Too much grunt!

[billstuewe]

I wonder if your whole bar could be resonating back and forth?

Nope--Its a 16" bar with about 3-4" of bar sticking out the far end and I can see the chain start to do a "wave" as it enters the wood.

 

[Brmorgan]

Low RPM can cause this problem (at least in my experience) with full-chisel chain left at ~25°. I think it has something to do with the sharp point of the cutter wanting to bite in sideways if the saw isn't pulling it fast enough. Even a nice, tight bar can allow some fairly significant play in the chain, and the chain by its nature can flex sideways a bit.

 

[BobL]

Watch next time to see if you notice the chain entering the cut doing that little wavey dance.

Yep I have seen the wavy dance and I also think it's a harmonic thingo, but rather than harmonic - which is one vibe frequency causing another - I now think it's more related to chain wave, cut rate and cant width synchronization.

For those that do not know, the cutters on a sharp chain do not act like little planes riding hard up against the bar and all making little shavings in a row - that does happen the chain is blunt and makes powder instead of chips. What happens when a sharp cutter grabs is, it is pulled into the wood to a depth determined by many factors and in doing so is pulled off the bar by around a 1/4". This cutter cuts a leading channel and one or more cutters trail behind it, cleaning up that channel but also doing very little cutting in the process. This action forms a triangular shaped wave on the chain which also propagates a pulse back up the chain out of the cut and can be seen as the "wavey dance" described by Bill above.

The wave length of the cutting wave will be different from the wavey dance. The wave length of the cutting wave depends on things like the raker depth setting, hardness of the wood, chain tension, cutter hook, chain length etc

If the log is narrow and the revs are right there is the possibility the cutting wave and cutting rate can synchronize with the wavey dance.

So if the cutting speed synchronizes with a cutter reaching the peak of the wavey dance just as it enters the wood this cutter can cut a fresh new channel and trail sufficient cutters behind it to make a single clear ripple in the wood all in the time that it takes for the bar to move forward that distance. So the cutter can start about 1/4" off the bar as it enters the wood and is down on the bar as it exits.

At 9000 rpm a 3/8, 7 sprocket chain speed is 800"/s
The forward cutting speed is around 1"/s
The wavelength of the wood ripple is 1/4" so one 1 wood ripple sees 200" of chain. which for a 42" bar is around 3 chain lengths worth.
It sounds like a lot of but the lovely big chunky chips that the saw was making under these conditions is consistent with this idea.

The narrowness of the log also means the cutter gullet does not fill completely. On a wider cut the gullet fills and quickly pulls the cutter away from the wood - this stops the cutter from continuing to make a channel and which is why it does not make as clear a washboard pattern as it otherwise wood.

Clear as mud?

 

[Brmorgan]

Bob - have you tried an 8-pin rim on that 880 in the smaller stuff (say under 30" dia.)? It should handle the load fine.

 

[billstuewe]

BobL, I like your thought process--Too bad you are so far away--I would love to get together with you and make some sawdust. Now if you would put that thought process to work and let me know how to keep this from happening. It seems to do it worse with some chain. Two Saturdays ago I was milling willow--about 15" logs and several times I would get one deep groove all the way across the cut. It was exactly like the washboard cut you pictured but it was only one single groove. Very strange???

 

[BobL]

Bob - have you tried an 8-pin rim on that 880 in the smaller stuff (say under 30" dia.)? It should handle the load fine.

Yeah I have the 8-pin sprocket - I'll have to give it a try!

 

[BobL]

BobL, I like your thought process--Too bad you are so far away--I would love to get together with you and make some sawdust.

I move around a fair bit so, never say never . . . . it could happen

Now if you would put that thought process to work and let me know how to keep this from happening.

I'm working on it!

It seems to do it worse with some chain. Two Saturdays ago I was milling willow--about 15" logs and several times I would get one deep groove all the way across the cut. It was exactly like the washboard cut you pictured but it was only one single groove. Very strange???

If while milling you slowly put pressure on the saw and slow it down there's a chance that it will pass through that synchronization speed and just do one big ripple as it passes through the wood as I describe.

 

[Kicker_92]

Yeah I have the 8-pin sprocket - I'll have to give it a try!

Bob, what sprocket do you usually run on your 880? I'm using an 8-pin with the 3/8" chain as well, but haven't tried any other combinations.

 

[BIG JAKE]

Maybe just out of practice Bob? Good to see you milling again anyway! opcorn:

 

[BobL]

Bob, what sprocket do you usually run on your 880? I'm using an 8-pin with the 3/8" chain as well, but haven't tried any other combinations.

Because our wood is so hard I have only used the 7 pin 3/8 with the 880. This also makes sense with the the 60" bar. I have an 8 pin for for the 42" but haven't tried it out yet.

 

[Brmorgan]

BobL, I like your thought process--Too bad you are so far away--I would love to get together with you and make some sawdust. Now if you would put that thought process to work and let me know how to keep this from happening. It seems to do it worse with some chain. Two Saturdays ago I was milling willow--about 15" logs and several times I would get one deep groove all the way across the cut. It was exactly like the washboard cut you pictured but it was only one single groove. Very strange???

This happens to me if I almost stall the chain. I posted this pic in another thread last summer because I couldn't figure it out at the time either:

After a few more cuts the next day I realized it only happened when the saw engine bogged out to really low RPM. This was cut using 3/8 full-chisel @ 25°, 28" bar in 16-18" of wood.

 

[deeker]

That pic is similar to what happens with the band-saw mill, if I have to stop pushing while cutting....

With the band-mill, I believe the cutters widen out as the rpm's go to a stop vs full speed......

It is easy to see where I have stopped while cutting...to adjust a dog or trim the log a bit.

Kevin

 

[BobL]

7 days later

Same mill, same chain (but with 2 stroke file touchup), same wood - but log is about 30% wider.

Last week on left - today's on the right!

 

[BobL]

Few milling pics

Nothing special - Just had a mate (forum member Martrix) handy with a camera to take a few pics of me milling a small lemon scented gum.
On days like these it's hard to believe we are in middle of winter.

 

[texx]

so what was the problem 2 stroke file touch up fixed ? did you change the angle a wee tad or just needed a sharpen ,or a bit more off the rakers ?

 

[BobL]

so what was the problem 2 stroke file touch up fixed ? did you change the angle a wee tad or just needed a sharpen ,or a bit more off the rakers ?

Rakers weren't changed. I don't think the two strokes touch on the cutters wouldn't have changed much - the biggest change was the width of the log being cut ie 30% wider. That means the chain speed would have been out of synch with the wavey dance thing that billstuewe refers to - well that's my guess anyway.

 

[Brmorgan]

I think it's probably a chain speed issue too. I once cut a 7" wide piece of pine with my 090 & full-comp .404 rip chain. It cut as fast as I could push it through the log, but produced a washboard cut even worse than yours. On a 30" spruce log though, it was almost as smooth as some finished lumber.

 

[SilverBox]

the biggest change was the width of the log being cut ie 30% wider.

Yup I agree. I've seen this before where you cut with alot of chain out of the cut and then go to a bigger log and it smooths right out.

 

[dancan]

Same mill, same chain (but with 2 stroke file touchup), same wood - but log is about 30% wider.

Last week on left - today's on the right!

Reminds me of machining a rotor , if the setup was a little off or just the right conditions you get similar marks on the rotor from the cutter vibrating at the right harmonics (don't know if this helps but I thought I'd throw it out there) .

 

[BobL]

Reminds me of machining a rotor , if the setup was a little off or just the right conditions you get similar marks on the rotor from the cutter vibrating at the right harmonics (don't know if this helps but I thought I'd throw it out there) .

Next time I find a suitable tree I will cut a long wedge and see what width this effect starts to kick in at.