First go with a 60" bar and 090

[dave k]

Got a chance to mill with my new 60" bar today and although the 090 handled it with no problems I have a question !
I was taking a slab off the stem so it can be put on my pals band mill so I decided to do one deep cut, 14", I set up the mill on a plank so no problem with setting up level. I was using standard .404 Stihl RSK which I was aware would be a bit aggeresive.
The rails were not as ridgid as I would have liked, I forgot the cross struts, but were well supported and level.
It was not the smoothest cut Ive ever made but didn't look to bad until I put a straight edge on it and found a fairly consistant dip along the middle of the cut ranging from 5mm to 8mm in depth.
I had checked the mill before starting and when I placed it up on the rails and all was ok.
So help ! was it just down to the RSK digging in to much, chain tension or ?

 

[Kicker_92]

Set your mill on the rails outside of the log, and place a straightedge on your bar. With a 60" bar, having that amount of sag in the middle is reasonable.

There's been several threads on ways to reduce this, might be able to search and find some info.

 

[BobL]

Firstly nice pics and nice wood!

As kicker says your problem is caused by bar sag and is not unusual on longer bars, although the 5 to 8 mm seems rather high given you have an hefty counterweight (the 090) that should help reduce the bar sag. In general if the bar starts with a sag it generally gets worse as the mill progresses down the slab. (BTW This puts a lot of pressure on the mill). Both my 60" bars sag about 2.5 mm at the 42" mark on the bar and if left "unsagged" will produce slab dip that starts at ~2.5 mm and ends up at ~5 mm by the end of an 8 ft slab.

Looking at these picture I can see why you have more that usual dip.

The first picture hints the bar is sagging while in the second picture both the mill uprights and rails are clearly bending, and being on such a long bar this translates into significant more dip.
The most obvious cause is the high depth of the cut (or long length of uprights between bar and mill rails), and the log rails being too close together don't help. This combination allows the whole mill to distort more easily than if a shallow cut is made on widely spaced log rails. The log rails also need better support along the log for such a deep cut. So lowering and nestled the log rails closer to the log - maybe something like this would be better.

The other reason, which you probably won't like to hear, is that the mill rails and uprights might be too flimsy for the weight of your saw when using a long bar. The easiest/cheapest fix is to get larger cross section profile rails for the mill but the uprights you are sort of stuck with. This is why strongly built mills like the GB are much better for longer bars.

There are several ways to reduce or even eliminate the effect of sag.

One way is to pretension the mill/bar. To do this undo one of the bar clamps (usually the outboard end) and then put tension on the bar/mill by somehow pulling on the bar or pushing on the bar clamp to put tension on the bar. To get the bar to reduce the sag from 2.5 to 1.25 mm I calculated I needed 1.3 tons of tension so this can really only be done with a very strong mill. Most mills I know would collapse under such weight. Even if you can't apply this amount of tension - some tension is always better than no tension.

Assuming the mill and bar are moderately tensioned, the other way (thanks Aggiewoodbutcher = AWB) is to hold the bar level with an anti-sag device just as the bar enters the wood. It's surprising but as long as the bar goes in straight it tends (but not always) to stay level. You should be able to get about 2/3rds of the bar into the wood before you remove the device

AWB uses a removable plastic hook but I first used a magnetic bracket like this

Full WIP here http://www.arboristsite.com/showthread.php?t=78905 (scroll down to post #5)

Now here is my delux version

(http://www.arboristsite.com/showthread.php?t=99773)
This one is really sweet to use - the bracket rotates up out of the way as soon as the bar is 2/3 of the way into the kerf and clips onto the mill so I leave it permanently on the mill. I also use it to stop the bar from wobbling when I'm touching up the chain.

I hope that helps.
BobL

 

[dave k]

Thanks Kicker and Bob nothing you boys said came as a suprise as I had already mentioned that I wasn't to happy with the rails and what I did'nt say was that I had approached the exercise as a bit of a test since it was not going to be to important how it turned out and as you say Bob that set up on the mill may not be strong enough for that depth of cut with that bar as when I tried it with my 41" bar it was ok.
The 090 has so much power that it finds the weakness else where ! I have more simillar bits to do so will reajust things and see how it turns out.
Thanks guys !

 

[BobL]

Thanks Kicker and Bob nothing you boys said came as a suprise as I had already mentioned that I wasn't to happy with the rails and what I did'nt say was that I had approached the exercise as a bit of a test since it was not going to be to important how it turned out and as you say Bob that set up on the mill may not be strong enough for that depth of cut with that bar as when I tried it with my 41" bar it was ok.
The 090 has so much power that it finds the weakness else where ! I have more simillar bits to do so will reajust things and see how it turns out.
Thanks guys !

Big saws are a real test for mills. When I was using the 3120 on the standard Granberg Mill I could see it flexing quite badly at times. Even the 880 gives the BIL mill a real shake and I cannot put any tension on the outboard vertical as it is effectively a 1" x 1/8" ally pipe.

The 880 also shook nuts and bolts and brackets off the mill that the 076 wouldn't touch.

Cheers