Pimped anti-big-bar-sag device

Last year I needed a quick anti-big-bar-sag device so I made this



When using it I found I had to add locking wing nuts above and below those two brass knurled adjustment knobs to stop from vibrating loose. The wing nuts also had to be tweaked with a spanner otherwise they would become loose. When the bar was buried into the log past half the bar width, I had to stop, get a spanner and undo the wing nuts and then take the whole device off the mill. The whole thing was also just a touch too flimsy for a mill, so I recently decided to make a new one.

One of the design aspects I wanted was a quick release and reinsert into position mechanism - so it now looks like this.


and it goes onto the mill at the same place as the previous device like this


At the bottom is a piece of 1" square ally rod into which is embedded a 1" diameter x 1/2" thick REE magnet. This is then covered with a soft steel plate screwed over the top of the magnet to hold it in place.


The adjustment is made by
- Undoing screw A
- Turning nut C so there is minimal brass thread exposed
- Undoing screw B so the vertical pipe can now be lowered so the magnet can contact the bar.
- Screw A is locked into place
- Nut C is turned to expose more thread which lifts the bar
- Once the bar is level Screw B is locked into place.

Part II

When the bar is buried in the log past half way, the device can simply be flipped upwards so that it clips into a horizontal out of the way position like this.

Because the magnet is slid off the bar it comes away a bit easier than a direct pull away from the bar.

View from other side.


What holds it in place horisontal is this inverted ally "clothes peg"


That took a bit of finessing to make but it seems to lock it very firmly because the vibe is pretty fierce.

When I next need to use it again the settings should not need adjusting since the screws should still be locked into the same place, I will just need to nudge the bar up to kiss the magnet which will grab it and hold it in place.

Another advantage of the anti sag device is that it can be used to hold the bar (and chain) steady when sharpening on the mill - otherwise the bar flexes back and forth like a yoyo.

One of the disadvantages of all this "fruit" hanging above the bar is that it is all the more to fall onto a moving chain, although most of the time it should just fall onto the board or slab being cut at the time.

As usual, proof will be in the cutting!

Cheers

Some nice machine work there.

I was curious -how long can a bar be before sag becomes a problem?

wavefreak said:

Some nice machine work there.

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No mill or lathe has been used in this construction. Just a 12" table saw with negative rake 80 tooth blade and a drill press and everything else has been done by hand and bending using a MAP torch.

I was curious -how long can a bar be before sag becomes a problem?

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It depends on how the bar is attached to the mill. If the saw is held by a conventional bar clamp on the inboard side of the saw then the weight of the saw seems to counter balance the sag in the bar up to ~ 60" but above that it can be a problem. However, the BIL mill is held at the bar bolts so even a 60" bar will sag by 0.09" at the 42" mark. Provided the bar is started straight it will stay straight for the rest of the slab.

Thanks - I often just rig things but I love making jigs and things.

Doesn't the bar sag become a non-issue once the bar enters the wood?

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Don't ask me how but somehow it seems to stay level once it half way into the wood!

Very clever.

As always fantastic ,simple and makes sense ! Thanks Bob

What a great idea, I'll have to come up with one for my mill.

Bob, you are nothing shy of amazing.

Mark

Cheers Guys.

Now I got to resist the urge to fiddle some more and get back to milling!

BobL said:

Cheers Guys.

Now I got to resist the urge to fiddle some more and get back to milling!

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Bob,your mind never stops working great stuff.I think you have extra long GB bars,i just got a Titanium extra long 50'' and its got on it R seires,do you know what this means ?.It seems to be very wide over 4'' is this the norm.The Stihl bar covers wont fit over it with the chain on the bar.

Stihlman441 said:

Bob,your mind never stops working great stuff.I think you have extra long GB bars,i just got a Titanium extra long 50'' and its got on it R seires,do you know what this means ?.It seems to be very wide over 4'' is this the norm.The Stihl bar covers wont fit over it with the chain on the bar.

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I saw your post on the CS forum about the TI bar but didn't respond. Is it a roller nose?
I suggest a PM to Rooshooter as he is a GB agent.

Of the two 60" bars I have, the Stihl bar I have is 110 mm wide whereas the GB ripper is 90mm wide.

I guess I could solve bar sag buy getting two power heads and a double ended bar.

Nice machine work you do!! Nothing ever looks like someone slaps something together....Always neat and clean job!!!!!!

wavefreak said:

I guess I could solve bar sag buy getting two power heads and a double ended bar.

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It will solve the sag problem but if conventional bar clamps are used it will then bow the other way!

stipes said:

Nice machine work you do!! Nothing ever looks like someone slaps something together....Always neat and clean job!!!!!!

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Cheers Stipes - work may be neat but unfortunately my shed is a real mess!!! Production will have to slow down as I have to go away twice in the next two months. Bummer - I'd rathe be playing with my saws!

BobL said:

up to ~ 60" but above that it can be a problem. However, the BIL mill is held at the bar bolts so even a 60" bar will sag by 0.09" at the 42" mark. Provided the bar is started straight it will stay straight for the rest of the slab.

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Mainly a bump for another great BobL thread, but I think that for many of us when we get beyond 60 sag could be a problem.

Bringing this one back from the archives with a quick question if I may.

I'm re-designing a slabbing attachment for my swing blade mill and was originally considering shims on the outer edges of the bar clamps to pull the sag out. Pro is it should stay straight throughout the cut and once set, will be pretty close to a set and forget option. Con is a bit fiddly to set up.

Then I read of Bob's magnet idea and thought that sounds like a clever solution. I just wonder if it will lead to the operation, when relying solely on the support in-the-cut, being less forgiving of other factors that might bring sag back into the cut before completing the cut? I figure that as much as I try, there are times when the chain or bar rails might not be 100% and the shimmed clamps might allow me just a bit more wriggle room.

If things were set up pretty good, then on a 72" bar in, say, 60" hard wood how long can the log be before sag coming back into play if the bar starts the cut straight?

Thanks all.

FWIW I added the pics back into the thread.

I worked out the amount of force need to take the sag out of a fixed 60" bar using shims and the amount required is surprisingly high (it's in the tons weight range) and although this is possible using shims I just found it easier to use the anti sag device. Conventional CSMs that clamp onto the bar have a slight advantage in that the weight of the powerhead does help counter balance bar sag but on my CSMs the mill bolts to the bar bolts so the counterbalance effect is less pronounced.

The widest stuff I've cut is ~55" about 8ft long and the magnetic anti sagging device worked fine for (now I'm guessing) 3 out of 4 big slabs but about 1 in 4 it did still end up sagging about 1/2 to 3/4 of the way through the cut.

It's surprising that it works but it does. My assessment of this is that to work correctly the operator needs to provide constant steady forward pressure in the cut and definitely do not move the bar around in the cut. this is why carriage CMS have an advantage over CSM i.e. can't move the bar around. If the bar is seesawed or pushed back and forth in the cut the constants stream of chips that escape from the chain and support the bar in the kerf is removed and the bar will sag into the empty kerf. I think that's why the 1 in 4 of my slabs probably went that way. using a winch or a sloped log will also help provide a more constant forward pressure.

Another possible way to deal with this I have thought about might be to use a slightly higher top plate angle (e.g. 15º) on the cutters on the top of the bar and a lower TPFA (e.g. 10º) for those on the bottom. My preference would be to use both this and the anti sagging device.

I find that as your bar size goes up, you have to proportionally spend more time sharpening and paying attention to you cut pressure.
Less than36" I will sharpen on the saw, less than 50 I will sharpen in the vise with a wood cut gauge, 84" bar chain is in the vise with the detail tools.
The larger cuts are not brain surgery, but every little bit helps. Trying to flatten a 55" board is NO fun!!!
BobL is correct with the pressure and top plate angle .

BobL said:

Another possible way to deal with this I have thought about might be to use a slightly higher top plate angle (e.g. 15º) on the cutters on the top of the bar and a lower TPFA (e.g. 10º) for those on the bottom. My preference would be to use both this and the anti sagging device.

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Is there any extra bar wear observed with the biased top plate angles? I'm thinking it might actually help a little rather than harm but don't have the benefit of experience.

And here's a wacky idea: Pack the bar with a few thin strips of hard wearing UHMWPE to almost match the kerfs experienced (which will vary on chain, cutter length, filing angles, bar groove wear) so that while it won't stop sag, it has a fighting chance of reducing it as the bar gets some support from the log underneath it earlier in the sagging than it otherwise would.