Why is a Granberg ripping chain designed the way it is?

[krawiea]

Hi all

New to milling, so I read what I could find on these forums about ripping chain.

I bought an Alaskan mill mk III and was sold a Granberg ripping chain to fit it.

After very few passes in fresh hard woods it became dull and I looked into how to sharpen it. It is clearly very different from other ripping chains and actually reminded me more of crosscut patterns on handsaws with which I am more familiar, where the alternate left and right teeth are more like knives than chisels ( for those who don't know, see the very clear explanation at Welcome to Vintage Saw's Saw Filing Treatise ) which are designed to slice fibres across. There is no concept of clearing the cut material other than the normal channel created by the set. Traditional ripping patterns for handsaws have teeth left and right which are like chisels, not slicers

In the case of the Granberg ripping chain, the rakers for the slicers are set lower, indicating that this is what should happen first, and then the chisels should plane out the waste that has been demarcated by the knives.

If you hold the chainsaw at right angles to the log you are most likely cutting into the end grain, or first slicing a line and then just chopping off the end of the fibres. If you angle the saw, as I have seen some people do, then you are closer to chiseling along/down the fibres as a handsaw might do.

So, the questions are:- What is teh point of the slicers on the Granberg? And why are the cutting angles of the clearers different? And can anyone enlighten me as to what that design is all about? People say you should get chips and not dust. I wonder if you ever will, given that you are cutting off the ends of teh fibres?

As I didn't have the right files to sharpen the Granberg, I put on a sharp crosscut chain and found it went through easily and produced a good clean cut. Now I wonder what I am missing that makes ripping chain that much better.

Any ideas gratefully received

Andy

 

[mad murdock]

I have never used Granberg's chain but I have used Woodsman Pro (Carlton) by Bailey"s, and Stihl's picco ripping chain, as well as regular full chisel chain in a milling application. The WP and the Stihl ripping chain produces a smoother cut, albeit slower than a regualr crosscut chain. If you lead the cut with either the tip of the bar or the handle of the saw in the cut at about a 15-20 deg angle, you will get a faster cut, without sacrificing smothness. The standard sawchain though faster leaves a rougher surface on the milled piece. I like the picco chain best, as the feed speed is very good, and it does lube better with the oil-o-matic feature on the drive links.

 

[BobL]

Before I get to the slicers it is important to understand that, apart from noodling, a chainsaw chain works more like a chisel than either a hand saw or a hand plane.

Cutting starts with one cutter grabbing the wood and (if it has been filed correctly) the cutter starts a high angle bite that pulls the cutter up off the bar. Then the raker makes contact with the wood slightly rocking the cutter causing the cutter to take a slightly shallower angle than before. The raker also penetrates slightly into the wood allowing the cutter to bite a little more. The cutter thus rocks it's way back and forth into the wood and eventually the raker resistance, saw power and chain tension snap the cutter back onto the bar pulling the chip out of the wood. If the chip was to hang together the resulting chip would be half crescent shaped. Think of it more like using a chisel firstly at a highish angle and the levering the chisel to a shallower angle and then levering hard on the chisel handle to bring out the chip. At WOT (60 mph or 88 ft/s) a 1" long chip takes 1 milliseconds to generate.

The chain itself porpoises in the cut over a length of 2 - 3 cutters so that on conventional chain the following 2-3 cutters cut a smaller chip or sometimes not at all whereas on a skip chain all or most of the cutters will be cutting. This is also a major reason not to push too hard on the bar.

The top plate filing angle (TPFA) determines the sideways movement of the cutter which contributes to the width of the kerf. On cross cutting chains a high TPFA is used to create a slightly wider kerf for two reasons
- if the tree/limb moves a little and narrows the kerf the B&C has some room to move before the B&C are jammed.
- with cross cutting is that the fibre ends hang inside the kerf and can resist the flow of chips from the kerf so a slightly wider kerf helps clear chips.
When CSM the latter is not a problem and the former is controllable with a few wedges hence a lower TPFA slightly reduces the kerf width and takes a bit of load off the saw. I have used TPFA angles as low as 5º without a problem but intuitively some TPFA must help cut end grain so I now tend to use about 10-12º. BTW Will Malloff uses TPFA of zero.
Lower TPFA are supposed to produce a better finish but I think this is more operator dependent since with a little finessing I can produce just as good a finish with 30º TPFA as a 10º TPFA.

I don't use Granberg chain but my understanding for the slicers is as follows. Using the chisel analogy, when cutting a mortise (even into into end grain) if you want to reduce tearout it makes sense to make a few nicks with the chisel along the sides of the mortise. This creates a cleaner mortise that is a bit easier to cut. Using slicers would theoretically take some load off a cutter breaking inter fibre bonds. I doubt they do much or anything in straight grained wood but could be helpful for cross grained timbers.

 

[mtngun]

The Granberg design was Will Malloff's first ripping chain. It's a complicated design that offers little advantage over simpler designs, though it does generally allow the engine to rev about 500 rpm faster, all other things being equal.

Malloff later switched to a different design that is much easier to grind and cuts faster anyway.

 

[krawiea]

Thanks for the very informative replies.

As flat straight boards is what I'm after, without skewing, the finish is pretty much secondary, a far as I am concerned. Though I imagine, if you are getting super smooth finish then your cutting is likely to have been efficient and you are not stressing your machine and you will save on fuel and lubricant. So, not a bad goal, I suppose.

I just took delivery of some green oak beams straight off the big bandsaw and a couple of passes with a handplane taking thickish shavings removes the sawmarks. About half a millimetre, if that. The boards I am getting from the csm are comparable, if I haven't done anything stupid.

With that in mind, I guess what I am really interested in is getting through the wood as quickly as possible. And I try and concentrate on making sure I don't introduce any bend.

Couple more questions if that's OK.

Why do you think milling with the saw at 15-20 degrees is faster? What is going on? I tried it and thought it was faster but wasn't sure if I;d imagined it.

I think I understand tpfa. Just. Are there resources on this site explaining it t the novice? As for wide kerf, is it because the cutters do not sit rigidly in the groove of the bar, and the wider the filing angle the more they dig in whilst being lifted of the bar, and can be deflected by the pressure? Is that what's going on with the wider kerf, do you think?

Thanks

Andy

 

[BobL]

Why do you think milling with the saw at 15-20 degrees is faster? What is going on? I tried it and thought it was faster but wasn't sure if I;d imagined it.

I haven't timed as many cuts as mntgun but the couple of times I tried angling the saw I found no significant difference in cutting speed. The fibres may be easier to cut when the saw is on the angle but there are also more of them to cut so its swings and roundabouts on this.

I think I understand tpfa. Just. Are there resources on this site explaining it t the novice?

I haven't seen much more detail about TPFA anywhere than I provided in my post.
The Carltonchainguide.pdf has one of the best explanations of how chain works - If you can't get hold of it send me a PM.

As for wide kerf, is it because the cutters do not sit rigidly in the groove of the bar, and the wider the filing angle the more they dig in whilst being lifted of the bar, and can be deflected by the pressure? Is that what's going on with the wider kerf, do you think?

Yep and if the groove is too large the chain can lay over on one side and it will cut a curve rather than a straight line.

 

[mad murdock]

Why do you think milling with the saw at 15-20 degrees is faster? What is going on? I tried it and thought it was faster but wasn't sure if I;d imagined it.

Andy

Have done a fair amount of milling, I find that leading the cut in most cases helps more with self feeding, not as much pushing is required, and so it takes less effort on part of the operator. Maybe feed speed is not effected, tho I can say that from my firsthand experience,'.050 low ripping chain cuts a bit faster than the 3/8 pitch WP ripping chain I used previously, by about 30%! It even cuts (feeds) faster than 3/8 pitch standard chisel chain in ripping mode. The other nice thing is 1/4" kerf which means less waste and more boards.

 

[BobL]

Have done a fair amount of milling, I find that leading the cut in most cases helps more with self feeding, not as much pushing is required, and so it takes less effort on part of the operator.

If you can slope the log then the saw will easily self feed.
The next best thing to do to improve self feeding is reduce the top plate cutting angle (TPCA).
Typical TPCA for cross cutting chain is 60º, ripping chain usually is around 50º and Will Malloff uses a 40º TPCA. Square ground chain is normally 40-50º - it feeds really well when first sharpened but I find it goes blunt too quickly in the woods I cut.

All this has been discussed in detail by mtngun and others in this forum - keyword to search for would be "hook"

 

[krawiea]

Hi All

Thanks for the useful info.

I got a copy of the Carlton Saw Chain Book as a PDF. It has a different name but seems to be the same 33page booklet. It has been uploaded on some other wood interest sites by some philanthropists. Otherwise we'd all be paying Scribd for what Carlton would probably want in the public domain anyway, or at least in their customer domain.

Bobl, I am not sure if PM means you would like a copy. If so, I can upload or send by email. Does this site not have a library where this kind of thing can be stored for all to see?


Andy

 

[mtngun]

Why do you think milling with the saw at 15-20 degrees is faster? What is going on? I tried it and thought it was faster but wasn't sure if I;d imagined it.

If you are cutting straight down the log and then switch to an angle, at first the saw will only be cutting on part of the width of the cut instead of the full width. There is more pressure per tooth so the teeth take bigger bites. Eventually the angled cut will hit full width and the speed advantage will be lost. So typically the operator angles the saw in the opposite direction to restart the process.

Show me an operator who needs to angle his saw to get it to bite, and I'll show you a chain with not enough raker angle.

 

[BobL]

Bobl, I am not sure if PM means you would like a copy. If so, I can upload or send by email. Does this site not have a library where this kind of thing can be stored for all to see?

PM = personal message, whereby I could then offer to send it to you if you could not find it.

 

[krawiea]

Thanks to everyone for all the useful stuff. I have plenty of theory. Now the practice...

Cheers

Andy