140” bar advice on bar sag

[Booo]

Hi,

We are a company in the Netherlands, focussed on milling local, big logs.
We have most of the logs up till 200 cm wide cut at the sawmill with a bandsaw, but for the bigger ones we have a XXL chainsawmill.
It is a 140”/350 cm bar made by Cannon powered by two 880’s.

Last week we milled a big Oak which measured 3 metres at the crotch and 6,5 in length.
The cuts were nice but every cut we had the bar sagging at the end. Not really a lot, it was like 1 cm max at a cut of 200+ but still we would like to tune it better.

It would be great if you guys have some ideas on how to get that last bit out.

Many thanks in advance, I dont post a lot/ever but already found some great info here!

Grtz from Holland

@xxlboomstamtafels

 

[Rob Stafari]

I think I have read somewhere about the bar being clamped with a slight upward curve so it sags into the straight position. Really just commenting to follow along and see what the experts might have to say.

 

[rarefish383]

BobL had a rare earth magnet set up at the back of the bar, in the center, to keep it straight until it was in the wood. Once it was in the cut it would stay straight.

 

[M.R.]

Hopefully BobL will chime in & perhaps provide
a link on this.
Here’s something in theory only as a 5’ bar has covered
my needs / uses so far. A holder to screw into the end of the log
holding a stick of wax on the leading edge underneath of the bar
with a star knob to adjust it upward to take the belly into a true plane.... the trailing side links then the cutters ought? to keep the
Bar in line as it enters the cut eating away the wax without deflection.
Photo to show the canning wax, a bearing was locking up
on the cutter head on this old planner & ate the shaft into a cam lobe w/ lots of vibe ration.

Edit ....reversed a sentence the wax across the face of the bar, then the back side of the chain going through the wax, clear as mud, flipping tinnitus!

 

[Booo]

Hi and thanks for the replies!

The problem is not at the start of the cut, getting it perfectly straight into the log is not an issue. The challenge is in keeping it straight through the rest of the log.
Especially on long cuts the bar will sag towards the end of the cut. I have been thinking about tilting the bar slightly to get it sawing upwards a fraction but I fear it will cause more problems than solve...

 

[Booo]

I think I have read somewhere about the bar being clamped with a slight upward curve so it sags into the straight position. Really just commenting to follow along and see what the experts might have to say.

Thanks for the reply Rob!
That might work a little bit with small bars like 60” but this bar is too long and as it is very bellied most of the weight is in the middle of the bar.

 

[KiwiBro]

I've always wondered if shimming the underside of the bar to match the kerf thickness, mid-span, would have any benefit.

 

[Booo]

I've always wondered if shimming the underside of the bar to match the kerf thickness, mid-span, would have any benefit.

Thanks for that idea!
My thoughts exactly! To avoid getting any room for the bar to tilt or drop that could be helpful. The reason I didn’t try it yet, is that if that would work, the angle you enter the log has to be 100% perfectly in line with the guides. Also sawdust could become an issue maybe.?

 

[BobL]

I've not been able to get shimming to work. When I worked out the forces needed by the clamps to bend the bar upwards they turned out to be of the order of many tons which the clamps simply cannot generate.

Angling the bar will just create more friction.

Besides none of this will fix the problem.

The reason its happening towards the end of a wide cut is because the chain is getting blunt and so it gets hotter and this heat is transferred to the bar making it hotter.

Steel has a coefficient of thermal expansion of 13x10^-6 /ºC so a 3m bar with a starting temperature of 15ºC heated to say 50ºC will expand by 3000 x 35 x 13x10^-6 = 1.4 mm (and if the bar reaches 85ºC the bar will have elongated by 2mm).

Elongation of a 3m bar by 1.4 mm sounds like NOTHING, BUT but if you do the math, Pythagoras says the bar will have drooped significantly in the middle.

If we consider just half the bar, a temp increase of 35ºC will elongate half the bar (1500mm) by only about 0.7mm, now treat the original half length of the bar (1500mm) as the adjacent side of a triangle and the heated half length of the bar (1500.7mm) as the hypotenuse of the triangle, then the opposite side (the droop) will turn out to be 46 mm or nearly 2". You cannot force this out of the bar by any mechanical mechanism. Anything you do will simply buckle the mill under these forces.

So try keeping the bar cool.
Something like this might work. NB this was not done to get around bar elongation but for lube purposes.
Back in 2007 I was cutting some really hard wood and I posted pics of a water cooling/lube system for milling but all the photos are gone, fortunately I have been able to find a few.
What I did was added a "click fit" garden hose fitting to the spare aux oiler hole (on the side before the chain goes around the bar) and attached a running garden hose
This kept the chain and bar cool but unfortunately but also washed off some of the oil added to the chain back at the drive sprocket.
The reduced wear and tear from reduced temps was negated by the wear and tear due to oil losses so I only did this for a few weeks, besides it did use a lot of water and where we live water is not cheap.
However for industrial purposes and where water is cheap it would definitely be worth a try.

 

[noodlewalker]

I've not been able to get shimming to work. When I worked out the forces needed by the clamps to bend the bar upwards they turned out to be of the order of many tons which the clamps simply cannot generate.

Angling the bar will just create more friction.

Besides none of this will fix the problem.

The reason its happening towards the end of a wide cut is because the chain is getting blunt and so it gets hotter and this heat is transferred to the bar making it hotter.

Steel has a coefficient of thermal expansion of 13x10^-6 /ºC so a 3m bar with a starting temperature of 15ºC heated to say 50ºC will expand by 3000 x 35 x 13x10^-6 = 1.4 mm (and if the bar reaches 85ºC the bar will have elongated by 2mm).

Elongation of a 3m bar by 1.4 mm sounds like NOTHING, BUT but if you do the math, Pythagoras says the bar will have drooped significantly in the middle.

If we consider just half the bar, a temp increase of 35ºC will elongate half the bar (1500mm) by only about 0.7mm, now treat the original half length of the bar (1500mm) as the adjacent side of a triangle and the heated half length of the bar (1500.7mm) as the hypotenuse of the triangle, then the opposite side (the droop) will turn out to be 46 mm or nearly 2". You cannot force this out of the bar by any mechanical mechanism. Anything you do will simply buckle the mill under these forces.

So try keeping the bar cool.
Something like this might work. NB this was not done to get around bar elongation but for lube purposes.
Back in 2007 I was cutting some really hard wood and I posted pics of a water cooling/lube system for milling but all the photos are gone, fortunately I have been able to find a few.
What I did was added a "click fit" garden hose fitting to the spare aux oiler hole (on the side before the chain goes around the bar) and attached a running garden hose
This kept the chain and bar cool but unfortunately but also washed off some of the oil added to the chain back at the drive sprocket.
The reduced wear and tear from reduced temps was negated by the wear and tear due to oil losses so I only did this for a few weeks, besides it did use a lot of water and where we live water is not cheap.
However for industrial purposes and where water is cheap it would definitely be worth a try.
View attachment 746626

And... Mic drop

 

[WidowMaker_1]

On the lucus mill slabbers we use shims at both ends of the bar/ at the steel frame post bar to frame fasteners to flatten out any sag, if using a chainsaw mill frame the frame needs to be rigid enough between upright posts for shimming to work.

 

[WidowMaker_1]

Shim at top of bar or bottom or both depending on your needs, we usually just place a thin shim both sides on top of bar on the outside of post fasteners

 

[KiwiBro]

Regarding expansion, why does the bar have to be locked in between two fixed points? Why can't it be mounted such that it is free to move horizontally but ultimately the mounting uses something like valve compression springs to maintain chain fit as the bar expanded and contracts. Bar mount could be fixed but bar nose mounting could be sprung, for example. Or the other way around. Less deflection loads if the bar is not between two fixed points.

 

[BobL]

Lucas mill is a lot more rigid than a standard CSM, also some i've seen milling hard Aussie timbers were using g after cooling

RE: why does the bar have to be locked in between two fixed points?
Like this. The hexagonal bolt on the bar end is fixed underneath but it could just as easily be on a sliding slot.
This is from my "latest project" in my sig - its currently in pieces in the ceiling of my shop - never been used!

 

[WidowMaker_1]

Standard alloy say csm have a lot of flex in the uprights. I use a rigid all steel csm with zero flex between uprights, yes it's a bit heavier but the weight in the rig also produces a better finish, there's basically no vibs and no moving about on the log or rails, the added weight also keeps the rig self feeding on a sloped/lifted log.. Hey I'm a farmer we like our gear strong and Im a gun stick welder

 

[kz1000]

It seems simple, set up an open center log support and mill vertically while using wedges. You can use dog style locks on the two sides for holding the sections.
At least that would take the weight factor of the bar out of the picture.

 

[Booo]

Thanks for all the interesting ideas and different views!

As for shinming, that really doesn’t work with this bar. The bar itself weighs about 50 kg and has a big belly of over 30 cm. There is no way shimming the bar would bring any positive result.

Thanks for the elaborate explanation on deformation/expansion because of the bar heating up Bob! That is something I never thought about and I admire the thoroughness you obviously put into getting to the bottom of it.
However, I don’t agree that it’s causing the bar to sag towards the end, not in a significant amount that is. There is enough room for the bar to expand to the sides where the bar is mounted to the uprights. Also if the expansion of the bar would make the bar want to sag about 4 cm it needs not bother, as gravity already makes the bar want to sag about that much.

Which brings me to the usual suspect for the problem: Gravity.
If the bar comes out of the log at the end of the cut, it doesn’t jump up or down. (When it’s a sharp chain, otherwise it will!) It just comes out and the curve in the cut is just the natural curve of the bar due to it’s own weight, not being supported by anything but the guides at the side of the cut. The wider the cut, the bigger the sag.

I would like to adjust something to the bar and/or the chain to prevent it from dropping as it cuts along the log.
I think the idea of shimming the bar to match the kerf thickness would be worth the try. Could be nothing, but worth a try anyway. Anybody ever tried that?

Vertical sawing is an interesting idea as well but it does create a whole lot of new, practical problems. I think getting out that last one cm sag in a 200 wide horizontal cut is not worth it in like ‘normal’ CSM conditions.

Looking forward to your replies!

 

[M.R.]

Seemly about the only perspective left might be to
Rethink on these wide slabs is the technique of how the wedges
are applied.... (& Center kerf support in narrower thickness..)
Too much lift may allow the natural belly in the bar to
track that way....

 

[Booo]

Seemly about the only perspective left might be to
Rethink on these wide slabs is the technique of how the wedges
are applied.... (& Center kerf support in narrower thickness..)
Too much lift may allow the natural belly in the bar to
track that way....

I agree about the wedges, that is why I hardly use them untill just before the end of the cut.

I don’t understand what you mean with the ‘center kerf support...’ part.?