Did you ever have someone tell you, "There's nothing like a new chain" and think he needed to learn to sharpen?
Well, he's actually right, if you always take the rakers down to 0.25" below the tooth.
I can't find a good picture on the internet, so here's a real life example from my workbench.
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Here's a fairly new cutter with most of the tooth length left.
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The old 0.25" raker gauge says this raker is way too low
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A FOP progressive says it is just about perfect, maybe a hair low
The calipers say the tooth is .505 high and the raker is .480 high, a difference of .035.
The calipers also say the distance from the tip of the raker to the tip of the cutter is .375.
Here is the triangle formed by the tip of the raker, the tip of the cutter, and the wood being cut. (I did the wrong color in the picture, .375 is the green line not the red one).
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Solve the right triangle to get the angle of attack there. A new chain generally bites at 5.7°.
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So that raker is .035, and making a 5.3° angle to the wood.... lower than a factory chain at 0.25"!
I put the digital angle finder on it and got 5.5, so close enough.
Now add some wear to the tooth.
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Now the tooth is .490 high, the raker is .445, a difference of 0.045. .375" In a straight line between the point of the tooth and the top of the raker.
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So 6.8°... this raker is actually a little aggressive but not bad enough that I'd file the tooth back to fix it.
If this depth gauge was set at 0.25"....
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We'd be cutting at 3.8 degrees and making tiny chips and sawdust.
The upshot is that if you always keep your rakers at 0.25", you will be cutting at around 2° at the end of the chain's life- barely any bite!
Progressive gauges like the FOPs are a huge improvement but they still don't keep you at 5.7 by the end of the tooth.
Hope this made sense. It's all about the angle between the raker and the place where the wood meets the tooth! Small angle, little bite. Big angle, big bite. Big bite cuts faster; too big gets rough and is hard on the saw, chain, and operator.
Interestingly, and this makes sense, I heard a guy say that you can have teeth twice as long on one side than the other and still cut straight if all your rakers are set to the same angle. (Haven't tried it, want to.)