This is a quick illustration of how the position and diameter of a round file affects the top plate cutting bevel angle (red line). Holding the file higher, or using a larger diameter file, widens this angle (more obtuse). Lowering the file, or using a smaller diameter, makes the angle narrow (more acute).
Some guys are only looking at the 'hook' or the gullet profile, but this edge is where half the cutting occurs (side plate edge does the other half). This angle makes a difference in the types of wood cut (hard, soft, green, dead, etc.), how fast the chain cuts, and how long an edge lasts. Different preferences for different applications. But you can still enlarge the gullet with separate passes, or different files, after sharpening this edge, just like you use a different file for the depth gauges, etc. Or, you can try using a larger diameter file, and holding it lower, to obtain a similar bevel angle.
The folks at STIHL, Oregon, etc. pick these angles / file diameters / positions for general use, based on their experience over 75 years. You don't have to follow their recommendations, but they are good starting points.
Philbert
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