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.
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
Another couple of good reason to use calipers are
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.
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)
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