Are FOP really progressive depth raker generators?

I have not used an FOP or their look alikes but, following discussions and some of the images posted in a previous post, I wonder if FOPs are really true progressive raker depth makers after all.

So I put the following up for your consideration - it's getting close to ultra geeky (a bit like the bar nut poll) for which I make no apology.

Compared to conventional raker gauges which generate the same raker depth irrespective of the cutter or length or gullet width, progressive raker generators make even greater raker depths as the cutter wears (or gullets get wider)

The whole idea of using progressive raker depth is to maintain a constant cutting angle (not to be confused with top plate filing angle).

This picture shows what I mean by constant cutting angle - it's the angle between the wood, the cutter edge and the top of the raker.

• The idea of a constant cutting angle is to produce the fastest cutting speed during the whole life of the cutter.
  
• Cutting angles for new cross cut and ripping chains are similar, around 6º , and determine the size of the chips being made, with higher angles making bigger chips , and very low angles making powder.
  
• Operators should consider varying their cutting angle to suit the size and hardness of the wood, and also the power of the saw.
  
• Higher angles (7- 7.5º) suit wider softer woods (some folks go as high as 9º) and bigger saws, while 6º works for harder wood and smaller saws. Some Lucas slab millers use 10º on their chains .

OK, now lets look at how an FOP works.

In this picture I have exaggerated all the angles so that things can be seen more clearly

• The Black line represents the FOP as would be set up for the current cutter length. The resulting cutter angle is 9.6º (NB this is actually not what an FOP would generate. An FOP would never let a raker angle be less than about 4.5º - the sharpener of this chain has generated this raker angle via other methods)
  
• As the cutter wears, as to be expected the FOP drops (blue line) but if it still pivots around the base of the back of the adjacent cutter as shown here the cutting angle does not remain constant - it gets shallower (in this image it drops to 7.3º) .
  
• The red line shows what true constant cutting angle would be (also 9.6º). It has to be parallel to the black line to maintain the same cutting angle because the line across the top of the cutters remains parallel at all times.
  
• If the FOP remains tucked up against the back of the adjacent cutter the result is the FOP can only make approximately progressive raker depths.
  
• The difference - between the red and blue lines shows what else is needed to be removed from the raker to create a true constant cutting angle - I agree it's not much but there is a difference
  
• As the cutter wears an even greater amount of extra raker will need to be removed to make a true constant cutting angle.

So all this suggests to me that although FOPs are better than regular gauges, I will probably stick with my old method of using vernier calipers to set my rakers even though it is a lot fiddlier.

Another couple of good reason to use calipers are

• I can change my cutting angle to whatever I want at the time, whereas one FOP only permits a fixed slowly dropping cutting angle as the cutter wears.
• FOPs cannot set raker height accurately on skip chain.[EDIT: this not correct the FOP does not pivot at the FOP pivot point shown in the image above. Instead it hooks onto the raker but the location of FOP Pivot remains the same contact point for the FOP and so prevents the same angle being constantly generated throughout the life of the saw and my conclusions still stand)

Clear as mud?

i wonder if the small change in distance between cutters warrants the small change in raker height.....as the tooth is sharpened the distance between cutting edges increases (one tooth to the next). Now is it enough to warrant a slightly less aggressive raker depth.

might be better in our hardwood?

Serg

So it progressively lowers the depth gauges Bob, it just doesn't quite maintain a constant cutting angle.

tdi-rick said:

So it progressively lowers the depth gauges Bob, it just doesn't quite maintain a constant cutting angle.

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it does both actually, because the cutter is angled and the depth gauge isn't the angle would change it you just dropped them 25 each because as the tooth becomes shorter the height changes.

husq2100 said:

i wonder if the small change in distance between cutters warrants the small change in raker height.....as the tooth is sharpened the distance between cutting edges increases (one tooth to the next). Now is it enough to warrant a slightly less aggressive raker depth.

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It's absolutely warranted.

On a new 3/8 chain the gullet is ~0.25" wide, a 0.025" raker depth then translates into a 6º cutting angle.

When the gullet width reaches 0.5" a 0.025" raker depth translates into a cutting angle of 2.9º, that means the cutter will take an initial bite of only half the amount of wood and make a lot more powder than chips.

In practice the chain will cut more than half but it will not cut as fast as if the cutting angle is maintained at 6º.

tdi-rick said:

So it progressively lowers the depth gauges Bob, it just doesn't quite maintain a constant cutting angle.

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That's correct.

I guess it adds a safety margin, but it's interesting to know that chains as they wear down more are capable of more than a FOP can deliver.

i get what your saying Bob, but im thinking more about cutting hardwood like ironbark...i wouldnt run skip, even on a bar over 28 inches long...now filling the tooth makes each cutter further apart....i wouldnt want to run full depth rakers on skip, so to speak....just a thought....

if you are taking things that far i would also be looking at the shape of the raker and the back of the tooth, depending on what type of cutting you do...

getting back to the FOP and raker depth, i think that guys cutting softwood in NA may run more raker depth than factory so it wont be good for them....

gets us back to how far you want to take a chain for what your doing.... FOP may not be absolute best, but is probably alot better than gussing by eye

Serg

sorry, had a brain fart and got it completely wrong, tooth to tooth does not change when sharpend......only distance from back of tooth to cutting edge of next tooth changes.......

ill shut up now

serg

jburlingham said:

it does both actually, because the cutter is angled and the depth gauge isn't the angle would change it you just dropped them 25 each because as the tooth becomes shorter the height changes.

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I can't really understand what you have said here.

If the reference line is the line between the top of two adjacent cutters then since as the cutters wear both cutters should go down in height at the same time so there is no rotation to that line.

However, the distance between a cutter edge and the back of the adjacent cutter does and this rotates the the cutting angle anti clockwise in my diagram - ie goes from black to blue.

While the cutter is relatively new the difference between the original cutting angle and the reduced angle made by the FOP is small <(1º) and translates to a raker height difference of less than 0.005"

But when cutter is close to worn out the FOP generated raker heights will be around 0.015" too high

husq2100 said:

i get what your saying Bob, but im thinking more about cutting hardwood like ironbark...i wouldnt run skip, even on a bar over 28 inches long...now filling the tooth makes each cutter further apart....i wouldnt want to run full depth rakers on skip, so to speak....just a thought....

Click to expand...

95% of what I do is milling and don't usually use run skip, but I have run 3 skip with 0.035" rakers on new chain (ie 8º cutting angle) in Tuart (a fairly hard wood) with a 42" bar no problems at all.

I've cut dry sugar gum with rakers as low as 0.050" but the cutters were quite worn (0.45" gullet) and it cut really well considering it was dry. Cutters go really blunt in that stuff though!

if you are taking things that far i would also be looking at the shape of the raker and the back of the tooth, depending on what type of cutting you do...

Click to expand...

Yep - that is really important to do some serious rounding and not leaving the raker too square

getting back to the FOP and raker depth, i think that guys cutting softwood in NA may run more raker depth than factory so it wont be good for them....

Click to expand...

I'm thinking that extra raker depth will really work for them.

gets us back to how far you want to take a chain for what your doing.... FOP may not be absolute best, but is probably alot better than gussing by eye

Click to expand...

Yep I agree, definitely a great field device but it won't help me much with a chain that has worn cutters because my rakers will already be 0.010" too low for the FOP

As I posted in the other thread, at an overall cutter length of approx. 7.3mm I have the depth gauges at 23-24 thou.

I have a stuffed Carlton A2EP chain, I'll file/grind a few cutters right back tomorrow and file the rakers with the FOP to see what height they end up.

tdi-rick said:

As I posted in the other thread, at an overall cutter length of approx. 7.3mm I have the depth gauges at 23-24 thou.

I have a stuffed Carlton A2EP chain, I'll file/grind a few cutters right back tomorrow and file the rakers with the FOP to see what height they end up.

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I'd like to work out your cutting angle but for that I need the gullet width not the cutter length.

it occurs to me that there will be an optimum cutting angle for every type of chain, but that it will actually fall into a range, dependent on a lot of variables. I'm not a physics major and know little about the physics of chainsaw cutters in action.

I have, however, come in contact with a bit of cutter geometry study in my capacity as team lead in a foundry pattern shop, and years ago as a cabinet maker. It occurs to me that chain geometry is quite different, as the cutter itself has a fair amount of latitude.

What you're describing as cutting angle is roughly correspondent to "rake" as well as "clearance" of, say, a saw tooth or milling cutter. In soft wood at high speeds, I believe the cutter more or less shaves it's way through several millimeters of wood, generally undulating in and out of maximum "dig".

But what about harder wood, what about the difference between winter and summer wood, relative spacing of growth rings, and hitting knots and other hard spots? We've all probably noticed the pattern of saw scratches on the end of a log, and how they vary, dependant upon chain speed, width of log, even the presence or absence of knots and etc.

This is an interesting thread, but I would expect that in the end research would show that progressive filing of rakers has only small effect compared to a regular approach. Other variables may tend to obscure the measurable effect.

how does the fraternal order of police control rakers??

BobL said:

I'd like to work out your cutting angle but for that I need the gullet width not the cutter length.

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Gullet lengths from the base of the leading cutter to the top of the trailing cutter are here http://www.arboristsite.com/showpost.php?p=1816825&postcount=76, as are a couple of more photos.
The other dimension I have is the height of the cutter.
With that particular filed A2EP chain it sits at 12.5mm high, the odd one as high as 12.65mm high, measured from where it rides on the rail.

eyolf said:

it occurs to me that there will be an optimum cutting angle for every type of chain, but that it will actually fall into a range, dependent on a lot of variables. I'm not a physics major and know little about the physics of chainsaw cutters in action.

I have, however, come in contact with a bit of cutter geometry study in my capacity as team lead in a foundry pattern shop, and years ago as a cabinet maker. It occurs to me that chain geometry is quite different, as the cutter itself has a fair amount of latitude.

Click to expand...

Yep, I agree, chain is surprisingly forgiving and I doubt whether in most softwood cases that the average Joe would notice using constant depth rakers versus an FOP, let alone using FOP versus true constant cutting angle geometry.

What you're describing as cutting angle is roughly correspondent to "rake" as well as "clearance" of, say, a saw tooth or milling cutter.

Click to expand...

Yep I use this term (not to be confused with tooth "rake" which is "the angle the gullet makes between the leading edge and the cutting surface.) I also prefer to use rake but I didn't want to confuse "constant raker depth" with "constant rake" because on a CS constant rake requires progressive raker depths.

I understand metal is quite different to wood but the analogy with rake in metal work is a very good one since it is fairly critical parameter and different rakes are used in cutting tools and for different materials. The key to successful metal work and to a lesser extent in wood working is using the right rake. The reasons in woodworking extend to more than just cutting and can relate to things like finish.

Because it is woodwork based, a better analogy is imagine trying to cross cut a log by hand with a 1/4" chisel but you were only allowed to use a very shallow cutting angle to chip away at the wood. The opposite would be trying to cut it but you had to use a very high cutting angle. Clearly there will be an optimum angle which will work best for the effort that can be applied and if the wood is similar all the way through that angle will by and large be a constant one.

In soft wood at high speeds, I believe the cutter more or less shaves it's way through several millimeters of wood, generally undulating in and out of maximum "dig".

Click to expand...

I think the process is the same in hardwood but the depth cutter penetrates less before the break out occurs so the cutting waves are shorter. Because I mill hardwood and stand above the bar sometimes I can see the chain form waves even on the exposed bar poking out of the side of the log, with the softer the wood the longer the waves. If the cutting waves length matches some value of the cutting speed a significant washboard effect is generated like the image on the left.

But what about harder wood, what about the difference between winter and summer wood, relative spacing of growth rings, and hitting knots and other hard spots? We've all probably noticed the pattern of saw scratches on the end of a log, and how they vary, dependant upon chain speed, width of log, even the presence or absence of knots and etc.

Click to expand...

Yep I agree these are all variables we have to contend with but I don't think hardwood is that different and doesn't negate the fact that a reasonable and constant rake is worth having.

There is no question that if the rakers are too high (ie cutting angle is too low) at any time of a cutters life that the chain makes more powder instead of chips. The chain has to make more passes to cut the same amount of wood, more fuel is used, and greater chain wear is experienced. Hence it makes sense to maintain an optimum and constant rake throughout the whole life of the cutter.

There is also no question that regular (constant depth) raker gauges reduces rake significantly during the life of the cutter. I started milling in mostly big Aussie hard wood about 3 years ago and used constant depth rakers because that is what I always used for cross cutting small logs. What I found was that my freshly sharpened chains began to cut a lot slower and make a lot more powder after the cutters were only about 1/3rd worn, and were virtually unusable after they were about half worn. Switching to a constant rake (progressive raker depth) allowed me to make great chips all the way to the end of the cutters life. I now stop using a chain when a cutter or two breaks off.

This is an interesting thread, but I would expect that in the end research would show that progressive filing of rakers has only small effect compared to a regular approach. Other variables may tend to obscure the measurable effect.

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I agree differences will be marginal for most softwood cutters, but in harder hardwoods and at the heavy duty end of the process I reckon the difference are very clear. Even for Aussie weekend warrior fire wood cutters constant rake is worth using. People have brought me their saws with reasonably correctly filed 1/2 worn cutters and constant depth rakers, and they tell me - it just doesn't cut like it used to". I don't touch the cutters and give the rakers a progressive treatment and the owners are blown away - "Wow just like when it was new" , is the common feedback I get.

Bobl, in breif, are you saying the important factor, given teeth being correctly and uniformly sharpend, that an angle of say 6degrees for hard wood be maintained. This angle being from the cutting edger to the raker.

what raker tools(make, model) do you recomend?

Serg

Husky combi gauge is better I think, its more progressive. The pivot point is a little farther back then the FOP gauge.

parrisw said:

Husky combi gauge is better I think, its more progressive. The pivot point is a little farther back then the FOP gauge.

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The full chisel FOP pivot point is further back than the semi one too so you will get lower rakers/depth gauges, and you need less raker depth for very hard timber than soft woods anyway.
The pivot point depends on the length of the cutters too, they do vary between brands.

Actually, where does the Husky pivot point end up with it in the hardwood position ?

tdi-rick said:

The full chisel FOP pivot point is further back than the semi one too so you will get lower rakers/depth gauges, and you need less raker depth for very hard timber than soft woods anyway.
Actually, where does the Husky pivot point end up with it in the hardwood position ?

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The pivot point is the same I think, but the hole is in a different position. I only use the soft setting.