opened up is 200mm wide inside sap to inside sap and you tried for two different products for the next board/boards - 2 of 100x50 or 1 of 200x50. Obiously the 100x50's have one edge from the outside (positive tension - tries to shorten) and one edge from the inside (negative (or less positive) tension - tries to lengthen (or tries to shorten less than other side). Off the saw, each end will pull away from the centre of the log (ie ouside of log edge of 100x50 concave) and the middle of the length will bow up. If you cut the 200x50 then both edges will have the same amount of positive tension and the centre of the board face will have negative tension. It will behave like a mini whole log and not spring (bend edgewise). It will however still bow (ends up, middle down) as the top has more positive tension than the bottom and will therefore shorten. It is always easier to straighten bow rather than spring!

These principles can be used to explain movement off the saw for any cut in a log with exceptions for leaning tree logs which have different tension wood distribution (maybe another day).

Hope this helps with your milling, Timboz

I've been told, by my better half, I should address some of this in the morning, if I still feel the need to.....

Originally Posted by Sigidi

I've been told, by my better half, I should address some of this in the morning, if I still feel the need to.....

Please do.
Last part of the puzzle for me.
I know you can't aquire experience overnight, but a quick heads up would cut the learning time down a bit.

Originally Posted by timboz

Handling tension. Small logs have high stress gradients and it is easiest to counteract this by sawing patterns that balance tension. When whole, a log has its tension in balance. The centre is trying to elongate and the outside is trying to shorten. It is like pushing down all around the edge of a stiff rod while the centre is trying to push up - all is balanced. Now if you lay the log on your mill bed and skim off the top as a flitch or a series of boards you have unbalanced the log - there are more fibres (cells) at the bottom trying to shorten than the top so the log will bow up in the centre. More if the stress gradient is high. This is why you place bearers under the very ends of logs being swing sawn as it allows the weight of the log to bow down the middle of the length and counteract this upward bow. Imagine the face you have opened up is 200mm wide inside sap to inside sap and you tried for two different products for the next board/boards - 2 of 100x50 or 1 of 200x50. Obiously the 100x50's have one edge from the outside (positive tension - tries to shorten) and one edge from the inside (negative (or less positive) tension - tries to lengthen (or tries to shorten less than other side). Off the saw, each end will pull away from the centre of the log (ie ouside of log edge of 100x50 concave) and the middle of the length will bow up. If you cut the 200x50 then both edges will have the same amount of positive tension and the centre of the board face will have negative tension. It will behave like a mini whole log and not spring (bend edgewise). It will however still bow (ends up, middle down) as the top has more positive tension than the bottom and will therefore shorten. It is always easier to straighten bow rather than spring!

These principles can be used to explain movement off the saw for any cut in a log with exceptions for leaning tree logs which have different tension wood distribution (maybe another day).

Hope this helps with your milling, Timboz

There are many other factors that vary tension. Leaning is only one of them.

Originally Posted by rustynail

There are many other factors that vary tension. Leaning is only one of them.

Rustynail,

If you read the third line of the first post you will see that I have listed some other factors. The comment at the end of second post regarding leaning trees was not suggesting that lean is the only source of variation in tension but rather that the distribution of tension is different to vertical trees. Everyone can quote exceptions to a rule. The post was intended to help novice millers demystify tension not to investigate every possible variant of it. Timboz

Originally Posted by timboz

Rustynail,

If you read the third line of the first post you will see that I have listed some other factors. The comment at the end of second post regarding leaning trees was not suggesting that lean is the only source of variation in tension but rather that the distribution of tension is different to vertical trees. Everyone can quote exceptions to a rule. The post was intended to help novice millers demystify tension not to investigate every possible variant of it. Timboz

My post was not to criticize or challenge a rule. I draw your attention to the first paragraph of your first post.
I felt the brief list of possible causes was falling into the same mode.
To a miller, the visible evidence of possible tension is his major concern. This will determin the method and sizes he will select to mill the log.This in no way detracts from the useful information you have supplied. To me, as a miller, I understand the importance of identifying possible problems early. And, as your thread was intended to help millers come to terms with a common, often problematic, occurrence, would have been of considerable assistance. After all, the ability to reading a log is what makes the difference between a good and bad miller.

Originally Posted by rustynail

My post was not to criticize or challenge a rule. I draw your attention to the first paragraph of your first post.
I felt the brief list of possible causes was falling into the same mode.
To a miller, the visible evidence of possible tension is his major concern. This will determin the method and sizes he will select to mill the log.This in no way detracts from the useful information you have supplied. To me, as a miller, I understand the importance of identifying possible problems early. And, as your thread was intended to help millers come to terms with a common, often problematic, occurrence, would have been of considerable assistance. After all, the ability to reading a log is what makes the difference between a good and bad miller.

Thanks Rustynail,

Sorry for sounding aggrieved. The curse of the internet - you can't read emotion in a post.There is no substitute for reading a face and tone of voice in a discussion! Perhaps you might like to pick up the thread and do a piece on resawing sprung sticks? Is your background in traditional breakdown and benches, swing milling, or both?
Timboz

Interesting reading and information fellas....I knew there was some reason I keep my log size to around 6'......in diameter! They mill like butter most of the time.

Cheers

James

Hi Tassie, it certainly is nice milling those big logs that have low stress gradients - just turn up with a cut list and take whatever you want, in whichever direction without having to think about distortion. Timboz

Nice bit of info Timboz. Very helpfull. What I've always wanted to know aswell is what rustynail referred to about identifying "visible evidence" of tensions in any particular log and what that means to the milling of the log???. So I'm hoping you or he might add some more info as well.

I know little about the resons for logs to have tension but i do know how to read a log and how to cut the best timber possible from it, where to place a board and what logs to use for what order.

a spotted gum log 12" in diamiter with a lot of sap (fast growing) will cut dead streight timber, a spotted gum log 400-600 in diamiter will spring like crazy.
an ironbark 12" in diamiter will spring a terrible amount where a large iron bark will cut good straight boards.
a big tallowwood log will cut nice streight timber where a tallowood under 400 will play up like crazy.
a log with a pipe will generaly cut straight timber.
A log growing on teh side of a mountain will behave no diferently to a log growing on a ridgeline but the heart will be offcenter.

BOLSTER PLACEMENT

bolsters should be placed as close as possible to the end of logs so that when the log "humps" the top can be easily be skimmed off, if your bolsters are more then a foot from the ends and the log humps the ends will bow down meaning to skimm the top off you have to drop each end of the mill down 5-10mm to skim the ends.

bow.png

it has nothing to do with using the weight of the log to hold the log down if this was the casee you would be causing excessive bow in each board as they were relesed from the log.


Board Placement
this applies to Australian hardwood only.

the board will always spring away from the heart of teh tree, that is all you need to know to cut good timber.
say we want to cut 6x2 boards joists

Board 1 & 4 will spring vertically and will have to be shot streight with a planer once laid to make the floor flat.
Board 2 & 3 will bow sideways but not spring this can easily be pulled out when laying on teh bearers.
Board 5 & 6 will bow and spring and have to be pulled streight on the bearers as well as shot streight.

boaard placing.jpg

you should always for your centers to get good streight timber, and use the quaters for battons, palings and low grade or small section timber that can be straitened when used.

typical cutting pattern for cuttign 4x1 on a lucas mill

cutting pttern 4 flooring.jpg

the green boards will all stay streight and perfect the yellow boards will spring slightly outwards but should still be useable and the red boards will generaly have a large amount of spring and would generaly be cut into 2x1 or roof batton.

Weissyboy,

Thanks for your input to the thread. I agree (with one exception) to what you have said and it is totally inline with the theory I put forward. The cutting patterns you speak of are what I use myself to deal with tension. I do however disagree with what you said about bolster placement -not the ends bow down bit which is correct and I should have mentioned, but the bit about the weight of the log. If you look at your bottom diagram you can see (as you and I know) that the board will be lens shaped (thicker in the middle). The weight of the log will tend to tend counteract this so works alongside the effect of stopping the ends from dropping and giving you boards of uniform thickness. Perhaps you could share your experience with regards to the best way to handle resawing sprung boards. Timboz.

Weissboy,

I meant to ask. Have you had any experience with Grey box or Grey gum ? Can you comment on how well they saw/behave. I imagine the box could be a little bit hard on blades? Interesting what you had to say about spotted gum - not what I would have expected (I haven't cut spotty). Do you have a favourite timber to mill?Timboz

between each layer of boards you should skim back over the log and take off the excess that has humped up in the middle, that way the boards are the size they are supposed to be, a board that varys in thickness is useless. the weight of the log is not enough to counterract the hump, it may lessen it ever so slightly, but teh log will still hump. puting the bolsters at the neds just means that you dont have to drop the mill to the level of the ends before you can skimm.

grey box cuts well when green but is hard as the hobs of hell when dry. as with all box logs it has a lot of tension and you have to be thinking and watching to get good boards.

grey gum depending on type, usually has a lot of defect and gum vein, most are hollow and grade poorly, its harder to cut than bluegum, probly about teh same as ironbark. normaly cuts fairly strait timber tho.