Thread: Log Tension by Timboz

Mine too,

Best all round timber in the world I reckon. I've heard the mills in NSW are paying $200 a cube at stump (and not for big trees either). Thanks for the info.

Regards, Timboz

Given the erroneous concept the weight of the log will counteract the heave exhibited once taking the sapwood off the top of a log when swingmilling. This would mean a longer log (heavier) would have less heave - when in actual fact it shows more....
Bolsters need to be in the last 200mm of the log ends to acheive maximum recovery in minimum time

You are correct Sigidi. A longer log with more weight will deflect up more but not because the log weight is not having an effect - it has to. Any beam sags because of it's weight. The answer lies in geometry. Imagine an extremely long high stress gradient log - say 40m - supported at both ends. You skim of the top and it bows up. Because it is so long you can imagine that it could concievabley take on the shape of a semi-circle (ridiculous -but for the sake of the argument). Now the the 'height'
of the bow is equal to the radius of the semi-circle. If you look at the second diagram a curved board from A to 1 is half the length of a board from B to 2 but the 'depth' of the board B to 2 is much greater than A to 1. It is this aspect of geometry that explains why a sprung board cut in two reduces the spring to much less than half of the original amount (see sprung board png). This geometrical effect simply increases the bow up at a greater rate than the weight of the log bows itself down. Try measuring the bow up of the top of an opened log then move the log supports closer together - the bow will increase. What changed? There is less unsupported weight so the log isn't pulling itself down as much. Timbozboard bow.png

You said placement of bearers was so weight of log would keep it down, that is incorrect as a heavier log heaves more

Did you read the post about geometry? I acknowledged that a longer (and commensurately heavier) log will bow up more and went on to explain why. We know the same things - a skimmed log bows up because of an imbalance in tension top to bottom. I'm saying that placing the supports at the end of the log allows the log to counteract that to some degree. Do you think that I have said that the weight of the log eliminates the upward bow? I haven't said that. You can't be suggesting that the weight of the log does not cause itself to sag to some degree can you?Timboz

I was afraid this would happen. The debate over spring and how to best compensate has been going on for years.
Unfortunately, there is no hard and fast rule to totaly eliminate the problem. Each log comes with its own inbuilt ability to do the unexpected. The important thing is the miller recognises what is happening and has the knowledge and ability to make the necessary adjustments. Each type of mill requires different action often to overcome the same problem. For example a bandsaw mill would turn the log. A gang saw can do bugger all. A swing mill can leave on the side to form an angle ridge of resistance and the list goes on. Log heave is always present and even when weight plays some small part in correction, the board produced will still respond to its original cell alignment once released from the parent log. Oversized cutting and resawing is usually the only sure method to gain straight timber. Personaly, I prefer to keep bearers as close to the log ends as possible and retrim log before each board to overcome heave. It aint perfect. I guess thats why the debate has been going on for years.

I absolutely agree with you Rustynail. I'm not sure why this is going south - we all know how spring affects logs and scantling as it comes of the saw but it seems to have evolved into some sort of circular argument specific to bearer placement when swing milling. For the sake of good forum tenor Sigidi, I think we should respectfully agree to disagree and let the valuable information supplied from all sides be digested by the novices out there. Timboz

P.S Sigidi, would you like to share your resawing methodology with the novices out there - I'm about typed out!

I don't resaw - I cut timber right the first time. If the log won't let me, I don't waste log chasing it...

And fortunately we don't know the same things, I know much less than yourself

Originally Posted by rustynail

I was afraid this would happen. The debate over spring and how to best compensate has been going on for years.
Unfortunately, there is no hard and fast rule to totaly eliminate the problem. Each log comes with its own inbuilt ability to do the unexpected. The important thing is the miller recognises what is happening and has the knowledge and ability to make the necessary adjustments. Each type of mill requires different action often to overcome the same problem. For example a bandsaw mill would turn the log. A gang saw can do bugger all. A swing mill can leave on the side to form an angle ridge of resistance and the list goes on. Log heave is always present and even when weight plays some small part in correction, the board produced will still respond to its original cell alignment once released from the parent log. Oversized cutting and resawing is usually the only sure method to gain straight timber. Personaly, I prefer to keep bearers as close to the log ends as possible and retrim log before each board to overcome heave. It aint perfect. I guess thats why the debate has been going on for years.

+1


One thing I learnt a long time ago is that if my timber is straight I can sell it. It doesn't have to be clean... gallery, kino, holes... just means I loose the premium I get for appearance grade. But if it's straight even with those things... it goes out the door no worries at all. Little bit of bow isn't an issue but spring is the quickest way to make good clean timber unsaleable, or at least miss the price premium it should garner. Most builders will go past appearance grade timber with spring to take gunbarrels with grade limiting characteristics in my experience

Am I the only one who's ever set a deck under a swingmill for the specific purpose of rolling logs I wonder? For sitting in one place for extended periods it's well worth the time. Y'know that last board that thins out in the middle when the log sags, even if you've left a bone coming down to it... add them up and it pays for itself in a matter of months, let alone the ability to reduce spring.

If I was in a different place - totally mobile milling and never in the same place for more then a week at a time - I'd be putting a swingmill on a trailer package with hydraulic features the same as a high end portable band. And backing it up with one of these:

http://www.cookssaw.com/index.php/ed...e-board-edgers.

Oversawing might cost a few points of recovery, but only on those that were going to spring regardless. In dollar terms it's still miles in front.

At home I have a deck set up, so that logs can be turned. On site, its usually get in and get out as quick as possible and often without mechanical asistance. Hence two bearers and into it. For some time I have been working towards a mobile Lucas and had just reconfigured my trailer to suit. Some Balfarsted has knocked the bloody thing off!!!!! Not happy Jan!

Originally Posted by Sigidi

I don't resaw - I cut timber right the first time. If the log won't let me, I don't waste log chasing it...

And fortunately we don't know the same things, I know much less than yourself

So what do you do with a difficult log?

Originally Posted by rustynail

So what do you do with a difficult log?

Rusty, like you'd know, some logs are not good quality and are only good enough to cut landscape grade from - the difficult ones. Hows that saying go "ya can't make a silk purse from a sows ear"

Originally Posted by Sigidi

Rusty, like you'd know, some logs are not good quality and are only good enough to cut landscape grade from - the difficult ones. Hows that saying go "ya can't make a silk purse from a sows ear"

Many years ago, I worked for a chap who was probably one of the best sawyers going. He was responsible for setting up most of the mills in PNG and also selection and milling of rifle stocks for Lithgow Small Arms. When I got to know him, he was marketing manager for an international whoesale timber company. I had just come off the tools and was moving into marketing and he was to be my mentor for the next couple of years. I couldnt believe my luck. I spent a lot of time knocking around the mills in most states sourcing timber for export. One of the things he said to me was, " Have a good look at their log dump. See what they are leaving uncut, then you'll get a bit of an idea how competant the sawyer is."
I guess if you have contracts for landscape stuff fair enough. But straight stick is easier to sell and a lot more profitable. It's worth learning the tricks, so that nobody would ever know your purse was once a sows ear.

Originally Posted by rustynail

Many years ago, I worked for a chap who was probably one of the best sawyers going. He was responsible for setting up most of the mills in PNG and also selection and milling of rifle stocks for Lithgow Small Arms. When I got to know him, he was marketing manager for an international whoesale timber company. I had just come off the tools and was moving into marketing and he was to be my mentor for the next couple of years. I couldnt believe my luck. I spent a lot of time knocking around the mills in most states sourcing timber for export. One of the things he said to me was, " Have a good look at their log dump. See what they are leaving uncut, then you'll get a bit of an idea how competant the sawyer is."
I guess if you have contracts for landscape stuff fair enough. But straight stick is easier to sell and a lot more profitable. It's worth learning the tricks, so that nobody would ever know your purse was once a sows ear.

I've thought on this quite a bit in the last few days. I agree with you in principal, and I've got to say I've been known to be pretty scathing about stuff people won't cut or whats in the waste pile myself.
But... there had to be a but....

I don't like how much I waste myself. I've worked everything from portable mills to a quad canter in a 300 MMBF per annum sawline, so I can make some value judgements about my own operation and others concerning objective performance evaluations.

The reality of running any small operation in the modern industry is that you're going to waste timber. Further I'd say that if you're not you're making a mistake. The why of that is simple.... it has to cost you less to recover the timber then you can sell it for. And the smaller you get, and the less automated your equipment is, the more it costs to recover on a per unit basis. We are forced to compete against product from countries with a far lower cost structure then ours, and when you reach the point of being a basicly manual operation that has to pay wages at $30 per hour against a similar mill paying wages at 30c an hour... you're up against it.
I make a living cutting stuff my grandfather would have left to rot as worthless... toothpicks and culverts he'd have called them. And he didn't have compo, leave loadings, super, EPA licensing fees, noise restrictions, carbon taxes, provisional taxes, fuel at 1.57 a litre, etc etc. He was as efficient as any of his contemporaries, but he'd go broke on wages alone today.
When you get back to basicly manual gear like a Lucas Mill... you can't afford the time it can cost to turn a sows ear into a silk purse. The only number that is more important then the recovery percentage in a modern small mill is recovery/man day. More better to get 80% of the theoretical yield on 20 cube a day then 100% of the theoretical yield on 10 cube... but you'll find that the cost to run those two very different types of operation given the same level of equipment will be very much the same. Because the wages and electricity bill will be similar, the rates, the mortgage and leasing payments... the only variable is log cost.

Sometimes it's more profitable to roast the pig and go buy another silk purse.